Internet Engineering Task Force (IETF) T. Haynes, Ed.
Request for Comments: 7530 Primary Data
Obsoletes: 3530 D. Noveck, Ed.
Category: Standards Track Dell
ISSN: 2070-1721 March 2015
Abstract
The Network File System (NFS) version 4 protocol is a distributed
file system protocol that builds on the heritage of NFS protocol
version 2 (RFC 1094) and version 3 (RFC 1813). Unlike earlier
versions, the NFS version 4 protocol supports traditional file access
while integrating support for file locking and the MOUNT protocol.
In addition, support for strong security (and its negotiation),
COMPOUND operations, client caching, and internationalization has
been added. Of course, attention has been applied to making NFS
version 4 operate well in an Internet environment.
This document, together with the companion External Data
Representation (XDR) description document, RFC 7531, obsoletes RFC
3530 as the definition of the NFS version 4 protocol.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7530.
Haynes & Noveck Standards Track [Page 1]
RFC 7530 NFSv4 March 2015
Copyright Notice
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Table of Contents
1. Introduction ....................................................8
1.1. Requirements Language ......................................8
1.2. NFS Version 4 Goals ........................................8
1.3. Definitions in the Companion Document RFC 7531 Are
Authoritative ..............................................9
1.4. Overview of NFSv4 Features .................................9
1.4.1. RPC and Security ....................................9
1.4.2. Procedure and Operation Structure ..................10
1.4.3. File System Model ..................................10
1.4.4. OPEN and CLOSE .....................................12
1.4.5. File Locking .......................................12
1.4.6. Client Caching and Delegation ......................13
1.5. General Definitions .......................................14
1.6. Changes since RFC 3530 ....................................16
1.7. Changes between RFC 3010 and RFC 3530 .....................16
2. Protocol Data Types ............................................18
2.1. Basic Data Types ..........................................18
2.2. Structured Data Types .....................................21
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3. RPC and Security Flavor ........................................25
3.1. Ports and Transports ......................................25
3.1.1. Client Retransmission Behavior .....................26
3.2. Security Flavors ..........................................27
3.2.1. Security Mechanisms for NFSv4 ......................27
3.3. Security Negotiation ......................................28
3.3.1. SECINFO ............................................29
3.3.2. Security Error .....................................29
3.3.3. Callback RPC Authentication ........................29
4. Filehandles ....................................................30
4.1. Obtaining the First Filehandle ............................30
4.1.1. Root Filehandle ....................................31
4.1.2. Public Filehandle ..................................31
4.2. Filehandle Types ..........................................31
4.2.1. General Properties of a Filehandle .................32
4.2.2. Persistent Filehandle ..............................32
4.2.3. Volatile Filehandle ................................33
4.2.4. One Method of Constructing a Volatile Filehandle ...34
4.3. Client Recovery from Filehandle Expiration ................35
5. Attributes .....................................................35
5.1. REQUIRED Attributes .......................................37
5.2. RECOMMENDED Attributes ....................................37
5.3. Named Attributes ..........................................37
5.4. Classification of Attributes ..............................39
5.5. Set-Only and Get-Only Attributes ..........................40
5.6. REQUIRED Attributes - List and Definition References ......40
5.7. RECOMMENDED Attributes - List and Definition References ...41
5.8. Attribute Definitions .....................................42
5.8.1. Definitions of REQUIRED Attributes .................42
5.8.2. Definitions of Uncategorized RECOMMENDED
Attributes .........................................45
5.9. Interpreting owner and owner_group ........................51
5.10. Character Case Attributes ................................53
6. Access Control Attributes ......................................54
6.1. Goals .....................................................54
6.2. File Attributes Discussion ................................55
6.2.1. Attribute 12: acl ..................................55
6.2.2. Attribute 33: mode .................................70
6.3. Common Methods ............................................71
6.3.1. Interpreting an ACL ................................71
6.3.2. Computing a mode Attribute from an ACL .............72
6.4. Requirements ..............................................73
6.4.1. Setting the mode and/or ACL Attributes .............74
6.4.2. Retrieving the mode and/or ACL Attributes ..........75
6.4.3. Creating New Objects ...............................75
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7. NFS Server Namespace ...........................................77
7.1. Server Exports ............................................77
7.2. Browsing Exports ..........................................77
7.3. Server Pseudo-File System .................................78
7.4. Multiple Roots ............................................79
7.5. Filehandle Volatility .....................................79
7.6. Exported Root .............................................79
7.7. Mount Point Crossing ......................................79
7.8. Security Policy and Namespace Presentation ................80
8. Multi-Server Namespace .........................................81
8.1. Location Attributes .......................................81
8.2. File System Presence or Absence ...........................81
8.3. Getting Attributes for an Absent File System ..............83
8.3.1. GETATTR within an Absent File System ...............83
8.3.2. READDIR and Absent File Systems ....................84
8.4. Uses of Location Information ..............................84
8.4.1. File System Replication ............................85
8.4.2. File System Migration ..............................86
8.4.3. Referrals ..........................................86
8.5. Location Entries and Server Identity ......................87
8.6. Additional Client-Side Considerations .....................88
8.7. Effecting File System Referrals ...........................89
8.7.1. Referral Example (LOOKUP) ..........................89
8.7.2. Referral Example (READDIR) .........................93
8.8. The Attribute fs_locations ................................96
9. File Locking and Share Reservations ............................98
9.1. Opens and Byte-Range Locks ................................99
9.1.1. Client ID ..........................................99
9.1.2. Server Release of Client ID .......................102
9.1.3. Use of Seqids .....................................103
9.1.4. Stateid Definition ................................104
9.1.5. Lock-Owner ........................................110
9.1.6. Use of the Stateid and Locking ....................110
9.1.7. Sequencing of Lock Requests .......................113
9.1.8. Recovery from Replayed Requests ...................114
9.1.9. Interactions of Multiple Sequence Values ..........114
9.1.10. Releasing State-Owner State ......................115
9.1.11. Use of Open Confirmation .........................116
9.2. Lock Ranges ..............................................117
9.3. Upgrading and Downgrading Locks ..........................117
9.4. Blocking Locks ...........................................118
9.5. Lease Renewal ............................................119
9.6. Crash Recovery ...........................................120
9.6.1. Client Failure and Recovery .......................120
9.6.2. Server Failure and Recovery .......................120
9.6.3. Network Partitions and Recovery ...................122
9.7. Recovery from a Lock Request Timeout or Abort ............130
9.8. Server Revocation of Locks ...............................130
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9.9. Share Reservations .......................................132
9.10. OPEN/CLOSE Operations ...................................132
9.10.1. Close and Retention of State Information .........133
9.11. Open Upgrade and Downgrade ..............................134
9.12. Short and Long Leases ...................................135
9.13. Clocks, Propagation Delay, and Calculating Lease
Expiration ..............................................135
9.14. Migration, Replication, and State .......................136
9.14.1. Migration and State ..............................136
9.14.2. Replication and State ............................137
9.14.3. Notification of Migrated Lease ...................137
9.14.4. Migration and the lease_time Attribute ...........138
10. Client-Side Caching ..........................................139
10.1. Performance Challenges for Client-Side Caching ..........139
10.2. Delegation and Callbacks ................................140
10.2.1. Delegation Recovery ..............................142
10.3. Data Caching ............................................147
10.3.1. Data Caching and OPENs ...........................147
10.3.2. Data Caching and File Locking ....................148
10.3.3. Data Caching and Mandatory File Locking ..........150
10.3.4. Data Caching and File Identity ...................150
10.4. Open Delegation .........................................151
10.4.1. Open Delegation and Data Caching .................154
10.4.2. Open Delegation and File Locks ...................155
10.4.3. Handling of CB_GETATTR ...........................155
10.4.4. Recall of Open Delegation ........................158
10.4.5. OPEN Delegation Race with CB_RECALL ..............160
10.4.6. Clients That Fail to Honor Delegation Recalls ....161
10.4.7. Delegation Revocation ............................162
10.5. Data Caching and Revocation .............................162
10.5.1. Revocation Recovery for Write Open Delegation ....163
10.6. Attribute Caching .......................................164
10.7. Data and Metadata Caching and Memory-Mapped Files .......166
10.8. Name Caching ............................................168
10.9. Directory Caching .......................................169
11. Minor Versioning .............................................170
12. Internationalization .........................................170
12.1. Introduction ............................................170
12.2. Limitations on Internationalization-Related
Processing in the NFSv4 Context .........................172
12.3. Summary of Server Behavior Types ........................173
12.4. String Encoding .........................................173
12.5. Normalization ...........................................174
12.6. Types with Processing Defined by Other Internet Areas ...175
12.7. Errors Related to UTF-8 .................................177
12.8. Servers That Accept File Component Names That
Are Not Valid UTF-8 Strings .............................177
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13. Error Values .................................................178
13.1. Error Definitions .......................................179
13.1.1. General Errors ...................................180
13.1.2. Filehandle Errors ................................181
13.1.3. Compound Structure Errors ........................183
13.1.4. File System Errors ...............................184
13.1.5. State Management Errors ..........................186
13.1.6. Security Errors ..................................187
13.1.7. Name Errors ......................................187
13.1.8. Locking Errors ...................................188
13.1.9. Reclaim Errors ...................................190
13.1.10. Client Management Errors ........................191
13.1.11. Attribute Handling Errors .......................191
13.1.12. Miscellaneous Errors ............................191
13.2. Operations and Their Valid Errors .......................192
13.3. Callback Operations and Their Valid Errors ..............200
13.4. Errors and the Operations That Use Them .................201
14. NFSv4 Requests ...............................................206
14.1. COMPOUND Procedure ......................................207
14.2. Evaluation of a COMPOUND Request ........................207
14.3. Synchronous Modifying Operations ........................208
14.4. Operation Values ........................................208
15. NFSv4 Procedures .............................................209
15.1. Procedure 0: NULL - No Operation ........................209
15.2. Procedure 1: COMPOUND - COMPOUND Operations .............210
16. NFSv4 Operations .............................................214
16.1. Operation 3: ACCESS - Check Access Rights ...............214
16.2. Operation 4: CLOSE - Close File .........................217
16.3. Operation 5: COMMIT - Commit Cached Data ................218
16.4. Operation 6: CREATE - Create a Non-regular File Object ..221
16.5. Operation 7: DELEGPURGE - Purge Delegations
Awaiting Recovery .......................................224
16.6. Operation 8: DELEGRETURN - Return Delegation ............226
16.7. Operation 9: GETATTR - Get Attributes ...................227
16.8. Operation 10: GETFH - Get Current Filehandle ............229
16.9. Operation 11: LINK - Create Link to a File ..............230
16.10. Operation 12: LOCK - Create Lock .......................232
16.11. Operation 13: LOCKT - Test for Lock ....................236
16.12. Operation 14: LOCKU - Unlock File ......................238
16.13. Operation 15: LOOKUP - Look Up Filename ................240
16.14. Operation 16: LOOKUPP - Look Up Parent Directory .......242
16.15. Operation 17: NVERIFY - Verify Difference in
Attributes .............................................243
16.16. Operation 18: OPEN - Open a Regular File ...............245
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16.17. Operation 19: OPENATTR - Open Named Attribute
Directory ..............................................256
16.18. Operation 20: OPEN_CONFIRM - Confirm Open ..............257
16.19. Operation 21: OPEN_DOWNGRADE - Reduce Open File
Access .................................................260
16.20. Operation 22: PUTFH - Set Current Filehandle ...........262
16.21. Operation 23: PUTPUBFH - Set Public Filehandle .........263
16.22. Operation 24: PUTROOTFH - Set Root Filehandle ..........265
16.23. Operation 25: READ - Read from File ....................266
16.24. Operation 26: READDIR - Read Directory .................269
16.25. Operation 27: READLINK - Read Symbolic Link ............273
16.26. Operation 28: REMOVE - Remove File System Object .......274
16.27. Operation 29: RENAME - Rename Directory Entry ..........276
16.28. Operation 30: RENEW - Renew a Lease ....................278
16.29. Operation 31: RESTOREFH - Restore Saved Filehandle .....280
16.30. Operation 32: SAVEFH - Save Current Filehandle .........281
16.31. Operation 33: SECINFO - Obtain Available Security ......282
16.32. Operation 34: SETATTR - Set Attributes .................286
16.33. Operation 35: SETCLIENTID - Negotiate Client ID ........289
16.34. Operation 36: SETCLIENTID_CONFIRM - Confirm Client ID ..293
16.35. Operation 37: VERIFY - Verify Same Attributes ..........297
16.36. Operation 38: WRITE - Write to File ....................299
16.37. Operation 39: RELEASE_LOCKOWNER - Release
Lock-Owner State .......................................304
16.38. Operation 10044: ILLEGAL - Illegal Operation ...........305
17. NFSv4 Callback Procedures ....................................306
17.1. Procedure 0: CB_NULL - No Operation .....................306
17.2. Procedure 1: CB_COMPOUND - COMPOUND Operations ..........307
18. NFSv4 Callback Operations ....................................309
18.1. Operation 3: CB_GETATTR - Get Attributes ................309
18.2. Operation 4: CB_RECALL - Recall an Open Delegation ......310
18.3. Operation 10044: CB_ILLEGAL - Illegal Callback
Operation ...............................................311
19. Security Considerations ......................................312
20. IANA Considerations ..........................................314
20.1. Named Attribute Definitions .............................314
20.1.1. Initial Registry .................................315
20.1.2. Updating Registrations ...........................315
20.2. Updates to Existing IANA Registries .....................315
21. References ...................................................316
21.1. Normative References ....................................316
21.2. Informative References ..................................318
Acknowledgments ..................................................322
Authors' Addresses ...............................................323
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1. Introduction
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119],
except where "REQUIRED" and "RECOMMENDED" are used as qualifiers to
distinguish classes of attributes as described in Sections 1.4.3.2
and 5 of this document.
1.2. NFS Version 4 Goals
The Network File System version 4 (NFSv4) protocol is a further
revision of the NFS protocol defined already by versions 2 [RFC1094]
and 3 [RFC1813]. It retains the essential characteristics of
previous versions: design for easy recovery; independent of transport
protocols, operating systems, and file systems; simplicity; and good
performance. The NFSv4 revision has the following goals:
o Improved access and good performance on the Internet.
The protocol is designed to transit firewalls easily, perform well
where latency is high and bandwidth is low, and scale to very
large numbers of clients per server.
o Strong security with negotiation built into the protocol.
The protocol builds on the work of the Open Network Computing
(ONC) Remote Procedure Call (RPC) working group in supporting the
RPCSEC_GSS protocol (see both [RFC2203] and [RFC5403]).
Additionally, the NFSv4 protocol provides a mechanism to allow
clients and servers the ability to negotiate security and require
clients and servers to support a minimal set of security schemes.
o Good cross-platform interoperability.
The protocol features a file system model that provides a useful,
common set of features that does not unduly favor one file system
or operating system over another.
o Designed for protocol extensions.
The protocol is designed to accept standard extensions that do not
compromise backward compatibility.
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This document, together with the companion External Data
Representation (XDR) description document [RFC7531], obsoletes
[RFC3530] as the authoritative document describing NFSv4. It does
not introduce any over-the-wire protocol changes, in the sense that
previously valid requests remain valid.
1.3. Definitions in the Companion Document RFC 7531 Are Authoritative
The "Network File System (NFS) Version 4 External Data Representation
Standard (XDR) Description" [RFC7531] contains the definitions in XDR
description language of the constructs used by the protocol. Inside
this document, several of the constructs are reproduced for purposes
of explanation. The reader is warned of the possibility of errors in
the reproduced constructs outside of [RFC7531]. For any part of the
document that is inconsistent with [RFC7531], [RFC7531] is to be
considered authoritative.
1.4. Overview of NFSv4 Features
To provide a reasonable context for the reader, the major features of
the NFSv4 protocol will be reviewed in brief. This is done to
provide an appropriate context for both the reader who is familiar
with the previous versions of the NFS protocol and the reader who is
new to the NFS protocols. For the reader new to the NFS protocols,
some fundamental knowledge is still expected. The reader should be
familiar with the XDR and RPC protocols as described in [RFC4506] and
[RFC5531]. A basic knowledge of file systems and distributed file
systems is expected as well.
1.4.1. RPC and Security
As with previous versions of NFS, the XDR and RPC mechanisms used for
the NFSv4 protocol are those defined in [RFC4506] and [RFC5531]. To
meet end-to-end security requirements, the RPCSEC_GSS framework (both
version 1 in [RFC2203] and version 2 in [RFC5403]) will be used to
extend the basic RPC security. With the use of RPCSEC_GSS, various
mechanisms can be provided to offer authentication, integrity, and
privacy to the NFSv4 protocol. Kerberos V5 will be used as described
in [RFC4121] to provide one security framework. With the use of
RPCSEC_GSS, other mechanisms may also be specified and used for NFSv4
security.
To enable in-band security negotiation, the NFSv4 protocol has added
a new operation that provides the client with a method of querying
the server about its policies regarding which security mechanisms
must be used for access to the server's file system resources. With
this, the client can securely match the security mechanism that meets
the policies specified at both the client and server.
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1.4.2. Procedure and Operation Structure
A significant departure from the previous versions of the NFS
protocol is the introduction of the COMPOUND procedure. For the
NFSv4 protocol, there are two RPC procedures: NULL and COMPOUND. The
COMPOUND procedure is defined in terms of operations, and these
operations correspond more closely to the traditional NFS procedures.
With the use of the COMPOUND procedure, the client is able to build
simple or complex requests. These COMPOUND requests allow for a
reduction in the number of RPCs needed for logical file system
operations. For example, without previous contact with a server a
client will be able to read data from a file in one request by
combining LOOKUP, OPEN, and READ operations in a single COMPOUND RPC.
With previous versions of the NFS protocol, this type of single
request was not possible.
The model used for COMPOUND is very simple. There is no logical OR
or ANDing of operations. The operations combined within a COMPOUND
request are evaluated in order by the server. Once an operation
returns a failing result, the evaluation ends and the results of all
evaluated operations are returned to the client.
The NFSv4 protocol continues to have the client refer to a file or
directory at the server by a "filehandle". The COMPOUND procedure
has a method of passing a filehandle from one operation to another
within the sequence of operations. There is a concept of a current
filehandle and a saved filehandle. Most operations use the current
filehandle as the file system object to operate upon. The saved
filehandle is used as temporary filehandle storage within a COMPOUND
procedure as well as an additional operand for certain operations.
1.4.3. File System Model
The general file system model used for the NFSv4 protocol is the same
as previous versions. The server file system is hierarchical, with
the regular files contained within being treated as opaque byte
streams. In a slight departure, file and directory names are encoded
with UTF-8 to deal with the basics of internationalization.
The NFSv4 protocol does not require a separate protocol to provide
for the initial mapping between pathname and filehandle. Instead of
using the older MOUNT protocol for this mapping, the server provides
a root filehandle that represents the logical root or top of the file
system tree provided by the server. The server provides multiple
file systems by gluing them together with pseudo-file systems. These
pseudo-file systems provide for potential gaps in the pathnames
between real file systems.
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1.4.3.1. Filehandle Types
In previous versions of the NFS protocol, the filehandle provided by
the server was guaranteed to be valid or persistent for the lifetime
of the file system object to which it referred. For some server
implementations, this persistence requirement has been difficult to
meet. For the NFSv4 protocol, this requirement has been relaxed by
introducing another type of filehandle -- volatile. With persistent
and volatile filehandle types, the server implementation can match
the abilities of the file system at the server along with the
operating environment. The client will have knowledge of the type of
filehandle being provided by the server and can be prepared to deal
with the semantics of each.
1.4.3.2. Attribute Types
The NFSv4 protocol has a rich and extensible file object attribute
structure, which is divided into REQUIRED, RECOMMENDED, and named
attributes (see Section 5).
Several (but not all) of the REQUIRED attributes are derived from the
attributes of NFSv3 (see the definition of the fattr3 data type in
[RFC1813]). An example of a REQUIRED attribute is the file object's
type (Section 5.8.1.2) so that regular files can be distinguished
from directories (also known as folders in some operating
environments) and other types of objects. REQUIRED attributes are
discussed in Section 5.1.
An example of the RECOMMENDED attributes is an acl (Section 6.2.1).
This attribute defines an Access Control List (ACL) on a file object.
An ACL provides file access control beyond the model used in NFSv3.
The ACL definition allows for specification of specific sets of
permissions for individual users and groups. In addition, ACL
inheritance allows propagation of access permissions and restriction
down a directory tree as file system objects are created.
RECOMMENDED attributes are discussed in Section 5.2.
A named attribute is an opaque byte stream that is associated with a
directory or file and referred to by a string name. Named attributes
are meant to be used by client applications as a method to associate
application-specific data with a regular file or directory. NFSv4.1
modifies named attributes relative to NFSv4.0 by tightening the
allowed operations in order to prevent the development of
non-interoperable implementations. Named attributes are discussed in
Section 5.3.
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1.4.3.3. Multi-Server Namespace
A single-server namespace is the file system hierarchy that the
server presents for remote access. It is a proper subset of all the
file systems available locally. NFSv4 contains a number of features
to allow implementation of namespaces that cross server boundaries
and that allow and facilitate a non-disruptive transfer of support
for individual file systems between servers. They are all based upon
attributes that allow one file system to specify alternative or new
locations for that file system. That is, just as a client might
traverse across local file systems on a single server, it can now
traverse to a remote file system on a different server.
These attributes may be used together with the concept of absent file
systems, which provide specifications for additional locations but no
actual file system content. This allows a number of important
facilities:
o Location attributes may be used with absent file systems to
implement referrals whereby one server may direct the client to a
file system provided by another server. This allows extensive
multi-server namespaces to be constructed.
o Location attributes may be provided for present file systems to
provide the locations of alternative file system instances or
replicas to be used in the event that the current file system
instance becomes unavailable.
o Location attributes may be provided when a previously present file
system becomes absent. This allows non-disruptive migration of
file systems to alternative servers.
1.4.4. OPEN and CLOSE
The NFSv4 protocol introduces OPEN and CLOSE operations. The OPEN
operation provides a single point where file lookup, creation, and
share semantics (see Section 9.9) can be combined. The CLOSE
operation also provides for the release of state accumulated by OPEN.
1.4.5. File Locking
With the NFSv4 protocol, the support for byte-range file locking is
part of the NFS protocol. The file locking support is structured so
that an RPC callback mechanism is not required. This is a departure
from the previous versions of the NFS file locking protocol, Network
Lock Manager (NLM) [RFC1813]. The state associated with file locks
is maintained at the server under a lease-based model. The server
defines a single lease period for all state held by an NFS client.
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RFC 7530 NFSv4 March 2015
If the client does not renew its lease within the defined period, all
state associated with the client's lease may be released by the
server. The client may renew its lease by use of the RENEW operation
or implicitly by use of other operations (primarily READ).
1.4.6. Client Caching and Delegation
The file, attribute, and directory caching for the NFSv4 protocol is
similar to previous versions. Attributes and directory information
are cached for a duration determined by the client. At the end of a
predefined timeout, the client will query the server to see if the
related file system object has been updated.
For file data, the client checks its cache validity when the file is
opened. A query is sent to the server to determine if the file has
been changed. Based on this information, the client determines if
the data cache for the file should be kept or released. Also, when
the file is closed, any modified data is written to the server.
If an application wants to serialize access to file data, file
locking of the file data ranges in question should be used.
The major addition to NFSv4 in the area of caching is the ability of
the server to delegate certain responsibilities to the client. When
the server grants a delegation for a file to a client, the client is
guaranteed certain semantics with respect to the sharing of that file
with other clients. At OPEN, the server may provide the client
either a read (OPEN_DELEGATE_READ) or a write (OPEN_DELEGATE_WRITE)
delegation for the file (see Section 10.4). If the client is granted
an OPEN_DELEGATE_READ delegation, it is assured that no other client
has the ability to write to the file for the duration of the
delegation. If the client is granted an OPEN_DELEGATE_WRITE
delegation, the client is assured that no other client has read or
write access to the file.
Delegations can be recalled by the server. If another client
requests access to the file in such a way that the access conflicts
with the granted delegation, the server is able to notify the initial
client and recall the delegation. This requires that a callback path
exist between the server and client. If this callback path does not
exist, then delegations cannot be granted. The essence of a
delegation is that it allows the client to locally service operations
such as OPEN, CLOSE, LOCK, LOCKU, READ, or WRITE without immediate
interaction with the server.
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1.5. General Definitions
The following definitions are provided for the purpose of providing
an appropriate context for the reader.
Absent File System: A file system is "absent" when a namespace
component does not have a backing file system.
Anonymous Stateid: The Anonymous Stateid is a special locking object
and is defined in Section 9.1.4.3.
Byte: In this document, a byte is an octet, i.e., a datum exactly
8 bits in length.
Client: The client is the entity that accesses the NFS server's
resources. The client may be an application that contains the
logic to access the NFS server directly. The client may also be
the traditional operating system client that provides remote file
system services for a set of applications.
With reference to byte-range locking, the client is also the
entity that maintains a set of locks on behalf of one or more
applications. This client is responsible for crash or failure
recovery for those locks it manages.
Note that multiple clients may share the same transport and
connection, and multiple clients may exist on the same network
node.
Client ID: The client ID is a 64-bit quantity used as a unique,
shorthand reference to a client-supplied verifier and ID. The
server is responsible for supplying the client ID.
File System: The file system is the collection of objects on a
server that share the same fsid attribute (see Section 5.8.1.9).
Lease: A lease is an interval of time defined by the server for
which the client is irrevocably granted a lock. At the end of a
lease period the lock may be revoked if the lease has not been
extended. The lock must be revoked if a conflicting lock has been
granted after the lease interval.
All leases granted by a server have the same fixed duration. Note
that the fixed interval duration was chosen to alleviate the
expense a server would have in maintaining state about variable-
length leases across server failures.
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Lock: The term "lock" is used to refer to record (byte-range) locks
as well as share reservations unless specifically stated
otherwise.
Lock-Owner: Each byte-range lock is associated with a specific
lock-owner and an open-owner. The lock-owner consists of a
client ID and an opaque owner string. The client presents this to
the server to establish the ownership of the byte-range lock as
needed.
Open-Owner: Each open file is associated with a specific open-owner,
which consists of a client ID and an opaque owner string. The
client presents this to the server to establish the ownership of
the open as needed.
READ Bypass Stateid: The READ Bypass Stateid is a special locking
object and is defined in Section 9.1.4.3.
Server: The "server" is the entity responsible for coordinating
client access to a set of file systems.
Stable Storage: NFSv4 servers must be able to recover without data
loss from multiple power failures (including cascading power
failures, that is, several power failures in quick succession),
operating system failures, and hardware failure of components
other than the storage medium itself (for example, disk,
non-volatile RAM).
Some examples of stable storage that are allowable for an NFS
server include:
(1) Media commit of data. That is, the modified data has been
successfully written to the disk media -- for example, the
disk platter.
(2) An immediate reply disk drive with battery-backed on-drive
intermediate storage or uninterruptible power system (UPS).
(3) Server commit of data with battery-backed intermediate
storage and recovery software.
(4) Cache commit with UPS and recovery software.
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Stateid: A stateid is a 128-bit quantity returned by a server that
uniquely identifies the open and locking states provided by the
server for a specific open-owner or lock-owner/open-owner pair for
a specific file and type of lock.
Verifier: A verifier is a 64-bit quantity generated by the client
that the server can use to determine if the client has restarted
and lost all previous lock state.
The main changes from RFC 3530 [RFC3530] are:
o The XDR definition has been moved to a companion document
[RFC7531].
o The IETF intellectual property statements were updated to the
latest version.
o There is a restructured and more complete explanation of multi-
server namespace features.
o The handling of domain names was updated to reflect
Internationalized Domain Names in Applications (IDNA) [RFC5891].
o The previously required LIPKEY and SPKM-3 security mechanisms have
been removed.
o Some clarification was provided regarding a client re-establishing
callback information to the new server if state has been migrated.
o A third edge case was added for courtesy locks and network
partitions.
o The definition of stateid was strengthened.
The definition of the NFSv4 protocol in [RFC3530] replaced and
obsoleted the definition present in [RFC3010]. While portions of the
two documents remained the same, there were substantive changes in
others. The changes made between [RFC3010] and [RFC3530] reflect
implementation experience and further review of the protocol.
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The following list is not inclusive of all changes but presents some
of the most notable changes or additions made:
o The state model has added an open_owner4 identifier. This was
done to accommodate POSIX-based clients and the model they use for
file locking. For POSIX clients, an open_owner4 would correspond
to a file descriptor potentially shared amongst a set of processes
and the lock_owner4 identifier would correspond to a process that
is locking a file.
o Added clarifications and error conditions for the handling of the
owner and group attributes. Since these attributes are string
based (as opposed to the numeric uid/gid of previous versions of
NFS), translations may not be available and hence the changes
made.
o Added clarifications for the ACL and mode attributes to address
evaluation and partial support.
o For identifiers that are defined as XDR opaque, set limits on
their size.
o Added the mounted_on_fileid attribute to allow POSIX clients to
correctly construct local mounts.
o Modified the SETCLIENTID/SETCLIENTID_CONFIRM operations to deal
correctly with confirmation details along with adding the ability
to specify new client callback information. Also added
clarification of the callback information itself.
o Added a new operation RELEASE_LOCKOWNER to enable notifying the
server that a lock_owner4 will no longer be used by the client.
o Added RENEW operation changes to identify the client correctly and
allow for additional error returns.
o Verified error return possibilities for all operations.
o Removed use of the pathname4 data type from LOOKUP and OPEN in
favor of having the client construct a sequence of LOOKUP
operations to achieve the same effect.
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2. Protocol Data Types
The syntax and semantics to describe the data types of the NFSv4
protocol are defined in the XDR [RFC4506] and RPC [RFC5531]
documents. The next sections build upon the XDR data types to define
types and structures specific to this protocol. As a reminder, the
size constants and authoritative definitions can be found in
[RFC7531].
2.1. Basic Data Types
Table 1 lists the base NFSv4 data types.
+-----------------+-------------------------------------------------+
| Data Type | Definition |
+-----------------+-------------------------------------------------+
| int32_t | typedef int int32_t; |
| | |
| uint32_t | typedef unsigned int uint32_t; |
| | |
| int64_t | typedef hyper int64_t; |
| | |
| uint64_t | typedef unsigned hyper uint64_t; |
| | |
| attrlist4 | typedef opaque attrlist4<>; |
| | |
| | Used for file/directory attributes. |
| | |
| bitmap4 | typedef uint32_t bitmap4<>; |
| | |
| | Used in attribute array encoding. |
| | |
| changeid4 | typedef uint64_t changeid4; |
| | |
| | Used in the definition of change_info4. |
| | |
| clientid4 | typedef uint64_t clientid4; |
| | |
| | Shorthand reference to client identification. |
| | |
| count4 | typedef uint32_t count4; |
| | |
| | Various count parameters (READ, WRITE, COMMIT). |
| | |
| length4 | typedef uint64_t length4; |
| | |
| | Describes LOCK lengths. |
| | |
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| mode4 | typedef uint32_t mode4; |
| | |
| | Mode attribute data type. |
| | |
| nfs_cookie4 | typedef uint64_t nfs_cookie4; |
| | |
| | Opaque cookie value for READDIR. |
| | |
| nfs_fh4 | typedef opaque nfs_fh4<NFS4_FHSIZE>; |
| | |
| | Filehandle definition. |
| | |
| nfs_ftype4 | enum nfs_ftype4; |
| | |
| | Various defined file types. |
| | |
| nfsstat4 | enum nfsstat4; |
| | |
| | Return value for operations. |
| | |
| nfs_lease4 | typedef uint32_t nfs_lease4; |
| | |
| | Duration of a lease in seconds. |
| | |
| offset4 | typedef uint64_t offset4; |
| | |
| | Various offset designations (READ, WRITE, LOCK, |
| | COMMIT). |
| | |
| qop4 | typedef uint32_t qop4; |
| | |
| | Quality of protection designation in SECINFO. |
| | |
| sec_oid4 | typedef opaque sec_oid4<>; |
| | |
| | Security Object Identifier. The sec_oid4 data |
| | type is not really opaque. Instead, it |
| | contains an ASN.1 OBJECT IDENTIFIER as used by |
| | GSS-API in the mech_type argument to |
| | GSS_Init_sec_context. See [RFC2743] for |
| | details. |
| | |
| seqid4 | typedef uint32_t seqid4; |
| | |
| | Sequence identifier used for file locking. |
| | |
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| utf8string | typedef opaque utf8string<>; |
| | |
| | UTF-8 encoding for strings. |
| | |
| utf8str_cis | typedef utf8string utf8str_cis; |
| | |
| | Case-insensitive UTF-8 string. |
| | |
| utf8str_cs | typedef utf8string utf8str_cs; |
| | |
| | Case-sensitive UTF-8 string. |
| | |
| utf8str_mixed | typedef utf8string utf8str_mixed; |
| | |
| | UTF-8 strings with a case-sensitive prefix and |
| | a case-insensitive suffix. |
| | |
| component4 | typedef utf8str_cs component4; |
| | |
| | Represents pathname components. |
| | |
| linktext4 | typedef opaque linktext4<>; |
| | |
| | Symbolic link contents ("symbolic link" is |
| | defined in an Open Group [openg_symlink] |
| | standard). |
| | |
| ascii_REQUIRED4 | typedef utf8string ascii_REQUIRED4; |
| | |
| | String is sent as ASCII and thus is |
| | automatically UTF-8. |
| | |
| pathname4 | typedef component4 pathname4<>; |
| | |
| | Represents pathname for fs_locations. |
| | |
| nfs_lockid4 | typedef uint64_t nfs_lockid4; |
| | |
| verifier4 | typedef opaque verifier4[NFS4_VERIFIER_SIZE]; |
| | |
| | Verifier used for various operations (COMMIT, |
| | CREATE, OPEN, READDIR, WRITE) |
| | NFS4_VERIFIER_SIZE is defined as 8. |
+-----------------+-------------------------------------------------+
Table 1: Base NFSv4 Data Types
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2.2. Structured Data Types
struct nfstime4 {
int64_t seconds;
uint32_t nseconds;
};
The nfstime4 structure gives the number of seconds and nanoseconds
since midnight or 0 hour January 1, 1970 Coordinated Universal Time
(UTC). Values greater than zero for the seconds field denote dates
after the 0 hour January 1, 1970. Values less than zero for the
seconds field denote dates before the 0 hour January 1, 1970. In
both cases, the nseconds field is to be added to the seconds field
for the final time representation. For example, if the time to be
represented is one-half second before 0 hour January 1, 1970, the
seconds field would have a value of negative one (-1) and the
nseconds fields would have a value of one-half second (500000000).
Values greater than 999,999,999 for nseconds are considered invalid.
This data type is used to pass time and date information. A server
converts to and from its local representation of time when processing
time values, preserving as much accuracy as possible. If the
precision of timestamps stored for a file system object is less than
defined, loss of precision can occur. An adjunct time maintenance
protocol is recommended to reduce client and server time skew.
2.2.2. time_how4
enum time_how4 {
SET_TO_SERVER_TIME4 = 0,
SET_TO_CLIENT_TIME4 = 1
};
union settime4 switch (time_how4 set_it) {
case SET_TO_CLIENT_TIME4:
nfstime4 time;
default:
void;
};
The above definitions are used as the attribute definitions to set
time values. If set_it is SET_TO_SERVER_TIME4, then the server uses
its local representation of time for the time value.
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2.2.4. specdata4
struct specdata4 {
uint32_t specdata1; /* major device number */
uint32_t specdata2; /* minor device number */
};
This data type represents additional information for the device file
types NF4CHR and NF4BLK.
struct fsid4 {
uint64_t major;
uint64_t minor;
};
This type is the file system identifier that is used as a REQUIRED
attribute.
2.2.6. fs_location4
struct fs_location4 {
utf8str_cis server<>;
pathname4 rootpath;
};
2.2.7. fs_locations4
struct fs_locations4 {
pathname4 fs_root;
fs_location4 locations<>;
};
The fs_location4 and fs_locations4 data types are used for the
fs_locations RECOMMENDED attribute, which is used for migration and
replication support.
struct fattr4 {
bitmap4 attrmask;
attrlist4 attr_vals;
};
The fattr4 structure is used to represent file and directory
attributes.
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The bitmap is a counted array of 32-bit integers used to contain bit
values. The position of the integer in the array that contains bit n
can be computed from the expression (n / 32), and its bit within that
integer is (n mod 32).
0 1
+-----------+-----------+-----------+--
| count | 31 .. 0 | 63 .. 32 |
+-----------+-----------+-----------+--
2.2.9. change_info4
struct change_info4 {
bool atomic;
changeid4 before;
changeid4 after;
};
This structure is used with the CREATE, LINK, REMOVE, and RENAME
operations to let the client know the value of the change attribute
for the directory in which the target file system object resides.
2.2.10. clientaddr4
struct clientaddr4 {
/* see struct rpcb in RFC 1833 */
string r_netid<>; /* network id */
string r_addr<>; /* universal address */
};
The clientaddr4 structure is used as part of the SETCLIENTID
operation, either (1) to specify the address of the client that is
using a client ID or (2) as part of the callback registration. The
r_netid and r_addr fields respectively contain a network id and
universal address. The network id and universal address concepts,
together with formats for TCP over IPv4 and TCP over IPv6, are
defined in [RFC5665], specifically Tables 2 and 3 and
Sections 5.2.3.3 and 5.2.3.4.
struct cb_client4 {
unsigned int cb_program;
clientaddr4 cb_location;
};
This structure is used by the client to inform the server of its
callback address; it includes the program number and client address.
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2.2.12. nfs_client_id4
struct nfs_client_id4 {
verifier4 verifier;
opaque id<NFS4_OPAQUE_LIMIT>;
};
This structure is part of the arguments to the SETCLIENTID operation.
2.2.13. open_owner4
struct open_owner4 {
clientid4 clientid;
opaque owner<NFS4_OPAQUE_LIMIT>;
};
This structure is used to identify the owner of open state.
2.2.14. lock_owner4
struct lock_owner4 {
clientid4 clientid;
opaque owner<NFS4_OPAQUE_LIMIT>;
};
This structure is used to identify the owner of file locking state.
2.2.15. open_to_lock_owner4
struct open_to_lock_owner4 {
seqid4 open_seqid;
stateid4 open_stateid;
seqid4 lock_seqid;
lock_owner4 lock_owner;
};
This structure is used for the first LOCK operation done for an
open_owner4. It provides both the open_stateid and lock_owner such
that the transition is made from a valid open_stateid sequence to
that of the new lock_stateid sequence. Using this mechanism avoids
the confirmation of the lock_owner/lock_seqid pair since it is tied
to established state in the form of the open_stateid/open_seqid.
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struct stateid4 {
uint32_t seqid;
opaque other[NFS4_OTHER_SIZE];
};
This structure is used for the various state-sharing mechanisms
between the client and server. For the client, this data structure
is read-only. The server is required to increment the seqid field
monotonically at each transition of the stateid. This is important
since the client will inspect the seqid in OPEN stateids to determine
the order of OPEN processing done by the server.
3. RPC and Security Flavor
The NFSv4 protocol is an RPC application that uses RPC version 2 and
the XDR as defined in [RFC5531] and [RFC4506]. The RPCSEC_GSS
security flavors as defined in version 1 ([RFC2203]) and version 2
([RFC5403]) MUST be implemented as the mechanism to deliver stronger
security for the NFSv4 protocol. However, deployment of RPCSEC_GSS
is optional.
3.1. Ports and Transports
Historically, NFSv2 and NFSv3 servers have resided on port 2049. The
registered port 2049 [RFC3232] for the NFS protocol SHOULD be the
default configuration. Using the registered port for NFS services
means the NFS client will not need to use the RPC binding protocols
as described in [RFC1833]; this will allow NFS to transit firewalls.
Where an NFSv4 implementation supports operation over the IP network
protocol, the supported transport layer between NFS and IP MUST be an
IETF standardized transport protocol that is specified to avoid
network congestion; such transports include TCP and the Stream
Control Transmission Protocol (SCTP). To enhance the possibilities
for interoperability, an NFSv4 implementation MUST support operation
over the TCP transport protocol.
If TCP is used as the transport, the client and server SHOULD use
persistent connections. This will prevent the weakening of TCP's
congestion control via short-lived connections and will improve
performance for the Wide Area Network (WAN) environment by
eliminating the need for SYN handshakes.
As noted in Section 19, the authentication model for NFSv4 has moved
from machine-based to principal-based. However, this modification of
the authentication model does not imply a technical requirement to
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move the TCP connection management model from whole machine-based to
one based on a per-user model. In particular, NFS over TCP client
implementations have traditionally multiplexed traffic for multiple
users over a common TCP connection between an NFS client and server.
This has been true, regardless of whether the NFS client is using
AUTH_SYS, AUTH_DH, RPCSEC_GSS, or any other flavor. Similarly, NFS
over TCP server implementations have assumed such a model and thus
scale the implementation of TCP connection management in proportion
to the number of expected client machines. It is intended that NFSv4
will not modify this connection management model. NFSv4 clients that
violate this assumption can expect scaling issues on the server and
hence reduced service.
3.1.1. Client Retransmission Behavior
When processing an NFSv4 request received over a reliable transport
such as TCP, the NFSv4 server MUST NOT silently drop the request,
except if the established transport connection has been broken.
Given such a contract between NFSv4 clients and servers, clients MUST
NOT retry a request unless one or both of the following are true:
o The transport connection has been broken
o The procedure being retried is the NULL procedure
Since reliable transports, such as TCP, do not always synchronously
inform a peer when the other peer has broken the connection (for
example, when an NFS server reboots), the NFSv4 client may want to
actively "probe" the connection to see if has been broken. Use of
the NULL procedure is one recommended way to do so. So, when a
client experiences a remote procedure call timeout (of some arbitrary
implementation-specific amount), rather than retrying the remote
procedure call, it could instead issue a NULL procedure call to the
server. If the server has died, the transport connection break will
eventually be indicated to the NFSv4 client. The client can then
reconnect, and then retry the original request. If the NULL
procedure call gets a response, the connection has not broken. The
client can decide to wait longer for the original request's response,
or it can break the transport connection and reconnect before
re-sending the original request.
For callbacks from the server to the client, the same rules apply,
but the server doing the callback becomes the client, and the client
receiving the callback becomes the server.
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3.2. Security Flavors
Traditional RPC implementations have included AUTH_NONE, AUTH_SYS,
AUTH_DH, and AUTH_KRB4 as security flavors. With [RFC2203], an
additional security flavor of RPCSEC_GSS has been introduced, which
uses the functionality of GSS-API [RFC2743]. This allows for the use
of various security mechanisms by the RPC layer without the
additional implementation overhead of adding RPC security flavors.
For NFSv4, the RPCSEC_GSS security flavor MUST be used to enable the
mandatory-to-implement security mechanism. Other flavors, such as
AUTH_NONE, AUTH_SYS, and AUTH_DH, MAY be implemented as well.
3.2.1. Security Mechanisms for NFSv4
RPCSEC_GSS, via GSS-API, supports multiple mechanisms that provide
security services. For interoperability, NFSv4 clients and servers
MUST support the Kerberos V5 security mechanism.
The use of RPCSEC_GSS requires selection of mechanism, quality of
protection (QOP), and service (authentication, integrity, privacy).
For the mandated security mechanisms, NFSv4 specifies that a QOP of
zero is used, leaving it up to the mechanism or the mechanism's
configuration to map QOP zero to an appropriate level of protection.
Each mandated mechanism specifies a minimum set of cryptographic
algorithms for implementing integrity and privacy. NFSv4 clients and
servers MUST be implemented on operating environments that comply
with the required cryptographic algorithms of each required
mechanism.
3.2.1.1. Kerberos V5 as a Security Triple
The Kerberos V5 GSS-API mechanism as described in [RFC4121] MUST be
implemented with the RPCSEC_GSS services as specified in Table 2.
Both client and server MUST support each of the pseudo-flavors.
+--------+-------+----------------------+-----------------------+
| Number | Name | Mechanism's OID | RPCSEC_GSS service |
+--------+-------+----------------------+-----------------------+
| 390003 | krb5 | 1.2.840.113554.1.2.2 | rpc_gss_svc_none |
| 390004 | krb5i | 1.2.840.113554.1.2.2 | rpc_gss_svc_integrity |
| 390005 | krb5p | 1.2.840.113554.1.2.2 | rpc_gss_svc_privacy |
+--------+-------+----------------------+-----------------------+
Table 2: Mapping Pseudo-Flavor to Service
Note that the pseudo-flavor is presented here as a mapping aid to the
implementer. Because this NFS protocol includes a method to
negotiate security and it understands the GSS-API mechanism, the
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pseudo-flavor is not needed. The pseudo-flavor is needed for NFSv3
since the security negotiation is done via the MOUNT protocol as
described in [RFC2623].
At the time this document was specified, the Advanced Encryption
Standard (AES) with HMAC-SHA1 was a required algorithm set for
Kerberos V5. In contrast, when NFSv4.0 was first specified in
[RFC3530], weaker algorithm sets were REQUIRED for Kerberos V5, and
were REQUIRED in the NFSv4.0 specification, because the Kerberos V5
specification at the time did not specify stronger algorithms. The
NFSv4 specification does not specify required algorithms for Kerberos
V5, and instead, the implementer is expected to track the evolution
of the Kerberos V5 standard if and when stronger algorithms are
specified.
3.2.1.1.1. Security Considerations for Cryptographic Algorithms in
Kerberos V5
When deploying NFSv4, the strength of the security achieved depends
on the existing Kerberos V5 infrastructure. The algorithms of
Kerberos V5 are not directly exposed to or selectable by the client
or server, so there is some due diligence required by the user of
NFSv4 to ensure that security is acceptable where needed. Guidance
is provided in [RFC6649] as to why weak algorithms should be disabled
by default.
3.3. Security Negotiation
With the NFSv4 server potentially offering multiple security
mechanisms, the client needs a method to determine or negotiate which
mechanism is to be used for its communication with the server. The
NFS server can have multiple points within its file system namespace
that are available for use by NFS clients. In turn, the NFS server
can be configured such that each of these entry points can have
different or multiple security mechanisms in use.
The security negotiation between client and server SHOULD be done
with a secure channel to eliminate the possibility of a third party
intercepting the negotiation sequence and forcing the client and
server to choose a lower level of security than required or desired.
See Section 19 for further discussion.
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The SECINFO operation will allow the client to determine, on a
per-filehandle basis, what security triple (see [RFC2743] and
Section 16.31.4) is to be used for server access. In general, the
client will not have to use the SECINFO operation, except during
initial communication with the server or when the client encounters a
new security policy as the client navigates the namespace. Either
condition will force the client to negotiate a new security triple.
3.3.2. Security Error
Based on the assumption that each NFSv4 client and server MUST
support a minimum set of security (i.e., Kerberos V5 under
RPCSEC_GSS), the NFS client will start its communication with the
server with one of the minimal security triples. During
communication with the server, the client can receive an NFS error of
NFS4ERR_WRONGSEC. This error allows the server to notify the client
that the security triple currently being used is not appropriate for
access to the server's file system resources. The client is then
responsible for determining what security triples are available at
the server and choosing one that is appropriate for the client. See
Section 16.31 for further discussion of how the client will respond
to the NFS4ERR_WRONGSEC error and use SECINFO.
3.3.3. Callback RPC Authentication
Except as noted elsewhere in this section, the callback RPC
(described later) MUST mutually authenticate the NFS server to the
principal that acquired the client ID (also described later), using
the security flavor of the original SETCLIENTID operation used.
For AUTH_NONE, there are no principals, so this is a non-issue.
AUTH_SYS has no notions of mutual authentication or a server
principal, so the callback from the server simply uses the AUTH_SYS
credential that the user used when he set up the delegation.
For AUTH_DH, one commonly used convention is that the server uses the
credential corresponding to this AUTH_DH principal:
unix.host@domain
where host and domain are variables corresponding to the name of the
server host and directory services domain in which it lives, such as
a Network Information System domain or a DNS domain.
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Regardless of what security mechanism under RPCSEC_GSS is being used,
the NFS server MUST identify itself in GSS-API via a
GSS_C_NT_HOSTBASED_SERVICE name type. GSS_C_NT_HOSTBASED_SERVICE
names are of the form:
service@hostname
For NFS, the "service" element is:
nfs
Implementations of security mechanisms will convert nfs@hostname to
various different forms. For Kerberos V5, the following form is
RECOMMENDED:
nfs/hostname
For Kerberos V5, nfs/hostname would be a server principal in the
Kerberos Key Distribution Center database. This is the same
principal the client acquired a GSS-API context for when it issued
the SETCLIENTID operation; therefore, the realm name for the server
principal must be the same for the callback as it was for the
SETCLIENTID.
4. Filehandles
The filehandle in the NFS protocol is a per-server unique identifier
for a file system object. The contents of the filehandle are opaque
to the client. Therefore, the server is responsible for translating
the filehandle to an internal representation of the file system
object.
4.1. Obtaining the First Filehandle
The operations of the NFS protocol are defined in terms of one or
more filehandles. Therefore, the client needs a filehandle to
initiate communication with the server. With the NFSv2 protocol
[RFC1094] and the NFSv3 protocol [RFC1813], there exists an ancillary
protocol to obtain this first filehandle. The MOUNT protocol, RPC
program number 100005, provides the mechanism of translating a
string-based file system pathname to a filehandle that can then be
used by the NFS protocols.
The MOUNT protocol has deficiencies in the area of security and use
via firewalls. This is one reason that the use of the public
filehandle was introduced in [RFC2054] and [RFC2055]. With the use
of the public filehandle in combination with the LOOKUP operation in
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the NFSv2 and NFSv3 protocols, it has been demonstrated that the
MOUNT protocol is unnecessary for viable interaction between the NFS
client and server.
Therefore, the NFSv4 protocol will not use an ancillary protocol for
translation from string-based pathnames to a filehandle. Two special
filehandles will be used as starting points for the NFS client.
4.1.1. Root Filehandle
The first of the special filehandles is the root filehandle. The
root filehandle is the "conceptual" root of the file system namespace
at the NFS server. The client uses or starts with the root
filehandle by employing the PUTROOTFH operation. The PUTROOTFH
operation instructs the server to set the current filehandle to the
root of the server's file tree. Once this PUTROOTFH operation is
used, the client can then traverse the entirety of the server's file
tree with the LOOKUP operation. A complete discussion of the server
namespace is in Section 7.
4.1.2. Public Filehandle
The second special filehandle is the public filehandle. Unlike the
root filehandle, the public filehandle may be bound or represent an
arbitrary file system object at the server. The server is
responsible for this binding. It may be that the public filehandle
and the root filehandle refer to the same file system object.
However, it is up to the administrative software at the server and
the policies of the server administrator to define the binding of the
public filehandle and server file system object. The client may not
make any assumptions about this binding. The client uses the public
filehandle via the PUTPUBFH operation.
4.2. Filehandle Types
In the NFSv2 and NFSv3 protocols, there was one type of filehandle
with a single set of semantics, of which the primary one was that it
was persistent across a server reboot. As such, this type of
filehandle is termed "persistent" in NFSv4. The semantics of a
persistent filehandle remain the same as before. A new type of
filehandle introduced in NFSv4 is the volatile filehandle, which
attempts to accommodate certain server environments.
The volatile filehandle type was introduced to address server
functionality or implementation issues that make correct
implementation of a persistent filehandle infeasible. Some server
environments do not provide a file system level invariant that can be
used to construct a persistent filehandle. The underlying server
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file system may not provide the invariant, or the server's file
system programming interfaces may not provide access to the needed
invariant. Volatile filehandles may ease the implementation of
server functionality, such as hierarchical storage management or file
system reorganization or migration. However, the volatile filehandle
increases the implementation burden for the client.
Since the client will need to handle persistent and volatile
filehandles differently, a file attribute is defined that may be used
by the client to determine the filehandle types being returned by the
server.
4.2.1. General Properties of a Filehandle
The filehandle contains all the information the server needs to
distinguish an individual file. To the client, the filehandle is
opaque. The client stores filehandles for use in a later request and
can compare two filehandles from the same server for equality by
doing a byte-by-byte comparison. However, the client MUST NOT
otherwise interpret the contents of filehandles. If two filehandles
from the same server are equal, they MUST refer to the same file.
However, it is not required that two different filehandles refer to
different file system objects. Servers SHOULD try to maintain a
one-to-one correspondence between filehandles and file system objects
but there may be situations in which the mapping is not one-to-one.
Clients MUST use filehandle comparisons only to improve performance,
not for correct behavior. All clients need to be prepared for
situations in which it cannot be determined whether two different
filehandles denote the same object and in such cases need to avoid
assuming that objects denoted are different, as this might cause
incorrect behavior. Further discussion of filehandle and attribute
comparison in the context of data caching is presented in
Section 10.3.4.
As an example, in the case that two different pathnames when
traversed at the server terminate at the same file system object, the
server SHOULD return the same filehandle for each path. This can
occur if a hard link is used to create two filenames that refer to
the same underlying file object and associated data. For example, if
paths /a/b/c and /a/d/c refer to the same file, the server SHOULD
return the same filehandle for both pathname traversals.
4.2.2. Persistent Filehandle
A persistent filehandle is defined as having a fixed value for the
lifetime of the file system object to which it refers. Once the
server creates the filehandle for a file system object, the server
MUST accept the same filehandle for the object for the lifetime of
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the object. If the server restarts or reboots, the NFS server must
honor the same filehandle value as it did in the server's previous
instantiation. Similarly, if the file system is migrated, the new
NFS server must honor the same filehandle as the old NFS server.
The persistent filehandle will become stale or invalid when the file
system object is removed. When the server is presented with a
persistent filehandle that refers to a deleted object, it MUST return
an error of NFS4ERR_STALE. A filehandle may become stale when the
file system containing the object is no longer available. The file
system may become unavailable if it exists on removable media and the
media is no longer available at the server, or if the file system in
whole has been destroyed, or if the file system has simply been
removed from the server's namespace (i.e., unmounted in a UNIX
environment).
4.2.3. Volatile Filehandle
A volatile filehandle does not share the same longevity
characteristics of a persistent filehandle. The server may determine
that a volatile filehandle is no longer valid at many different
points in time. If the server can definitively determine that a
volatile filehandle refers to an object that has been removed, the
server should return NFS4ERR_STALE to the client (as is the case for
persistent filehandles). In all other cases where the server
determines that a volatile filehandle can no longer be used, it
should return an error of NFS4ERR_FHEXPIRED.
The REQUIRED attribute "fh_expire_type" is used by the client to
determine what type of filehandle the server is providing for a
particular file system. This attribute is a bitmask with the
following values:
FH4_PERSISTENT: The value of FH4_PERSISTENT is used to indicate a
persistent filehandle, which is valid until the object is removed
from the file system. The server will not return
NFS4ERR_FHEXPIRED for this filehandle. FH4_PERSISTENT is defined
as a value in which none of the bits specified below are set.
FH4_VOLATILE_ANY: The filehandle may expire at any time, except as
specifically excluded (i.e., FH4_NOEXPIRE_WITH_OPEN).
FH4_NOEXPIRE_WITH_OPEN: May only be set when FH4_VOLATILE_ANY is
set. If this bit is set, then the meaning of FH4_VOLATILE_ANY
is qualified to exclude any expiration of the filehandle when it
is open.
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FH4_VOL_MIGRATION: The filehandle will expire as a result of
migration. If FH4_VOLATILE_ANY is set, FH4_VOL_MIGRATION is
redundant.
FH4_VOL_RENAME: The filehandle will expire during rename. This
includes a rename by the requesting client or a rename by any
other client. If FH4_VOLATILE_ANY is set, FH4_VOL_RENAME is
redundant.
Servers that provide volatile filehandles that may expire while open
(i.e., if FH4_VOL_MIGRATION or FH4_VOL_RENAME is set or if
FH4_VOLATILE_ANY is set and FH4_NOEXPIRE_WITH_OPEN is not set) should
deny a RENAME or REMOVE that would affect an OPEN file of any of the
components leading to the OPEN file. In addition, the server SHOULD
deny all RENAME or REMOVE requests during the grace period upon
server restart.
Note that the bits FH4_VOL_MIGRATION and FH4_VOL_RENAME allow the
client to determine that expiration has occurred whenever a specific
event occurs, without an explicit filehandle expiration error from
the server. FH4_VOLATILE_ANY does not provide this form of
information. In situations where the server will expire many, but
not all, filehandles upon migration (e.g., all but those that are
open), FH4_VOLATILE_ANY (in this case, with FH4_NOEXPIRE_WITH_OPEN)
is a better choice since the client may not assume that all
filehandles will expire when migration occurs, and it is likely that
additional expirations will occur (as a result of file CLOSE) that
are separated in time from the migration event itself.
4.2.4. One Method of Constructing a Volatile Filehandle
A volatile filehandle, while opaque to the client, could contain:
[volatile bit = 1 | server boot time | slot | generation number]
o slot is an index in the server volatile filehandle table
o generation number is the generation number for the table
entry/slot
When the client presents a volatile filehandle, the server makes the
following checks, which assume that the check for the volatile bit
has passed. If the server boot time is less than the current server
boot time, return NFS4ERR_FHEXPIRED. If slot is out of range, return
NFS4ERR_BADHANDLE. If the generation number does not match, return
NFS4ERR_FHEXPIRED.
When the server reboots, the table is gone (it is volatile).
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If the volatile bit is 0, then it is a persistent filehandle with a
different structure following it.
4.3. Client Recovery from Filehandle Expiration
If possible, the client should recover from the receipt of an
NFS4ERR_FHEXPIRED error. The client must take on additional
responsibility so that it may prepare itself to recover from the
expiration of a volatile filehandle. If the server returns
persistent filehandles, the client does not need these additional
steps.
For volatile filehandles, most commonly the client will need to store
the component names leading up to and including the file system
object in question. With these names, the client should be able to
recover by finding a filehandle in the namespace that is still
available or by starting at the root of the server's file system
namespace.
If the expired filehandle refers to an object that has been removed
from the file system, obviously the client will not be able to
recover from the expired filehandle.
It is also possible that the expired filehandle refers to a file that
has been renamed. If the file was renamed by another client, again
it is possible that the original client will not be able to recover.
However, in the case that the client itself is renaming the file and
the file is open, it is possible that the client may be able to
recover. The client can determine the new pathname based on the
processing of the rename request. The client can then regenerate the
new filehandle based on the new pathname. The client could also use
the COMPOUND operation mechanism to construct a set of operations
like:
RENAME A B
LOOKUP B
GETFH
Note that the COMPOUND procedure does not provide atomicity. This
example only reduces the overhead of recovering from an expired
filehandle.
5. Attributes
To meet the requirements of extensibility and increased
interoperability with non-UNIX platforms, attributes need to be
handled in a flexible manner. The NFSv3 fattr3 structure contains a
fixed list of attributes that not all clients and servers are able to
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support or care about. The fattr3 structure cannot be extended as
new needs arise, and it provides no way to indicate non-support.
With the NFSv4.0 protocol, the client is able to query what
attributes the server supports and construct requests with only those
supported attributes (or a subset thereof).
To this end, attributes are divided into three groups: REQUIRED,
RECOMMENDED, and named. Both REQUIRED and RECOMMENDED attributes are
supported in the NFSv4.0 protocol by a specific and well-defined
encoding and are identified by number. They are requested by setting
a bit in the bit vector sent in the GETATTR request; the server
response includes a bit vector to list what attributes were returned
in the response. New REQUIRED or RECOMMENDED attributes may be added
to the NFSv4 protocol as part of a new minor version by publishing a
Standards Track RFC that allocates a new attribute number value and
defines the encoding for the attribute. See Section 11 for further
discussion.
Named attributes are accessed by the OPENATTR operation, which
accesses a hidden directory of attributes associated with a file
system object. OPENATTR takes a filehandle for the object and
returns the filehandle for the attribute hierarchy. The filehandle
for the named attributes is a directory object accessible by LOOKUP
or READDIR and contains files whose names represent the named
attributes and whose data bytes are the value of the attribute. For
example:
+----------+-----------+---------------------------------+
| LOOKUP | "foo" | ; look up file |
| GETATTR | attrbits | |
| OPENATTR | | ; access foo's named attributes |
| LOOKUP | "x11icon" | ; look up specific attribute |
| READ | 0,4096 | ; read stream of bytes |
+----------+-----------+---------------------------------+
Named attributes are intended for data needed by applications rather
than by an NFS client implementation. NFS implementers are strongly
encouraged to define their new attributes as RECOMMENDED attributes
by bringing them to the IETF Standards Track process.
The set of attributes that are classified as REQUIRED is deliberately
small since servers need to do whatever it takes to support them. A
server should support as many of the RECOMMENDED attributes as
possible; however, by their definition, the server is not required to
support all of them. Attributes are deemed REQUIRED if the data is
both needed by a large number of clients and is not otherwise
reasonably computable by the client when support is not provided on
the server.
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Note that the hidden directory returned by OPENATTR is a convenience
for protocol processing. The client should not make any assumptions
about the server's implementation of named attributes and whether or
not the underlying file system at the server has a named attribute
directory. Therefore, operations such as SETATTR and GETATTR on the
named attribute directory are undefined.
5.1. REQUIRED Attributes
These attributes MUST be supported by every NFSv4.0 client and server
in order to ensure a minimum level of interoperability. The server
MUST store and return these attributes, and the client MUST be able
to function with an attribute set limited to these attributes. With
just the REQUIRED attributes, some client functionality can be
impaired or limited in some ways. A client can ask for any of these
attributes to be returned by setting a bit in the GETATTR request.
For each such bit set, the server MUST return the corresponding
attribute value.
5.2. RECOMMENDED Attributes
These attributes are understood well enough to warrant support in the
NFSv4.0 protocol. However, they may not be supported on all clients
and servers. A client MAY ask for any of these attributes to be
returned by setting a bit in the GETATTR request but MUST handle the
case where the server does not return them. A client MAY ask for the
set of attributes the server supports and SHOULD NOT request
attributes the server does not support. A server should be tolerant
of requests for unsupported attributes and simply not return them,
rather than considering the request an error. It is expected that
servers will support all attributes they comfortably can and only
fail to support attributes that are difficult to support in their
operating environments. A server should provide attributes whenever
they don't have to "tell lies" to the client. For example, a file
modification time either should be an accurate time or should not be
supported by the server. At times this will be difficult for
clients, but a client is better positioned to decide whether and how
to fabricate or construct an attribute or whether to do without the
attribute.
5.3. Named Attributes
These attributes are not supported by direct encoding in the NFSv4
protocol but are accessed by string names rather than numbers and
correspond to an uninterpreted stream of bytes that are stored with
the file system object. The namespace for these attributes may be
accessed by using the OPENATTR operation. The OPENATTR operation
returns a filehandle for a virtual "named attribute directory", and
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further perusal and modification of the namespace may be done using
operations that work on more typical directories. In particular,
READDIR may be used to get a list of such named attributes, and
LOOKUP and OPEN may select a particular attribute. Creation of a new
named attribute may be the result of an OPEN specifying file
creation.
Once an OPEN is done, named attributes may be examined and changed by
normal READ and WRITE operations using the filehandles and stateids
returned by OPEN.
Named attributes and the named attribute directory may have their own
(non-named) attributes. Each of these objects must have all of the
REQUIRED attributes and may have additional RECOMMENDED attributes.
However, the set of attributes for named attributes and the named
attribute directory need not be, and typically will not be, as large
as that for other objects in that file system.
Named attributes might be the target of delegations. However, since
granting of delegations is at the server's discretion, a server need
not support delegations on named attributes.
It is RECOMMENDED that servers support arbitrary named attributes.
A client should not depend on the ability to store any named
attributes in the server's file system. If a server does support
named attributes, a client that is also able to handle them should be
able to copy a file's data and metadata with complete transparency
from one location to another; this would imply that names allowed for
regular directory entries are valid for named attribute names
as well.
In NFSv4.0, the structure of named attribute directories is
restricted in a number of ways, in order to prevent the development
of non-interoperable implementations in which some servers support a
fully general hierarchical directory structure for named attributes
while others support a limited but adequate structure for named
attributes. In such an environment, clients or applications might
come to depend on non-portable extensions. The restrictions are:
o CREATE is not allowed in a named attribute directory. Thus, such
objects as symbolic links and special files are not allowed to be
named attributes. Further, directories may not be created in a
named attribute directory, so no hierarchical structure of named
attributes for a single object is allowed.
o If OPENATTR is done on a named attribute directory or on a named
attribute, the server MUST return an error.
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o Doing a RENAME of a named attribute to a different named attribute
directory or to an ordinary (i.e., non-named-attribute) directory
is not allowed.
o Creating hard links between named attribute directories or between
named attribute directories and ordinary directories is not
allowed.
Names of attributes will not be controlled by this document or other
IETF Standards Track documents. See Section 20 for further
discussion.
5.4. Classification of Attributes
Each of the attributes accessed using SETATTR and GETATTR (i.e.,
REQUIRED and RECOMMENDED attributes) can be classified in one of
three categories:
1. per-server attributes for which the value of the attribute will
be the same for all file objects that share the same server.
2. per-file system attributes for which the value of the attribute
will be the same for some or all file objects that share the same
server and fsid attribute (Section 5.8.1.9). See below for
details regarding when such sharing is in effect.
3. per-file system object attributes.
The handling of per-file system attributes depends on the particular
attribute and the setting of the homogeneous (Section 5.8.2.12)
attribute. The following rules apply:
1. The values of the attributes supported_attrs, fsid, homogeneous,
link_support, and symlink_support are always common to all
objects within the given file system.
2. For other attributes, different values may be returned for
different file system objects if the attribute homogeneous is
supported within the file system in question and has the value
false.
The classification of attributes is as follows. Note that the
attributes time_access_set and time_modify_set are not listed in this
section, because they are write-only attributes corresponding to
time_access and time_modify and are used in a special instance of
SETATTR.
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o The per-server attribute is:
lease_time
o The per-file system attributes are:
supported_attrs, fh_expire_type, link_support, symlink_support,
unique_handles, aclsupport, cansettime, case_insensitive,
case_preserving, chown_restricted, files_avail, files_free,
files_total, fs_locations, homogeneous, maxfilesize, maxname,
maxread, maxwrite, no_trunc, space_avail, space_free,
space_total, and time_delta
o The per-file system object attributes are:
type, change, size, named_attr, fsid, rdattr_error, filehandle,
acl, archive, fileid, hidden, maxlink, mimetype, mode,
numlinks, owner, owner_group, rawdev, space_used, system,
time_access, time_backup, time_create, time_metadata,
time_modify, and mounted_on_fileid
For quota_avail_hard, quota_avail_soft, and quota_used, see their
definitions below for the appropriate classification.
5.5. Set-Only and Get-Only Attributes
Some REQUIRED and RECOMMENDED attributes are set-only; i.e., they can
be set via SETATTR but not retrieved via GETATTR. Similarly, some
REQUIRED and RECOMMENDED attributes are get-only; i.e., they can be
retrieved via GETATTR but not set via SETATTR. If a client attempts
to set a get-only attribute or get a set-only attribute, the server
MUST return NFS4ERR_INVAL.
5.6. REQUIRED Attributes - List and Definition References
The list of REQUIRED attributes appears in Table 3. The meanings of
the columns of the table are:
o Name: The name of the attribute.
o ID: The number assigned to the attribute. In the event of
conflicts between the assigned number and [RFC7531], the latter is
authoritative, but in such an event, it should be resolved with
errata to this document and/or [RFC7531]. See [IESG_ERRATA] for
the errata process.
o Data Type: The XDR data type of the attribute.
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o Acc: Access allowed to the attribute. R means read-only (GETATTR
may retrieve, SETATTR may not set). W means write-only (SETATTR
may set, GETATTR may not retrieve). R W means read/write (GETATTR
may retrieve, SETATTR may set).
o Defined in: The section of this specification that describes the
attribute.
+-----------------+----+------------+-----+-------------------+
| Name | ID | Data Type | Acc | Defined in |
+-----------------+----+------------+-----+-------------------+
| supported_attrs | 0 | bitmap4 | R | Section 5.8.1.1 |
| type | 1 | nfs_ftype4 | R | Section 5.8.1.2 |
| fh_expire_type | 2 | uint32_t | R | Section 5.8.1.3 |
| change | 3 | changeid4 | R | Section 5.8.1.4 |
| size | 4 | uint64_t | R W | Section 5.8.1.5 |
| link_support | 5 | bool | R | Section 5.8.1.6 |
| symlink_support | 6 | bool | R | Section 5.8.1.7 |
| named_attr | 7 | bool | R | Section 5.8.1.8 |
| fsid | 8 | fsid4 | R | Section 5.8.1.9 |
| unique_handles | 9 | bool | R | Section 5.8.1.10 |
| lease_time | 10 | nfs_lease4 | R | Section 5.8.1.11 |
| rdattr_error | 11 | nfsstat4 | R | Section 5.8.1.12 |
| filehandle | 19 | nfs_fh4 | R | Section 5.8.1.13 |
+-----------------+----+------------+-----+-------------------+
Table 3: REQUIRED Attributes
5.7. RECOMMENDED Attributes - List and Definition References
The RECOMMENDED attributes are defined in Table 4. The meanings of
the column headers are the same as Table 3; see Section 5.6 for the
meanings.
+-------------------+----+-----------------+-----+------------------+
| Name | ID | Data Type | Acc | Defined in |
+-------------------+----+-----------------+-----+------------------+
| acl | 12 | nfsace4<> | R W | Section 6.2.1 |
| aclsupport | 13 | uint32_t | R | Section 6.2.1.2 |
| archive | 14 | bool | R W | Section 5.8.2.1 |
| cansettime | 15 | bool | R | Section 5.8.2.2 |
| case_insensitive | 16 | bool | R | Section 5.8.2.3 |
| case_preserving | 17 | bool | R | Section 5.8.2.4 |
| chown_restricted | 18 | bool | R | Section 5.8.2.5 |
| fileid | 20 | uint64_t | R | Section 5.8.2.6 |
| files_avail | 21 | uint64_t | R | Section 5.8.2.7 |
| files_free | 22 | uint64_t | R | Section 5.8.2.8 |
| files_total | 23 | uint64_t | R | Section 5.8.2.9 |
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| fs_locations | 24 | fs_locations4 | R | Section 5.8.2.10 |
| hidden | 25 | bool | R W | Section 5.8.2.11 |
| homogeneous | 26 | bool | R | Section 5.8.2.12 |
| maxfilesize | 27 | uint64_t | R | Section 5.8.2.13 |
| maxlink | 28 | uint32_t | R | Section 5.8.2.14 |
| maxname | 29 | uint32_t | R | Section 5.8.2.15 |
| maxread | 30 | uint64_t | R | Section 5.8.2.16 |
| maxwrite | 31 | uint64_t | R | Section 5.8.2.17 |
| mimetype | 32 | ascii_ | R W | Section 5.8.2.18 |
| | | REQUIRED4<> | | |
| mode | 33 | mode4 | R W | Section 6.2.2 |
| mounted_on_fileid | 55 | uint64_t | R | Section 5.8.2.19 |
| no_trunc | 34 | bool | R | Section 5.8.2.20 |
| numlinks | 35 | uint32_t | R | Section 5.8.2.21 |
| owner | 36 | utf8str_mixed | R W | Section 5.8.2.22 |
| owner_group | 37 | utf8str_mixed | R W | Section 5.8.2.23 |
| quota_avail_hard | 38 | uint64_t | R | Section 5.8.2.24 |
| quota_avail_soft | 39 | uint64_t | R | Section 5.8.2.25 |
| quota_used | 40 | uint64_t | R | Section 5.8.2.26 |
| rawdev | 41 | specdata4 | R | Section 5.8.2.27 |
| space_avail | 42 | uint64_t | R | Section 5.8.2.28 |
| space_free | 43 | uint64_t | R | Section 5.8.2.29 |
| space_total | 44 | uint64_t | R | Section 5.8.2.30 |
| space_used | 45 | uint64_t | R | Section 5.8.2.31 |
| system | 46 | bool | R W | Section 5.8.2.32 |
| time_access | 47 | nfstime4 | R | Section 5.8.2.33 |
| time_access_set | 48 | settime4 | W | Section 5.8.2.34 |
| time_backup | 49 | nfstime4 | R W | Section 5.8.2.35 |
| time_create | 50 | nfstime4 | R W | Section 5.8.2.36 |
| time_delta | 51 | nfstime4 | R | Section 5.8.2.37 |
| time_metadata | 52 | nfstime4 | R | Section 5.8.2.38 |
| time_modify | 53 | nfstime4 | R | Section 5.8.2.39 |
| time_modify_set | 54 | settime4 | W | Section 5.8.2.40 |
+-------------------+----+-----------------+-----+------------------+
Table 4: RECOMMENDED Attributes
5.8. Attribute Definitions
5.8.1. Definitions of REQUIRED Attributes
5.8.1.1. Attribute 0: supported_attrs
The bit vector that would retrieve all REQUIRED and RECOMMENDED
attributes that are supported for this object. The scope of this
attribute applies to all objects with a matching fsid.
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5.8.1.2. Attribute 1: type
Designates the type of an object in terms of one of a number of
special constants:
o NF4REG designates a regular file.
o NF4DIR designates a directory.
o NF4BLK designates a block device special file.
o NF4CHR designates a character device special file.
o NF4LNK designates a symbolic link.
o NF4SOCK designates a named socket special file.
o NF4FIFO designates a fifo special file.
o NF4ATTRDIR designates a named attribute directory.
o NF4NAMEDATTR designates a named attribute.
Within the explanatory text and operation descriptions, the following
phrases will be used with the meanings given below:
o The phrase "is a directory" means that the object's type attribute
is NF4DIR or NF4ATTRDIR.
o The phrase "is a special file" means that the object's type
attribute is NF4BLK, NF4CHR, NF4SOCK, or NF4FIFO.
o The phrase "is a regular file" means that the object's type
attribute is NF4REG or NF4NAMEDATTR.
o The phrase "is a symbolic link" means that the object's type
attribute is NF4LNK.
5.8.1.3. Attribute 2: fh_expire_type
The server uses this to specify filehandle expiration behavior to the
client. See Section 4 for additional description.
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5.8.1.4. Attribute 3: change
A value created by the server that the client can use to determine if
file data, directory contents, or attributes of the object have been
modified. The server MAY return the object's time_metadata attribute
for this attribute's value but only if the file system object cannot
be updated more frequently than the resolution of time_metadata.
5.8.1.5. Attribute 4: size
The size of the object in bytes.
5.8.1.6. Attribute 5: link_support
TRUE, if the object's file system supports hard links.
5.8.1.7. Attribute 6: symlink_support
TRUE, if the object's file system supports symbolic links.
5.8.1.8. Attribute 7: named_attr
TRUE, if this object has named attributes. In other words, this
object has a non-empty named attribute directory.
5.8.1.9. Attribute 8: fsid
Unique file system identifier for the file system holding this
object. The fsid attribute has major and minor components, each of
which are of data type uint64_t.
5.8.1.10. Attribute 9: unique_handles
TRUE, if two distinct filehandles are guaranteed to refer to two
different file system objects.
5.8.1.11. Attribute 10: lease_time
Duration of the lease at the server in seconds.
5.8.1.12. Attribute 11: rdattr_error
Error returned from an attempt to retrieve attributes during a
READDIR operation.
5.8.1.13. Attribute 19: filehandle
The filehandle of this object (primarily for READDIR requests).
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5.8.2. Definitions of Uncategorized RECOMMENDED Attributes
The definitions of most of the RECOMMENDED attributes follow.
Collections that share a common category are defined in other
sections.
5.8.2.1. Attribute 14: archive
TRUE, if this file has been archived since the time of the last
modification (deprecated in favor of time_backup).
5.8.2.2. Attribute 15: cansettime
TRUE, if the server is able to change the times for a file system
object as specified in a SETATTR operation.
5.8.2.3. Attribute 16: case_insensitive
TRUE, if filename comparisons on this file system are case
insensitive. This refers only to comparisons, and not to the case in
which filenames are stored.
5.8.2.4. Attribute 17: case_preserving
TRUE, if the filename case on this file system is preserved. This
refers only to how filenames are stored, and not to how they are
compared. Filenames stored in mixed case might be compared using
either case-insensitive or case-sensitive comparisons.
5.8.2.5. Attribute 18: chown_restricted
If TRUE, the server will reject any request to change either the
owner or the group associated with a file if the caller is not a
privileged user (for example, "root" in UNIX operating environments
or the "Take Ownership" privilege in Windows 2000).
5.8.2.6. Attribute 20: fileid
A number uniquely identifying the file within the file system.
5.8.2.7. Attribute 21: files_avail
File slots available to this user on the file system containing this
object -- this should be the smallest relevant limit.
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5.8.2.8. Attribute 22: files_free
Free file slots on the file system containing this object -- this
should be the smallest relevant limit.
5.8.2.9. Attribute 23: files_total
Total file slots on the file system containing this object.
5.8.2.10. Attribute 24: fs_locations
Locations where this file system may be found. If the server returns
NFS4ERR_MOVED as an error, this attribute MUST be supported.
The server specifies the rootpath for a given server by returning a
path consisting of zero path components.
5.8.2.11. Attribute 25: hidden
TRUE, if the file is considered hidden with respect to the
Windows API.
5.8.2.12. Attribute 26: homogeneous
TRUE, if this object's file system is homogeneous, i.e., all objects
in the file system (all objects on the server with the same fsid)
have common values for all per-file system attributes.
5.8.2.13. Attribute 27: maxfilesize
Maximum supported file size for the file system of this object.
5.8.2.14. Attribute 28: maxlink
Maximum number of hard links for this object.
5.8.2.15. Attribute 29: maxname
Maximum filename size supported for this object.
5.8.2.16. Attribute 30: maxread
Maximum amount of data the READ operation will return for this
object.
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5.8.2.17. Attribute 31: maxwrite
Maximum amount of data the WRITE operation will accept for this
object. This attribute SHOULD be supported if the file is writable.
Lack of this attribute can lead to the client either wasting
bandwidth or not receiving the best performance.
5.8.2.18. Attribute 32: mimetype
MIME media type/subtype of this object.
5.8.2.19. Attribute 55: mounted_on_fileid
Like fileid, but if the target filehandle is the root of a file
system, this attribute represents the fileid of the underlying
directory.
UNIX-based operating environments connect a file system into the
namespace by connecting (mounting) the file system onto the existing
file object (the mount point, usually a directory) of an existing
file system. When the mount point's parent directory is read via an
API such as readdir() [readdir_api], the return results are directory
entries, each with a component name and a fileid. The fileid of the
mount point's directory entry will be different from the fileid that
the stat() [stat] system call returns. The stat() system call is
returning the fileid of the root of the mounted file system, whereas
readdir() is returning the fileid that stat() would have returned
before any file systems were mounted on the mount point.
Unlike NFSv3, NFSv4.0 allows a client's LOOKUP request to cross other
file systems. The client detects the file system crossing whenever
the filehandle argument of LOOKUP has an fsid attribute different
from that of the filehandle returned by LOOKUP. A UNIX-based client
will consider this a "mount point crossing". UNIX has a legacy
scheme for allowing a process to determine its current working
directory. This relies on readdir() of a mount point's parent and
stat() of the mount point returning fileids as previously described.
The mounted_on_fileid attribute corresponds to the fileid that
readdir() would have returned, as described previously.
While the NFSv4.0 client could simply fabricate a fileid
corresponding to what mounted_on_fileid provides (and if the server
does not support mounted_on_fileid, the client has no choice), there
is a risk that the client will generate a fileid that conflicts with
one that is already assigned to another object in the file system.
Instead, if the server can provide the mounted_on_fileid, the
potential for client operational problems in this area is eliminated.
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If the server detects that there is nothing mounted on top of the
target file object, then the value for mounted_on_fileid that it
returns is the same as that of the fileid attribute.
The mounted_on_fileid attribute is RECOMMENDED, so the server SHOULD
provide it if possible, and for a UNIX-based server, this is
straightforward. Usually, mounted_on_fileid will be requested during
a READDIR operation, in which case it is trivial (at least for
UNIX-based servers) to return mounted_on_fileid since it is equal to
the fileid of a directory entry returned by readdir(). If
mounted_on_fileid is requested in a GETATTR operation, the server
should obey an invariant that has it returning a value that is equal
to the file object's entry in the object's parent directory, i.e.,
what readdir() would have returned. Some operating environments
allow a series of two or more file systems to be mounted onto a
single mount point. In this case, for the server to obey the
aforementioned invariant, it will need to find the base mount point,
and not the intermediate mount points.
5.8.2.20. Attribute 34: no_trunc
If this attribute is TRUE, then if the client uses a filename longer
than name_max, an error will be returned instead of the name being
truncated.
5.8.2.21. Attribute 35: numlinks
Number of hard links to this object.
5.8.2.22. Attribute 36: owner
The string name of the owner of this object.
5.8.2.23. Attribute 37: owner_group
The string name of the group ownership of this object.
5.8.2.24. Attribute 38: quota_avail_hard
The value in bytes that represents the amount of additional disk
space beyond the current allocation that can be allocated to this
file or directory before further allocations will be refused. It is
understood that this space may be consumed by allocations to other
files or directories.
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5.8.2.25. Attribute 39: quota_avail_soft
The value in bytes that represents the amount of additional disk
space that can be allocated to this file or directory before the user
may reasonably be warned. It is understood that this space may be
consumed by allocations to other files or directories, though there
may exist server-side rules as to which other files or directories.
5.8.2.26. Attribute 40: quota_used
The value in bytes that represents the amount of disk space used by
this file or directory and possibly a number of other similar files
or directories, where the set of "similar" meets at least the
criterion that allocating space to any file or directory in the set
will reduce the "quota_avail_hard" of every other file or directory
in the set.
Note that there may be a number of distinct but overlapping sets of
files or directories for which a quota_used value is maintained,
e.g., "all files with a given owner", "all files with a given group
owner", etc. The server is at liberty to choose any of those sets
when providing the content of the quota_used attribute but should do
so in a repeatable way. The rule may be configured per file system
or may be "choose the set with the smallest quota".
5.8.2.27. Attribute 41: rawdev
Raw device number of file of type NF4BLK or NF4CHR. The device
number is split into major and minor numbers. If the file's type
attribute is not NF4BLK or NF4CHR, this attribute SHOULD NOT be
returned, and any value returned SHOULD NOT be considered useful.
5.8.2.28. Attribute 42: space_avail
Disk space in bytes available to this user on the file system
containing this object -- this should be the smallest relevant limit.
5.8.2.29. Attribute 43: space_free
Free disk space in bytes on the file system containing this object --
this should be the smallest relevant limit.
5.8.2.30. Attribute 44: space_total
Total disk space in bytes on the file system containing this object.
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5.8.2.31. Attribute 45: space_used
Number of file system bytes allocated to this object.
5.8.2.32. Attribute 46: system
TRUE, if this file is a "system" file with respect to the Windows
operating environment.
5.8.2.33. Attribute 47: time_access
Represents the time of last access to the object by a READ operation
sent to the server. The notion of what is an "access" depends on the
server's operating environment and/or the server's file system
semantics. For example, for servers obeying Portable Operating
System Interface (POSIX) semantics, time_access would be updated only
by the READ and READDIR operations and not any of the operations that
modify the content of the object [read_api], [readdir_api],
[write_api]. Of course, setting the corresponding time_access_set
attribute is another way to modify the time_access attribute.
Whenever the file object resides on a writable file system, the
server should make its best efforts to record time_access into stable
storage. However, to mitigate the performance effects of doing so,
and most especially whenever the server is satisfying the read of the
object's content from its cache, the server MAY cache access time
updates and lazily write them to stable storage. It is also
acceptable to give administrators of the server the option to disable
time_access updates.
5.8.2.34. Attribute 48: time_access_set
Sets the time of last access to the object. SETATTR use only.
5.8.2.35. Attribute 49: time_backup
The time of last backup of the object.
5.8.2.36. Attribute 50: time_create
The time of creation of the object. This attribute does not have
any relation to the traditional UNIX file attribute "ctime"
("change time").
5.8.2.37. Attribute 51: time_delta
Smallest useful server time granularity.
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5.8.2.38. Attribute 52: time_metadata
The time of last metadata modification of the object.
5.8.2.39. Attribute 53: time_modify
The time of last modification to the object.
5.8.2.40. Attribute 54: time_modify_set
Sets the time of last modification to the object. SETATTR use only.
5.9. Interpreting owner and owner_group
The RECOMMENDED attributes "owner" and "owner_group" (and also users
and groups used as values of the who field within nfs4ace structures
used in the acl attribute) are represented in the form of UTF-8
strings. This format avoids the use of a representation that is tied
to a particular underlying implementation at the client or server.
Note that Section 6.1 of [RFC2624] provides additional rationale. It
is expected that the client and server will have their own local
representation of owners and groups that is used for local storage or
presentation to the application via APIs that expect such a
representation. Therefore, the protocol requires that when these
attributes are transferred between the client and server, the local
representation is translated to a string of the form
"identifier@dns_domain". This allows clients and servers that do not
use the same local representation to effectively interoperate since
they both use a common syntax that can be interpreted by both.
Similarly, security principals may be represented in different ways
by different security mechanisms. Servers normally translate these
representations into a common format, generally that used by local
storage, to serve as a means of identifying the users corresponding
to these security principals. When these local identifiers are
translated to the form of the owner attribute, associated with files
created by such principals, they identify, in a common format, the
users associated with each corresponding set of security principals.
The translation used to interpret owner and group strings is not
specified as part of the protocol. This allows various solutions to
be employed. For example, a local translation table may be consulted
that maps a numeric identifier to the user@dns_domain syntax. A name
service may also be used to accomplish the translation. A server may
provide a more general service, not limited by any particular
translation (which would only translate a limited set of possible
strings) by storing the owner and owner_group attributes in local
storage without any translation, or it may augment a translation
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method by storing the entire string for attributes for which no
translation is available while using the local representation for
those cases in which a translation is available.
Servers that do not provide support for all possible values of user
and group strings SHOULD return an error (NFS4ERR_BADOWNER) when a
string is presented that has no translation, as the value to be set
for a SETATTR of the owner or owner_group attributes or as part of
the value of the acl attribute. When a server does accept a user or
group string as valid on a SETATTR, it is promising to return that
same string (see below) when a corresponding GETATTR is done, as long
as there has been no further change in the corresponding attribute
before the GETATTR. For some internationalization-related exceptions
where this is not possible, see below. Configuration changes
(including changes from the mapping of the string to the local
representation) and ill-constructed name translations (those that
contain aliasing) may make that promise impossible to honor. Servers
should make appropriate efforts to avoid a situation in which these
attributes have their values changed when no real change to either
ownership or acls has occurred.
The "dns_domain" portion of the owner string is meant to be a DNS
domain name -- for example, "user@example.org". Servers should
accept as valid a set of users for at least one domain. A server may
treat other domains as having no valid translations. A more general
service is provided when a server is capable of accepting users for
multiple domains, or for all domains, subject to security
constraints.
As an implementation guide, both clients and servers may provide a
means to configure the "dns_domain" portion of the owner string. For
example, the DNS domain name of the host running the NFS server might
be "lab.example.org", but the user names are defined in
"example.org". In the absence of such a configuration, or as a
default, the current DNS domain name of the server should be the
value used for the "dns_domain".
As mentioned above, it is desirable that a server, when accepting a
string of the form "user@domain" or "group@domain" in an attribute,
return this same string when that corresponding attribute is fetched.
Internationalization issues make this impossible under certain
circumstances, and the client needs to take note of these. See
Section 12 for a detailed discussion of these issues.
In the case where there is no translation available to the client or
server, the attribute value will be constructed without the "@".
Therefore, the absence of the "@" from the owner or owner_group
attribute signifies that no translation was available at the sender
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and that the receiver of the attribute should not use that string as
a basis for translation into its own internal format. Even though
the attribute value cannot be translated, it may still be useful. In
the case of a client, the attribute string may be used for local
display of ownership.
To provide a greater degree of compatibility with NFSv3, which
identified users and groups by 32-bit unsigned user identifiers and
group identifiers, owner and group strings that consist of ASCII-
encoded decimal numeric values with no leading zeros can be given a
special interpretation by clients and servers that choose to provide
such support. The receiver may treat such a user or group string as
representing the same user as would be represented by an NFSv3 uid or
gid having the corresponding numeric value.
A server SHOULD reject such a numeric value if the security mechanism
is using Kerberos. That is, in such a scenario, the client will
already need to form "user@domain" strings. For any other security
mechanism, the server SHOULD accept such numeric values. As an
implementation note, the server could make such an acceptance be
configurable. If the server does not support numeric values or if it
is configured off, then it MUST return an NFS4ERR_BADOWNER error. If
the security mechanism is using Kerberos and the client attempts to
use the special form, then the server SHOULD return an
NFS4ERR_BADOWNER error when there is a valid translation for the user
or owner designated in this way. In that case, the client must use
the appropriate user@domain string and not the special form for
compatibility.
The client MUST always accept numeric values if the security
mechanism is not RPCSEC_GSS. A client can determine if a server
supports numeric identifiers by first attempting to provide a numeric
identifier. If this attempt is rejected with an NFS4ERR_BADOWNER
error, then the client should only use named identifiers of the form
"user@dns_domain".
The owner string "nobody" may be used to designate an anonymous user,
which will be associated with a file created by a security principal
that cannot be mapped through normal means to the owner attribute.
5.10. Character Case Attributes
With respect to the case_insensitive and case_preserving attributes,
case-insensitive comparisons of Unicode characters SHOULD use Unicode
Default Case Folding as defined in Chapter 3 of the Unicode Standard
[UNICODE] and MAY override that behavior for specific selected
characters with the case folding defined in the SpecialCasing.txt
[SPECIALCASING] file; see Section 3.13 of the Unicode Standard.
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The SpecialCasing.txt file replaces the Default Case Folding with
locale- and context-dependent case folding for specific situations.
An example of locale- and context-dependent case folding is that
LATIN CAPITAL LETTER I ("I", U+0049) is default case folded to LATIN
SMALL LETTER I ("i", U+0069). However, several languages (e.g.,
Turkish) treat an "I" character with a dot as a different letter than
an "I" character without a dot; therefore, in such languages, unless
an I is before a dot_above, the "I" (U+0049) character should be case
folded to a different character, LATIN SMALL LETTER DOTLESS I
(U+0131).
The [UNICODE] and [SPECIALCASING] references in this RFC are for
version 7.0.0 of the Unicode standard, as that was the latest version
of Unicode when this RFC was published. Implementations SHOULD
always use the latest version of Unicode
(<http://www.unicode.org/versions/latest/>).
6. Access Control Attributes
Access Control Lists (ACLs) are file attributes that specify fine-
grained access control. This section covers the "acl", "aclsupport",
and "mode" file attributes, and their interactions. Note that file
attributes may apply to any file system object.
6.1. Goals
ACLs and modes represent two well-established models for specifying
permissions. This section specifies requirements that attempt to
meet the following goals:
o If a server supports the mode attribute, it should provide
reasonable semantics to clients that only set and retrieve the
mode attribute.
o If a server supports ACL attributes, it should provide reasonable
semantics to clients that only set and retrieve those attributes.
o On servers that support the mode attribute, if ACL attributes have
never been set on an object, via inheritance or explicitly, the
behavior should be traditional UNIX-like behavior.
o On servers that support the mode attribute, if the ACL attributes
have been previously set on an object, either explicitly or via
inheritance:
* Setting only the mode attribute should effectively control the
traditional UNIX-like permissions of read, write, and execute
on owner, owner_group, and other.
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* Setting only the mode attribute should provide reasonable
security. For example, setting a mode of 000 should be enough
to ensure that future opens for read or write by any principal
fail, regardless of a previously existing or inherited ACL.
o When a mode attribute is set on an object, the ACL attributes may
need to be modified so as to not conflict with the new mode. In
such cases, it is desirable that the ACL keep as much information
as possible. This includes information about inheritance, AUDIT
and ALARM access control entries (ACEs), and permissions granted
and denied that do not conflict with the new mode.
6.2. File Attributes Discussion
Support for each of the ACL attributes is RECOMMENDED and not
required, since file systems accessed using NFSv4 might not
support ACLs.
6.2.1. Attribute 12: acl
The NFSv4.0 ACL attribute contains an array of ACEs that are
associated with the file system object. Although the client can read
and write the acl attribute, the server is responsible for using the
ACL to perform access control. The client can use the OPEN or ACCESS
operations to check access without modifying or reading data or
metadata.
The NFS ACE structure is defined as follows:
typedef uint32_t acetype4;
typedef uint32_t aceflag4;
typedef uint32_t acemask4;
struct nfsace4 {
acetype4 type;
aceflag4 flag;
acemask4 access_mask;
utf8str_mixed who;
};
To determine if a request succeeds, the server processes each nfsace4
entry in order. Only ACEs that have a "who" that matches the
requester are considered. Each ACE is processed until all of the
bits of the requester's access have been ALLOWED. Once a bit (see
below) has been ALLOWED by an ACCESS_ALLOWED_ACE, it is no longer
considered in the processing of later ACEs. If an ACCESS_DENIED_ACE
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is encountered where the requester's access still has unALLOWED bits
in common with the "access_mask" of the ACE, the request is denied.
When the ACL is fully processed, if there are bits in the requester's
mask that have not been ALLOWED or DENIED, access is denied.
Unlike the ALLOW and DENY ACE types, the ALARM and AUDIT ACE types do
not affect a requester's access and instead are for triggering events
as a result of a requester's access attempt. Therefore, AUDIT and
ALARM ACEs are processed only after processing ALLOW and DENY ACEs.
The NFSv4.0 ACL model is quite rich. Some server platforms may
provide access control functionality that goes beyond the UNIX-style
mode attribute but that is not as rich as the NFS ACL model. So that
users can take advantage of this more limited functionality, the
server may support the acl attributes by mapping between its ACL
model and the NFSv4.0 ACL model. Servers must ensure that the ACL
they actually store or enforce is at least as strict as the NFSv4 ACL
that was set. It is tempting to accomplish this by rejecting any ACL
that falls outside the small set that can be represented accurately.
However, such an approach can render ACLs unusable without special
client-side knowledge of the server's mapping, which defeats the
purpose of having a common NFSv4 ACL protocol. Therefore, servers
should accept every ACL that they can without compromising security.
To help accomplish this, servers may make a special exception, in the
case of unsupported permission bits, to the rule that bits not
ALLOWED or DENIED by an ACL must be denied. For example, a UNIX-
style server might choose to silently allow read attribute
permissions even though an ACL does not explicitly allow those
permissions. (An ACL that explicitly denies permission to read
attributes should still result in a denial.)
The situation is complicated by the fact that a server may have
multiple modules that enforce ACLs. For example, the enforcement for
NFSv4.0 access may be different from, but not weaker than, the
enforcement for local access, and both may be different from the
enforcement for access through other protocols such as Server Message
Block (SMB) [MS-SMB]. So it may be useful for a server to accept an
ACL even if not all of its modules are able to support it.
The guiding principle with regard to NFSv4 access is that the server
must not accept ACLs that give an appearance of more restricted
access to a file than what is actually enforced.
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The constants used for the type field (acetype4) are as follows:
const ACE4_ACCESS_ALLOWED_ACE_TYPE = 0x00000000;
const ACE4_ACCESS_DENIED_ACE_TYPE = 0x00000001;
const ACE4_SYSTEM_AUDIT_ACE_TYPE = 0x00000002;
const ACE4_SYSTEM_ALARM_ACE_TYPE = 0x00000003;
All four bit types are permitted in the acl attribute.
+------------------------------+--------------+---------------------+
| Value | Abbreviation | Description |
+------------------------------+--------------+---------------------+
| ACE4_ACCESS_ALLOWED_ACE_TYPE | ALLOW | Explicitly grants |
| | | the access defined |
| | | in acemask4 to the |
| | | file or directory. |
| | | |
| ACE4_ACCESS_DENIED_ACE_TYPE | DENY | Explicitly denies |
| | | the access defined |
| | | in acemask4 to the |
| | | file or directory. |
| | | |
| ACE4_SYSTEM_AUDIT_ACE_TYPE | AUDIT | LOG (in a system- |
| | | dependent way) any |
| | | access attempt to a |
| | | file or directory |
| | | that uses any of |
| | | the access methods |
| | | specified in |
| | | acemask4. |
| | | |
| ACE4_SYSTEM_ALARM_ACE_TYPE | ALARM | Generate a system |
| | | ALARM (system |
| | | dependent) when any |
| | | access attempt is |
| | | made to a file or |
| | | directory for the |
| | | access methods |
| | | specified in |
| | | acemask4. |
+------------------------------+--------------+---------------------+
The "Abbreviation" column denotes how the types will be referred to
throughout the rest of this section.
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6.2.1.2. Attribute 13: aclsupport
A server need not support all of the above ACE types. This attribute
indicates which ACE types are supported for the current file system.
The bitmask constants used to represent the above definitions within
the aclsupport attribute are as follows:
const ACL4_SUPPORT_ALLOW_ACL = 0x00000001;
const ACL4_SUPPORT_DENY_ACL = 0x00000002;
const ACL4_SUPPORT_AUDIT_ACL = 0x00000004;
const ACL4_SUPPORT_ALARM_ACL = 0x00000008;
Servers that support either the ALLOW or DENY ACE type SHOULD support
both ALLOW and DENY ACE types.
Clients should not attempt to set an ACE unless the server claims
support for that ACE type. If the server receives a request to set
an ACE that it cannot store, it MUST reject the request with
NFS4ERR_ATTRNOTSUPP. If the server receives a request to set an ACE
that it can store but cannot enforce, the server SHOULD reject the
request with NFS4ERR_ATTRNOTSUPP.
6.2.1.3. ACE Access Mask
The bitmask constants used for the access mask field are as follows:
const ACE4_READ_DATA = 0x00000001;
const ACE4_LIST_DIRECTORY = 0x00000001;
const ACE4_WRITE_DATA = 0x00000002;
const ACE4_ADD_FILE = 0x00000002;
const ACE4_APPEND_DATA = 0x00000004;
const ACE4_ADD_SUBDIRECTORY = 0x00000004;
const ACE4_READ_NAMED_ATTRS = 0x00000008;
const ACE4_WRITE_NAMED_ATTRS = 0x00000010;
const ACE4_EXECUTE = 0x00000020;
const ACE4_DELETE_CHILD = 0x00000040;
const ACE4_READ_ATTRIBUTES = 0x00000080;
const ACE4_WRITE_ATTRIBUTES = 0x00000100;
const ACE4_DELETE = 0x00010000;
const ACE4_READ_ACL = 0x00020000;
const ACE4_WRITE_ACL = 0x00040000;
const ACE4_WRITE_OWNER = 0x00080000;
const ACE4_SYNCHRONIZE = 0x00100000;
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Note that some masks have coincident values -- for example,
ACE4_READ_DATA and ACE4_LIST_DIRECTORY. The mask entries
ACE4_LIST_DIRECTORY, ACE4_ADD_FILE, and ACE4_ADD_SUBDIRECTORY are
intended to be used with directory objects, while ACE4_READ_DATA,
ACE4_WRITE_DATA, and ACE4_APPEND_DATA are intended to be used with
non-directory objects.
6.2.1.3.1. Discussion of Mask Attributes
ACE4_READ_DATA
Operation(s) affected:
READ
OPEN
Discussion:
Permission to read the data of the file.
Servers SHOULD allow a user the ability to read the data of the
file when only the ACE4_EXECUTE access mask bit is set.
ACE4_LIST_DIRECTORY
Operation(s) affected:
READDIR
Discussion:
Permission to list the contents of a directory.
ACE4_WRITE_DATA
Operation(s) affected:
WRITE
OPEN
SETATTR of size
Discussion:
Permission to modify a file's data.
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ACE4_ADD_FILE
Operation(s) affected:
CREATE
LINK
OPEN
RENAME
Discussion:
Permission to add a new file in a directory. The CREATE
operation is affected when nfs_ftype4 is NF4LNK, NF4BLK,
NF4CHR, NF4SOCK, or NF4FIFO. (NF4DIR is not listed because it
is covered by ACE4_ADD_SUBDIRECTORY.) OPEN is affected when
used to create a regular file. LINK and RENAME are always
affected.
ACE4_APPEND_DATA
Operation(s) affected:
WRITE
OPEN
SETATTR of size
Discussion:
The ability to modify a file's data, but only starting at EOF.
This allows for the notion of append-only files, by allowing
ACE4_APPEND_DATA and denying ACE4_WRITE_DATA to the same user
or group. If a file has an ACL such as the one described above
and a WRITE request is made for somewhere other than EOF, the
server SHOULD return NFS4ERR_ACCESS.
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ACE4_ADD_SUBDIRECTORY
Operation(s) affected:
CREATE
RENAME
Discussion:
Permission to create a subdirectory in a directory. The CREATE
operation is affected when nfs_ftype4 is NF4DIR. The RENAME
operation is always affected.
ACE4_READ_NAMED_ATTRS
Operation(s) affected:
OPENATTR
Discussion:
Permission to read the named attributes of a file or to look up
the named attributes directory. OPENATTR is affected when it
is not used to create a named attribute directory. This is
when 1) createdir is TRUE but a named attribute directory
already exists or 2) createdir is FALSE.
ACE4_WRITE_NAMED_ATTRS
Operation(s) affected:
OPENATTR
Discussion:
Permission to write the named attributes of a file or to create
a named attribute directory. OPENATTR is affected when it is
used to create a named attribute directory. This is when
createdir is TRUE and no named attribute directory exists. The
ability to check whether or not a named attribute directory
exists depends on the ability to look it up; therefore, users
also need the ACE4_READ_NAMED_ATTRS permission in order to
create a named attribute directory.
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ACE4_EXECUTE
Operation(s) affected:
READ
Discussion:
Permission to execute a file.
Servers SHOULD allow a user the ability to read the data of the
file when only the ACE4_EXECUTE access mask bit is set. This
is because there is no way to execute a file without reading
the contents. Though a server may treat ACE4_EXECUTE and
ACE4_READ_DATA bits identically when deciding to permit a READ
operation, it SHOULD still allow the two bits to be set
independently in ACLs and MUST distinguish between them when
replying to ACCESS operations. In particular, servers SHOULD
NOT silently turn on one of the two bits when the other is set,
as that would make it impossible for the client to correctly
enforce the distinction between read and execute permissions.
As an example, following a SETATTR of the following ACL:
nfsuser:ACE4_EXECUTE:ALLOW
A subsequent GETATTR of ACL for that file SHOULD return:
nfsuser:ACE4_EXECUTE:ALLOW
Rather than:
nfsuser:ACE4_EXECUTE/ACE4_READ_DATA:ALLOW
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ACE4_EXECUTE
Operation(s) affected:
LOOKUP
OPEN
REMOVE
RENAME
LINK
CREATE
Discussion:
Permission to traverse/search a directory.
ACE4_DELETE_CHILD
Operation(s) affected:
REMOVE
RENAME
Discussion:
Permission to delete a file or directory within a directory.
See Section 6.2.1.3.2 for information on how ACE4_DELETE and
ACE4_DELETE_CHILD interact.
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ACE4_READ_ATTRIBUTES
Operation(s) affected:
GETATTR of file system object attributes
VERIFY
NVERIFY
READDIR
Discussion:
The ability to read basic attributes (non-ACLs) of a file.
On a UNIX system, basic attributes can be thought of as the
stat-level attributes. Allowing this access mask bit would
mean the entity can execute "ls -l" and stat. If a READDIR
operation requests attributes, this mask must be allowed for
the READDIR to succeed.
ACE4_WRITE_ATTRIBUTES
Operation(s) affected:
SETATTR of time_access_set, time_backup, time_create,
time_modify_set, mimetype, hidden, and system
Discussion:
Permission to change the times associated with a file or
directory to an arbitrary value. Also, permission to change
the mimetype, hidden and system attributes. A user having
ACE4_WRITE_DATA or ACE4_WRITE_ATTRIBUTES will be allowed to set
the times associated with a file to the current server time.
ACE4_DELETE
Operation(s) affected:
REMOVE
Discussion:
Permission to delete the file or directory. See
Section 6.2.1.3.2 for information on ACE4_DELETE and
ACE4_DELETE_CHILD interact.
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ACE4_READ_ACL
Operation(s) affected:
GETATTR of acl
NVERIFY
VERIFY
Discussion:
Permission to read the ACL.
ACE4_WRITE_ACL
Operation(s) affected:
SETATTR of acl and mode
Discussion:
Permission to write the acl and mode attributes.
ACE4_WRITE_OWNER
Operation(s) affected:
SETATTR of owner and owner_group
Discussion:
Permission to write the owner and owner_group attributes. On
UNIX systems, this is the ability to execute chown() and
chgrp().
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ACE4_SYNCHRONIZE
Operation(s) affected:
NONE
Discussion:
Permission to use the file object as a synchronization
primitive for interprocess communication. This permission is
not enforced or interpreted by the NFSv4.0 server on behalf of
the client.
Typically, the ACE4_SYNCHRONIZE permission is only meaningful
on local file systems, i.e., file systems not accessed via
NFSv4.0. The reason that the permission bit exists is that
some operating environments, such as Windows, use
ACE4_SYNCHRONIZE.
For example, if a client copies a file that has
ACE4_SYNCHRONIZE set from a local file system to an NFSv4.0
server, and then later copies the file from the NFSv4.0 server
to a local file system, it is likely that if ACE4_SYNCHRONIZE
was set in the original file, the client will want it set in
the second copy. The first copy will not have the permission
set unless the NFSv4.0 server has the means to set the
ACE4_SYNCHRONIZE bit. The second copy will not have the
permission set unless the NFSv4.0 server has the means to
retrieve the ACE4_SYNCHRONIZE bit.
Server implementations need not provide the granularity of control
that is implied by this list of masks. For example, POSIX-based
systems might not distinguish ACE4_APPEND_DATA (the ability to append
to a file) from ACE4_WRITE_DATA (the ability to modify existing
contents); both masks would be tied to a single "write" permission.
When such a server returns attributes to the client, it would show
both ACE4_APPEND_DATA and ACE4_WRITE_DATA if and only if the write
permission is enabled.
If a server receives a SETATTR request that it cannot accurately
implement, it should err in the direction of more restricted access,
except in the previously discussed cases of execute and read. For
example, suppose a server cannot distinguish overwriting data from
appending new data, as described in the previous paragraph. If a
client submits an ALLOW ACE where ACE4_APPEND_DATA is set but
ACE4_WRITE_DATA is not (or vice versa), the server should either turn
off ACE4_APPEND_DATA or reject the request with NFS4ERR_ATTRNOTSUPP.
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6.2.1.3.2. ACE4_DELETE versus ACE4_DELETE_CHILD
Two access mask bits govern the ability to delete a directory entry:
ACE4_DELETE on the object itself (the "target") and ACE4_DELETE_CHILD
on the containing directory (the "parent").
Many systems also take the "sticky bit" (MODE4_SVTX) on a directory
to allow unlink only to a user that owns either the target or the
parent; on some such systems, the decision also depends on whether
the target is writable.
Servers SHOULD allow unlink if either ACE4_DELETE is permitted on the
target or ACE4_DELETE_CHILD is permitted on the parent. (Note that
this is true even if the parent or target explicitly denies the other
of these permissions.)
If the ACLs in question neither explicitly ALLOW nor DENY either of
the above, and if MODE4_SVTX is not set on the parent, then the
server SHOULD allow the removal if and only if ACE4_ADD_FILE is
permitted. In the case where MODE4_SVTX is set, the server may also
require the remover to own either the parent or the target, or may
require the target to be writable.
This allows servers to support something close to traditional
UNIX-like semantics, with ACE4_ADD_FILE taking the place of the
write bit.
The bitmask constants used for the flag field are as follows:
const ACE4_FILE_INHERIT_ACE = 0x00000001;
const ACE4_DIRECTORY_INHERIT_ACE = 0x00000002;
const ACE4_NO_PROPAGATE_INHERIT_ACE = 0x00000004;
const ACE4_INHERIT_ONLY_ACE = 0x00000008;
const ACE4_SUCCESSFUL_ACCESS_ACE_FLAG = 0x00000010;
const ACE4_FAILED_ACCESS_ACE_FLAG = 0x00000020;
const ACE4_IDENTIFIER_GROUP = 0x00000040;
A server need not support any of these flags. If the server supports
flags that are similar to, but not exactly the same as, these flags,
the implementation may define a mapping between the protocol-defined
flags and the implementation-defined flags.
For example, suppose a client tries to set an ACE with
ACE4_FILE_INHERIT_ACE set but not ACE4_DIRECTORY_INHERIT_ACE. If the
server does not support any form of ACL inheritance, the server
should reject the request with NFS4ERR_ATTRNOTSUPP. If the server
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supports a single "inherit ACE" flag that applies to both files and
directories, the server may reject the request (i.e., requiring the
client to set both the file and directory inheritance flags). The
server may also accept the request and silently turn on the
ACE4_DIRECTORY_INHERIT_ACE flag.
6.2.1.4.1. Discussion of Flag Bits
ACE4_FILE_INHERIT_ACE
Any non-directory file in any subdirectory will get this ACE
inherited.
ACE4_DIRECTORY_INHERIT_ACE
Can be placed on a directory and indicates that this ACE should be
added to each new directory created.
If this flag is set in an ACE in an ACL attribute to be set on a
non-directory file system object, the operation attempting to set
the ACL SHOULD fail with NFS4ERR_ATTRNOTSUPP.
ACE4_INHERIT_ONLY_ACE
Can be placed on a directory but does not apply to the directory;
ALLOW and DENY ACEs with this bit set do not affect access to the
directory, and AUDIT and ALARM ACEs with this bit set do not
trigger log or alarm events. Such ACEs only take effect once they
are applied (with this bit cleared) to newly created files and
directories as specified by the above two flags.
If this flag is present on an ACE, but neither
ACE4_DIRECTORY_INHERIT_ACE nor ACE4_FILE_INHERIT_ACE is present,
then an operation attempting to set such an attribute SHOULD fail
with NFS4ERR_ATTRNOTSUPP.
ACE4_NO_PROPAGATE_INHERIT_ACE
Can be placed on a directory. This flag tells the server that
inheritance of this ACE should stop at newly created child
directories.
ACE4_SUCCESSFUL_ACCESS_ACE_FLAG
ACE4_FAILED_ACCESS_ACE_FLAG
The ACE4_SUCCESSFUL_ACCESS_ACE_FLAG (SUCCESS) and
ACE4_FAILED_ACCESS_ACE_FLAG (FAILED) flag bits may be set only on
ACE4_SYSTEM_AUDIT_ACE_TYPE (AUDIT) and ACE4_SYSTEM_ALARM_ACE_TYPE
(ALARM) ACE types. If, during the processing of the file's ACL,
the server encounters an AUDIT or ALARM ACE that matches the
principal attempting the OPEN, the server notes that fact and
notes the presence, if any, of the SUCCESS and FAILED flags
encountered in the AUDIT or ALARM ACE. Once the server completes
the ACL processing, it then notes if the operation succeeded or
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failed. If the operation succeeded, and if the SUCCESS flag was
set for a matching AUDIT or ALARM ACE, then the appropriate AUDIT
or ALARM event occurs. If the operation failed, and if the FAILED
flag was set for the matching AUDIT or ALARM ACE, then the
appropriate AUDIT or ALARM event occurs. Either or both of the
SUCCESS or FAILED can be set, but if neither is set, the AUDIT or
ALARM ACE is not useful.
The previously described processing applies to ACCESS operations
even when they return NFS4_OK. For the purposes of AUDIT and
ALARM, we consider an ACCESS operation to be a "failure" if it
fails to return a bit that was requested and supported.
ACE4_IDENTIFIER_GROUP
Indicates that the "who" refers to a GROUP as defined under UNIX
or a GROUP ACCOUNT as defined under Windows. Clients and servers
MUST ignore the ACE4_IDENTIFIER_GROUP flag on ACEs with a who
value equal to one of the special identifiers outlined in
Section 6.2.1.5.
The who field of an ACE is an identifier that specifies the principal
or principals to whom the ACE applies. It may refer to a user or a
group, with the flag bit ACE4_IDENTIFIER_GROUP specifying which.
There are several special identifiers that need to be understood
universally, rather than in the context of a particular DNS domain.
Some of these identifiers cannot be understood when an NFS client
accesses the server but have meaning when a local process accesses
the file. The ability to display and modify these permissions is
permitted over NFS, even if none of the access methods on the server
understand the identifiers.
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+---------------+---------------------------------------------------+
| Who | Description |
+---------------+---------------------------------------------------+
| OWNER | The owner of the file. |
| GROUP | The group associated with the file. |
| EVERYONE | The world, including the owner and owning group. |
| INTERACTIVE | Accessed from an interactive terminal. |
| NETWORK | Accessed via the network. |
| DIALUP | Accessed as a dialup user to the server. |
| BATCH | Accessed from a batch job. |
| ANONYMOUS | Accessed without any authentication. |
| AUTHENTICATED | Any authenticated user (opposite of ANONYMOUS). |
| SERVICE | Access from a system service. |
+---------------+---------------------------------------------------+
Table 5: Special Identifiers
To avoid conflict, these special identifiers are distinguished by an
appended "@" and should appear in the form "xxxx@" (with no domain
name after the "@") -- for example, ANONYMOUS@.
The ACE4_IDENTIFIER_GROUP flag MUST be ignored on entries with these
special identifiers. When encoding entries with these special
identifiers, the ACE4_IDENTIFIER_GROUP flag SHOULD be set to zero.
6.2.1.5.1. Discussion of EVERYONE@
It is important to note that "EVERYONE@" is not equivalent to the
UNIX "other" entity. This is because, by definition, UNIX "other"
does not include the owner or owning group of a file. "EVERYONE@"
means literally everyone, including the owner or owning group.
6.2.2. Attribute 33: mode
The NFSv4.0 mode attribute is based on the UNIX mode bits. The
following bits are defined:
const MODE4_SUID = 0x800; /* set user id on execution */
const MODE4_SGID = 0x400; /* set group id on execution */
const MODE4_SVTX = 0x200; /* save text even after use */
const MODE4_RUSR = 0x100; /* read permission: owner */
const MODE4_WUSR = 0x080; /* write permission: owner */
const MODE4_XUSR = 0x040; /* execute permission: owner */
const MODE4_RGRP = 0x020; /* read permission: group */
const MODE4_WGRP = 0x010; /* write permission: group */
const MODE4_XGRP = 0x008; /* execute permission: group */
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const MODE4_ROTH = 0x004; /* read permission: other */
const MODE4_WOTH = 0x002; /* write permission: other */
const MODE4_XOTH = 0x001; /* execute permission: other */
Bits MODE4_RUSR, MODE4_WUSR, and MODE4_XUSR apply to the principal
identified in the owner attribute. Bits MODE4_RGRP, MODE4_WGRP, and
MODE4_XGRP apply to principals identified in the owner_group
attribute but who are not identified in the owner attribute. Bits
MODE4_ROTH, MODE4_WOTH, and MODE4_XOTH apply to any principal that
does not match that in the owner attribute and does not have a group
matching that of the owner_group attribute.
Bits within the mode other than those specified above are not defined
by this protocol. A server MUST NOT return bits other than those
defined above in a GETATTR or READDIR operation, and it MUST return
NFS4ERR_INVAL if bits other than those defined above are set in a
SETATTR, CREATE, OPEN, VERIFY, or NVERIFY operation.
6.3. Common Methods
The requirements in this section will be referred to in future
sections, especially Section 6.4.
6.3.1. Interpreting an ACL
6.3.1.1. Server Considerations
The server uses the algorithm described in Section 6.2.1 to determine
whether an ACL allows access to an object. However, the ACL may not
be the sole determiner of access. For example:
o In the case of a file system exported as read-only, the server may
deny write permissions even though an object's ACL grants it.
o Server implementations MAY grant ACE4_WRITE_ACL and ACE4_READ_ACL
permissions to prevent a situation from arising in which there is
no valid way to ever modify the ACL.
o All servers will allow a user the ability to read the data of the
file when only the execute permission is granted (i.e., if the ACL
denies the user ACE4_READ_DATA access and allows the user
ACE4_EXECUTE, the server will allow the user to read the data of
the file).
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o Many servers have the notion of owner-override, in which the owner
of the object is allowed to override accesses that are denied by
the ACL. This may be helpful, for example, to allow users
continued access to open files on which the permissions have
changed.
o Many servers have the notion of a "superuser" that has privileges
beyond an ordinary user. The superuser may be able to read or
write data or metadata in ways that would not be permitted by
the ACL.
6.3.1.2. Client Considerations
Clients SHOULD NOT do their own access checks based on their
interpretation of the ACL but rather use the OPEN and ACCESS
operations to do access checks. This allows the client to act on the
results of having the server determine whether or not access should
be granted based on its interpretation of the ACL.
Clients must be aware of situations in which an object's ACL will
define a certain access even though the server will not have adequate
information to enforce it. For example, the server has no way of
determining whether a particular OPEN reflects a user's open for read
access or is done as part of executing the file in question. In such
situations, the client needs to do its part in the enforcement of
access as defined by the ACL. To do this, the client will send the
appropriate ACCESS operation (or use a cached previous determination)
prior to servicing the request of the user or application in order to
determine whether the user or application should be granted the
access requested. For examples in which the ACL may define accesses
that the server does not enforce, see Section 6.3.1.1.
6.3.2. Computing a mode Attribute from an ACL
The following method can be used to calculate the MODE4_R*, MODE4_W*,
and MODE4_X* bits of a mode attribute, based upon an ACL.
First, for each of the special identifiers OWNER@, GROUP@, and
EVERYONE@, evaluate the ACL in order, considering only ALLOW and DENY
ACEs for the identifier EVERYONE@ and for the identifier under
consideration. The result of the evaluation will be an NFSv4 ACL
mask showing exactly which bits are permitted to that identifier.
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Then translate the calculated mask for OWNER@, GROUP@, and EVERYONE@
into mode bits for the user, group, and other, respectively, as
follows:
1. Set the read bit (MODE4_RUSR, MODE4_RGRP, or MODE4_ROTH) if and
only if ACE4_READ_DATA is set in the corresponding mask.
2. Set the write bit (MODE4_WUSR, MODE4_WGRP, or MODE4_WOTH) if and
only if ACE4_WRITE_DATA and ACE4_APPEND_DATA are both set in the
corresponding mask.
3. Set the execute bit (MODE4_XUSR, MODE4_XGRP, or MODE4_XOTH), if
and only if ACE4_EXECUTE is set in the corresponding mask.
Some server implementations also add bits permitted to named users
and groups to the group bits (MODE4_RGRP, MODE4_WGRP, and
MODE4_XGRP).
Implementations are discouraged from doing this, because it has been
found to cause confusion for users who see members of a file's group
denied access that the mode bits appear to allow. (The presence of
DENY ACEs may also lead to such behavior, but DENY ACEs are expected
to be more rarely used.)
The same user confusion seen when fetching the mode also results if
setting the mode does not effectively control permissions for the
owner, group, and other users; this motivates some of the
requirements that follow.
6.4. Requirements
The server that supports both mode and ACL must take care to
synchronize the MODE4_*USR, MODE4_*GRP, and MODE4_*OTH bits with the
ACEs that have respective who fields of "OWNER@", "GROUP@", and
"EVERYONE@" so that the client can see that semantically equivalent
access permissions exist whether the client asks for just the ACL or
any of the owner, owner_group, and mode attributes.
Many requirements refer to Section 6.3.2, but note that the methods
have behaviors specified with "SHOULD". This is intentional, to
avoid invalidating existing implementations that compute the mode
according to the withdrawn POSIX ACL draft ([P1003.1e]), rather than
by actual permissions on owner, group, and other.
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6.4.1. Setting the mode and/or ACL Attributes
6.4.1.1. Setting mode and Not ACL
When any of the nine low-order mode bits are changed because the mode
attribute was set, and no ACL attribute is explicitly set, the acl
attribute must be modified in accordance with the updated value of
those bits. This must happen even if the value of the low-order bits
is the same after the mode is set as before.
Note that any AUDIT or ALARM ACEs are unaffected by changes to the
mode.
In cases in which the permissions bits are subject to change, the acl
attribute MUST be modified such that the mode computed via the method
described in Section 6.3.2 yields the low-order nine bits (MODE4_R*,
MODE4_W*, MODE4_X*) of the mode attribute as modified by the change
attribute. The ACL attributes SHOULD also be modified such that:
1. If MODE4_RGRP is not set, entities explicitly listed in the ACL
other than OWNER@ and EVERYONE@ SHOULD NOT be granted
ACE4_READ_DATA.
2. If MODE4_WGRP is not set, entities explicitly listed in the ACL
other than OWNER@ and EVERYONE@ SHOULD NOT be granted
ACE4_WRITE_DATA or ACE4_APPEND_DATA.
3. If MODE4_XGRP is not set, entities explicitly listed in the ACL
other than OWNER@ and EVERYONE@ SHOULD NOT be granted
ACE4_EXECUTE.
Access mask bits other than those listed above, appearing in ALLOW
ACEs, MAY also be disabled.
Note that ACEs with the flag ACE4_INHERIT_ONLY_ACE set do not affect
the permissions of the ACL itself, nor do ACEs of the types AUDIT and
ALARM. As such, it is desirable to leave these ACEs unmodified when
modifying the ACL attributes.
Also note that the requirement may be met by discarding the acl in
favor of an ACL that represents the mode and only the mode. This is
permitted, but it is preferable for a server to preserve as much of
the ACL as possible without violating the above requirements.
Discarding the ACL makes it effectively impossible for a file created
with a mode attribute to inherit an ACL (see Section 6.4.3).
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6.4.1.2. Setting ACL and Not mode
When setting the acl and not setting the mode attribute, the
permission bits of the mode need to be derived from the ACL. In this
case, the ACL attribute SHOULD be set as given. The nine low-order
bits of the mode attribute (MODE4_R*, MODE4_W*, MODE4_X*) MUST be
modified to match the result of the method described in
Section 6.3.2. The three high-order bits of the mode (MODE4_SUID,
MODE4_SGID, MODE4_SVTX) SHOULD remain unchanged.
6.4.1.3. Setting Both ACL and mode
When setting both the mode and the acl attribute in the same
operation, the attributes MUST be applied in this order: mode, then
ACL. The mode-related attribute is set as given, then the ACL
attribute is set as given, possibly changing the final mode, as
described above in Section 6.4.1.2.
6.4.2. Retrieving the mode and/or ACL Attributes
This section applies only to servers that support both the mode and
ACL attributes.
Some server implementations may have a concept of "objects without
ACLs", meaning that all permissions are granted and denied according
to the mode attribute, and that no ACL attribute is stored for that
object. If an ACL attribute is requested of such a server, the
server SHOULD return an ACL that does not conflict with the mode;
that is to say, the ACL returned SHOULD represent the nine low-order
bits of the mode attribute (MODE4_R*, MODE4_W*, MODE4_X*) as
described in Section 6.3.2.
For other server implementations, the ACL attribute is always present
for every object. Such servers SHOULD store at least the three
high-order bits of the mode attribute (MODE4_SUID, MODE4_SGID,
MODE4_SVTX). The server SHOULD return a mode attribute if one is
requested, and the low-order nine bits of the mode (MODE4_R*,
MODE4_W*, MODE4_X*) MUST match the result of applying the method in
Section 6.3.2 to the ACL attribute.
6.4.3. Creating New Objects
If a server supports any ACL attributes, it may use the ACL
attributes on the parent directory to compute an initial ACL
attribute for a newly created object. This will be referred to as
the inherited ACL within this section. The act of adding one or more
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ACEs to the inherited ACL that are based upon ACEs in the parent
directory's ACL will be referred to as inheriting an ACE within this
section.
In the presence or absence of the mode and ACL attributes, the
behavior of CREATE and OPEN SHOULD be:
1. If just the mode is given in the call:
In this case, inheritance SHOULD take place, but the mode MUST be
applied to the inherited ACL as described in Section 6.4.1.1,
thereby modifying the ACL.
2. If just the ACL is given in the call:
In this case, inheritance SHOULD NOT take place, and the ACL as
defined in the CREATE or OPEN will be set without modification,
and the mode modified as in Section 6.4.1.2.
3. If both mode and ACL are given in the call:
In this case, inheritance SHOULD NOT take place, and both
attributes will be set as described in Section 6.4.1.3.
4. If neither mode nor ACL is given in the call:
In the case where an object is being created without any initial
attributes at all, e.g., an OPEN operation with an opentype4 of
OPEN4_CREATE and a createmode4 of EXCLUSIVE4, inheritance SHOULD
NOT take place. Instead, the server SHOULD set permissions to
deny all access to the newly created object. It is expected that
the appropriate client will set the desired attributes in a
subsequent SETATTR operation, and the server SHOULD allow that
operation to succeed, regardless of what permissions the object
is created with. For example, an empty ACL denies all
permissions, but the server should allow the owner's SETATTR to
succeed even though WRITE_ACL is implicitly denied.
In other cases, inheritance SHOULD take place, and no
modifications to the ACL will happen. The mode attribute, if
supported, MUST be as computed via the method described in
Section 6.3.2, with the MODE4_SUID, MODE4_SGID, and MODE4_SVTX
bits clear. If no inheritable ACEs exist on the parent
directory, the rules for creating acl attributes are
implementation defined.
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6.4.3.1. The Inherited ACL
If the object being created is not a directory, the inherited ACL
SHOULD NOT inherit ACEs from the parent directory ACL unless the
ACE4_FILE_INHERIT_FLAG is set.
If the object being created is a directory, the inherited ACL should
inherit all inheritable ACEs from the parent directory, i.e., those
that have the ACE4_FILE_INHERIT_ACE or ACE4_DIRECTORY_INHERIT_ACE
flag set. If the inheritable ACE has ACE4_FILE_INHERIT_ACE set, but
ACE4_DIRECTORY_INHERIT_ACE is clear, the inherited ACE on the newly
created directory MUST have the ACE4_INHERIT_ONLY_ACE flag set to
prevent the directory from being affected by ACEs meant for
non-directories.
When a new directory is created, the server MAY split any inherited
ACE that is both inheritable and effective (in other words, that has
neither ACE4_INHERIT_ONLY_ACE nor ACE4_NO_PROPAGATE_INHERIT_ACE set)
into two ACEs -- one with no inheritance flags, and one with
ACE4_INHERIT_ONLY_ACE set. This makes it simpler to modify the
effective permissions on the directory without modifying the ACE that
is to be inherited to the new directory's children.
7. NFS Server Namespace
7.1. Server Exports
On a UNIX server, the namespace describes all the files reachable by
pathnames under the root directory or "/". On a Windows server, the
namespace constitutes all the files on disks named by mapped disk
letters. NFS server administrators rarely make the entire server's
file system namespace available to NFS clients. More often, portions
of the namespace are made available via an "export" feature. In
previous versions of the NFS protocol, the root filehandle for each
export is obtained through the MOUNT protocol; the client sends a
string that identifies an object in the exported namespace, and the
server returns the root filehandle for it. The MOUNT protocol
supports an EXPORTS procedure that will enumerate the server's
exports.
7.2. Browsing Exports
The NFSv4 protocol provides a root filehandle that clients can use to
obtain filehandles for these exports via a multi-component LOOKUP. A
common user experience is to use a graphical user interface (perhaps
a file "Open" dialog window) to find a file via progressive browsing
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through a directory tree. The client must be able to move from one
export to another export via single-component, progressive LOOKUP
operations.
This style of browsing is not well supported by the NFSv2 and NFSv3
protocols. The client expects all LOOKUP operations to remain within
a single-server file system. For example, the device attribute will
not change. This prevents a client from taking namespace paths that
span exports.
An automounter on the client can obtain a snapshot of the server's
namespace using the EXPORTS procedure of the MOUNT protocol. If it
understands the server's pathname syntax, it can create an image of
the server's namespace on the client. The parts of the namespace
that are not exported by the server are filled in with a "pseudo-file
system" that allows the user to browse from one mounted file system
to another. There is a drawback to this representation of the
server's namespace on the client: it is static. If the server
administrator adds a new export, the client will be unaware of it.
7.3. Server Pseudo-File System
NFSv4 servers avoid this namespace inconsistency by presenting all
the exports within the framework of a single-server namespace. An
NFSv4 client uses LOOKUP and READDIR operations to browse seamlessly
from one export to another. Portions of the server namespace that
are not exported are bridged via a "pseudo-file system" that provides
a view of exported directories only. A pseudo-file system has a
unique fsid and behaves like a normal, read-only file system.
Based on the construction of the server's namespace, it is possible
that multiple pseudo-file systems may exist. For example:
/a pseudo-file system
/a/b real file system
/a/b/c pseudo-file system
/a/b/c/d real file system
Each of the pseudo-file systems are considered separate entities and
therefore will have a unique fsid.
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7.4. Multiple Roots
The DOS and Windows operating environments are sometimes described as
having "multiple roots". File systems are commonly represented as
disk letters. MacOS represents file systems as top-level names.
NFSv4 servers for these platforms can construct a pseudo-file system
above these root names so that disk letters or volume names are
simply directory names in the pseudo-root.
7.5. Filehandle Volatility
The nature of the server's pseudo-file system is that it is a logical
representation of file system(s) available from the server.
Therefore, the pseudo-file system is most likely constructed
dynamically when the server is first instantiated. It is expected
that the pseudo-file system may not have an on-disk counterpart from
which persistent filehandles could be constructed. Even though it is
preferable that the server provide persistent filehandles for the
pseudo-file system, the NFS client should expect that pseudo-file
system filehandles are volatile. This can be confirmed by checking
the associated "fh_expire_type" attribute for those filehandles in
question. If the filehandles are volatile, the NFS client must be
prepared to recover a filehandle value (e.g., with a multi-component
LOOKUP) when receiving an error of NFS4ERR_FHEXPIRED.
7.6. Exported Root
If the server's root file system is exported, one might conclude that
a pseudo-file system is not needed. This would be wrong. Assume the
following file systems on a server:
/ disk1 (exported)
/a disk2 (not exported)
/a/b disk3 (exported)
Because disk2 is not exported, disk3 cannot be reached with simple
LOOKUPs. The server must bridge the gap with a pseudo-file system.
7.7. Mount Point Crossing
The server file system environment may be constructed in such a way
that one file system contains a directory that is 'covered' or
mounted upon by a second file system. For example:
/a/b (file system 1)
/a/b/c/d (file system 2)
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The pseudo-file system for this server may be constructed to
look like:
/ (placeholder/not exported)
/a/b (file system 1)
/a/b/c/d (file system 2)
It is the server's responsibility to present the pseudo-file system
that is complete to the client. If the client sends a LOOKUP request
for the path "/a/b/c/d", the server's response is the filehandle of
the file system "/a/b/c/d". In previous versions of the NFS
protocol, the server would respond with the filehandle of directory
"/a/b/c/d" within the file system "/a/b".
The NFS client will be able to determine if it crosses a server mount
point by a change in the value of the "fsid" attribute.
7.8. Security Policy and Namespace Presentation
Because NFSv4 clients possess the ability to change the security
mechanisms used, after determining what is allowed, by using SECINFO
the server SHOULD NOT present a different view of the namespace based
on the security mechanism being used by a client. Instead, it should
present a consistent view and return NFS4ERR_WRONGSEC if an attempt
is made to access data with an inappropriate security mechanism.
If security considerations make it necessary to hide the existence of
a particular file system, as opposed to all of the data within it,
the server can apply the security policy of a shared resource in the
server's namespace to components of the resource's ancestors. For
example:
/ (placeholder/not exported)
/a/b (file system 1)
/a/b/MySecretProject (file system 2)
The /a/b/MySecretProject directory is a real file system and is the
shared resource. Suppose the security policy for /a/b/
MySecretProject is Kerberos with integrity and it is desired to limit
knowledge of the existence of this file system. In this case, the
server should apply the same security policy to /a/b. This allows
for knowledge of the existence of a file system to be secured when
desirable.
For the case of the use of multiple, disjoint security mechanisms in
the server's resources, applying that sort of policy would result in
the higher-level file system not being accessible using any security
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flavor. Therefore, that sort of configuration is not compatible with
hiding the existence (as opposed to the contents) from clients using
multiple disjoint sets of security flavors.
In other circumstances, a desirable policy is for the security of a
particular object in the server's namespace to include the union of
all security mechanisms of all direct descendants. A common and
convenient practice, unless strong security requirements dictate
otherwise, is to make the entire pseudo-file system accessible by all
of the valid security mechanisms.
Where there is concern about the security of data on the network,
clients should use strong security mechanisms to access the
pseudo-file system in order to prevent man-in-the-middle attacks.
8. Multi-Server Namespace
NFSv4 supports attributes that allow a namespace to extend beyond the
boundaries of a single server. It is RECOMMENDED that clients and
servers support construction of such multi-server namespaces. Use of
such multi-server namespaces is optional, however, and for many
purposes, single-server namespaces are perfectly acceptable. Use of
multi-server namespaces can provide many advantages, however, by
separating a file system's logical position in a namespace from the
(possibly changing) logistical and administrative considerations that
result in particular file systems being located on particular
servers.
8.1. Location Attributes
NFSv4 contains RECOMMENDED attributes that allow file systems on one
server to be associated with one or more instances of that file
system on other servers. These attributes specify such file system
instances by specifying a server address target (as either a DNS name
representing one or more IP addresses, or a literal IP address),
together with the path of that file system within the associated
single-server namespace.
The fs_locations RECOMMENDED attribute allows specification of the
file system locations where the data corresponding to a given file
system may be found.
8.2. File System Presence or Absence
A given location in an NFSv4 namespace (typically but not necessarily
a multi-server namespace) can have a number of file system instance
locations associated with it via the fs_locations attribute. There
may also be an actual current file system at that location,
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accessible via normal namespace operations (e.g., LOOKUP). In this
case, the file system is said to be "present" at that position in the
namespace, and clients will typically use it, reserving use of
additional locations specified via the location-related attributes to
situations in which the principal location is no longer available.
When there is no actual file system at the namespace location in
question, the file system is said to be "absent". An absent file
system contains no files or directories other than the root. Any
reference to it, except to access a small set of attributes useful in
determining alternative locations, will result in an error,
NFS4ERR_MOVED. Note that if the server ever returns the error
NFS4ERR_MOVED, it MUST support the fs_locations attribute.
While the error name suggests that we have a case of a file system
that once was present, and has only become absent later, this is only
one possibility. A position in the namespace may be permanently
absent with the set of file system(s) designated by the location
attributes being the only realization. The name NFS4ERR_MOVED
reflects an earlier, more limited conception of its function, but
this error will be returned whenever the referenced file system is
absent, whether it has moved or simply never existed.
Except in the case of GETATTR-type operations (to be discussed
later), when the current filehandle at the start of an operation is
within an absent file system, that operation is not performed and the
error NFS4ERR_MOVED is returned, to indicate that the file system is
absent on the current server.
Because a GETFH cannot succeed if the current filehandle is within an
absent file system, filehandles within an absent file system cannot
be transferred to the client. When a client does have filehandles
within an absent file system, it is the result of obtaining them when
the file system was present, and having the file system become absent
subsequently.
It should be noted that because the check for the current filehandle
being within an absent file system happens at the start of every
operation, operations that change the current filehandle so that it
is within an absent file system will not result in an error. This
allows such combinations as PUTFH-GETATTR and LOOKUP-GETATTR to be
used to get attribute information, particularly location attribute
information, as discussed below.
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8.3. Getting Attributes for an Absent File System
When a file system is absent, most attributes are not available, but
it is necessary to allow the client access to the small set of
attributes that are available, and most particularly that which gives
information about the correct current locations for this file system,
fs_locations.
8.3.1. GETATTR within an Absent File System
As mentioned above, an exception is made for GETATTR in that
attributes may be obtained for a filehandle within an absent file
system. This exception only applies if the attribute mask contains
at least the fs_locations attribute bit, which indicates that the
client is interested in a result regarding an absent file system. If
it is not requested, GETATTR will result in an NFS4ERR_MOVED error.
When a GETATTR is done on an absent file system, the set of supported
attributes is very limited. Many attributes, including those that
are normally REQUIRED, will not be available on an absent file
system. In addition to the fs_locations attribute, the following
attributes SHOULD be available on absent file systems. In the case
of RECOMMENDED attributes, they should be available at least to the
same degree that they are available on present file systems.
fsid: This attribute should be provided so that the client can
determine file system boundaries, including, in particular, the
boundary between present and absent file systems. This value must
be different from any other fsid on the current server and need
have no particular relationship to fsids on any particular
destination to which the client might be directed.
mounted_on_fileid: For objects at the top of an absent file system,
this attribute needs to be available. Since the fileid is within
the present parent file system, there should be no need to
reference the absent file system to provide this information.
Other attributes SHOULD NOT be made available for absent file
systems, even when it is possible to provide them. The server should
not assume that more information is always better and should avoid
gratuitously providing additional information.
When a GETATTR operation includes a bitmask for the attribute
fs_locations, but where the bitmask includes attributes that are not
supported, GETATTR will not return an error but will return the mask
of the actual attributes supported with the results.
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Handling of VERIFY/NVERIFY is similar to GETATTR in that if the
attribute mask does not include fs_locations the error NFS4ERR_MOVED
will result. It differs in that any appearance in the attribute mask
of an attribute not supported for an absent file system (and note
that this will include some normally REQUIRED attributes) will also
cause an NFS4ERR_MOVED result.
8.3.2. READDIR and Absent File Systems
A READDIR performed when the current filehandle is within an absent
file system will result in an NFS4ERR_MOVED error, since, unlike the
case of GETATTR, no such exception is made for READDIR.
Attributes for an absent file system may be fetched via a READDIR for
a directory in a present file system, when that directory contains
the root directories of one or more absent file systems. In this
case, the handling is as follows:
o If the attribute set requested includes fs_locations, then the
fetching of attributes proceeds normally, and no NFS4ERR_MOVED
indication is returned even when the rdattr_error attribute is
requested.
o If the attribute set requested does not include fs_locations, then
if the rdattr_error attribute is requested, each directory entry
for the root of an absent file system will report NFS4ERR_MOVED as
the value of the rdattr_error attribute.
o If the attribute set requested does not include either of the
attributes fs_locations or rdattr_error, then the occurrence of
the root of an absent file system within the directory will result
in the READDIR failing with an NFS4ERR_MOVED error.
o The unavailability of an attribute because of a file system's
absence, even one that is ordinarily REQUIRED, does not result in
any error indication. The set of attributes returned for the root
directory of the absent file system in that case is simply
restricted to those actually available.
8.4. Uses of Location Information
The location-bearing attribute of fs_locations provides, together
with the possibility of absent file systems, a number of important
facilities in providing reliable, manageable, and scalable data
access.
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When a file system is present, these attributes can provide
alternative locations, to be used to access the same data, in the
event of server failures, communications problems, or other
difficulties that make continued access to the current file system
impossible or otherwise impractical. Under some circumstances,
multiple alternative locations may be used simultaneously to provide
higher-performance access to the file system in question. Provision
of such alternative locations is referred to as "replication",
although there are cases in which replicated sets of data are not in
fact present and the replicas are instead different paths to the same
data.
When a file system is present and subsequently becomes absent,
clients can be given the opportunity to have continued access to
their data, at an alternative location. Transfer of the file system
contents to the new location is referred to as "migration". See
Section 8.4.2 for details.
Alternative locations may be physical replicas of the file system
data or alternative communication paths to the same server or, in the
case of various forms of server clustering, another server providing
access to the same physical file system. The client's
responsibilities in dealing with this transition depend on the
specific nature of the new access path as well as how and whether
data was in fact migrated. These issues will be discussed in detail
below.
Where a file system was not previously present, specification of file
system location provides a means by which file systems located on one
server can be associated with a namespace defined by another server,
thus allowing a general multi-server namespace facility. A
designation of such a location, in place of an absent file system, is
called a "referral".
Because client support for location-related attributes is OPTIONAL, a
server may (but is not required to) take action to hide migration and
referral events from such clients, by acting as a proxy, for example.
8.4.1. File System Replication
The fs_locations attribute provides alternative locations, to be used
to access data in place of, or in addition to, the current file
system instance. On first access to a file system, the client should
obtain the value of the set of alternative locations by interrogating
the fs_locations attribute.
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In the event that server failures, communications problems, or other
difficulties make continued access to the current file system
impossible or otherwise impractical, the client can use the
alternative locations as a way to get continued access to its data.
Multiple locations may be used simultaneously, to provide higher
performance through the exploitation of multiple paths between client
and target file system.
Multiple server addresses, whether they are derived from a single
entry with a DNS name representing a set of IP addresses or from
multiple entries each with its own server address, may correspond to
the same actual server.
8.4.2. File System Migration
When a file system is present and becomes absent, clients can be
given the opportunity to have continued access to their data, at an
alternative location, as specified by the fs_locations attribute.
Typically, a client will be accessing the file system in question,
get an NFS4ERR_MOVED error, and then use the fs_locations attribute
to determine the new location of the data.
Such migration can be helpful in providing load balancing or general
resource reallocation. The protocol does not specify how the file
system will be moved between servers. It is anticipated that a
number of different server-to-server transfer mechanisms might be
used, with the choice left to the server implementer. The NFSv4
protocol specifies the method used to communicate the migration event
between client and server.
When an alternative location is designated as the target for
migration, it must designate the same data. Where file systems are
writable, a change made on the original file system must be visible
on all migration targets. Where a file system is not writable but
represents a read-only copy (possibly periodically updated) of a
writable file system, similar requirements apply to the propagation
of updates. Any change visible in the original file system must
already be effected on all migration targets, to avoid any
possibility that a client, in effecting a transition to the migration
target, will see any reversion in file system state.
8.4.3. Referrals
Referrals provide a way of placing a file system in a location within
the namespace essentially without respect to its physical location on
a given server. This allows a single server or a set of servers to
present a multi-server namespace that encompasses file systems
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located on multiple servers. Some likely uses of this include
establishment of site-wide or organization-wide namespaces, or even
knitting such together into a truly global namespace.
Referrals occur when a client determines, upon first referencing a
position in the current namespace, that it is part of a new file
system and that the file system is absent. When this occurs,
typically by receiving the error NFS4ERR_MOVED, the actual location
or locations of the file system can be determined by fetching the
fs_locations attribute.
The location-related attribute may designate a single file system
location or multiple file system locations, to be selected based on
the needs of the client.
Use of multi-server namespaces is enabled by NFSv4 but is not
required. The use of multi-server namespaces and their scope will
depend on the applications used and system administration
preferences.
Multi-server namespaces can be established by a single server
providing a large set of referrals to all of the included file
systems. Alternatively, a single multi-server namespace may be
administratively segmented with separate referral file systems (on
separate servers) for each separately administered portion of the
namespace. The top-level referral file system or any segment may use
replicated referral file systems for higher availability.
Generally, multi-server namespaces are for the most part uniform, in
that the same data made available to one client at a given location
in the namespace is made available to all clients at that location.
8.5. Location Entries and Server Identity
As mentioned above, a single location entry may have a server address
target in the form of a DNS name that may represent multiple IP
addresses, while multiple location entries may have their own server
address targets that reference the same server.
When multiple addresses for the same server exist, the client may
assume that for each file system in the namespace of a given server
network address, there exist file systems at corresponding namespace
locations for each of the other server network addresses. It may do
this even in the absence of explicit listing in fs_locations. Such
corresponding file system locations can be used as alternative
locations, just as those explicitly specified via the fs_locations
attribute.
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If a single location entry designates multiple server IP addresses,
the client should choose a single one to use. When two server
addresses are designated by a single location entry and they
correspond to different servers, this normally indicates some sort of
misconfiguration, and so the client should avoid using such location
entries when alternatives are available. When they are not, clients
should pick one of the IP addresses and use it, without using others
that are not directed to the same server.
8.6. Additional Client-Side Considerations
When clients make use of servers that implement referrals,
replication, and migration, care should be taken that a user who
mounts a given file system that includes a referral or a relocated
file system continues to see a coherent picture of that user-side
file system despite the fact that it contains a number of server-side
file systems that may be on different servers.
One important issue is upward navigation from the root of a
server-side file system to its parent (specified as ".." in UNIX), in
the case in which it transitions to that file system as a result of
referral, migration, or a transition as a result of replication.
When the client is at such a point, and it needs to ascend to the
parent, it must go back to the parent as seen within the multi-server
namespace rather than sending a LOOKUPP operation to the server,
which would result in the parent within that server's single-server
namespace. In order to do this, the client needs to remember the
filehandles that represent such file system roots and use these
instead of issuing a LOOKUPP operation to the current server. This
will allow the client to present to applications a consistent
namespace, where upward navigation and downward navigation are
consistent.
Another issue concerns refresh of referral locations. When referrals
are used extensively, they may change as server configurations
change. It is expected that clients will cache information related
to traversing referrals so that future client-side requests are
resolved locally without server communication. This is usually
rooted in client-side name lookup caching. Clients should
periodically purge this data for referral points in order to detect
changes in location information.
A potential problem exists if a client were to allow an open-owner to
have state on multiple file systems on a server, in that it is
unclear how the sequence numbers associated with open-owners are to
be dealt with, in the event of transparent state migration. A client
can avoid such a situation if it ensures that any use of an
open-owner is confined to a single file system.
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A server MAY decline to migrate state associated with open-owners
that span multiple file systems. In cases in which the server
chooses not to migrate such state, the server MUST return
NFS4ERR_BAD_STATEID when the client uses those stateids on the new
server.
The server MUST return NFS4ERR_STALE_STATEID when the client uses
those stateids on the old server, regardless of whether migration has
occurred or not.
8.7. Effecting File System Referrals
Referrals are effected when an absent file system is encountered and
one or more alternative locations are made available by the
fs_locations attribute. The client will typically get an
NFS4ERR_MOVED error, fetch the appropriate location information, and
proceed to access the file system on a different server, even though
it retains its logical position within the original namespace.
Referrals differ from migration events in that they happen only when
the client has not previously referenced the file system in question
(so there is nothing to transition). Referrals can only come into
effect when an absent file system is encountered at its root.
The examples given in the sections below are somewhat artificial in
that an actual client will not typically do a multi-component lookup
but will have cached information regarding the upper levels of the
name hierarchy. However, these example are chosen to make the
required behavior clear and easy to put within the scope of a small
number of requests, without getting unduly into details of how
specific clients might choose to cache things.
8.7.1. Referral Example (LOOKUP)
Let us suppose that the following COMPOUND is sent in an environment
in which /this/is/the/path is absent from the target server. This
may be for a number of reasons. It may be the case that the file
system has moved, or it may be the case that the target server is
functioning mainly, or solely, to refer clients to the servers on
which various file systems are located.
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o PUTROOTFH
o LOOKUP "this"
o LOOKUP "is"
o LOOKUP "the"
o LOOKUP "path"
o GETFH
o GETATTR(fsid, fileid, size, time_modify)
Under the given circumstances, the following will be the result:
o PUTROOTFH --> NFS_OK. The current fh is now the root of the
pseudo-fs.
o LOOKUP "this" --> NFS_OK. The current fh is for /this and is
within the pseudo-fs.
o LOOKUP "is" --> NFS_OK. The current fh is for /this/is and is
within the pseudo-fs.
o LOOKUP "the" --> NFS_OK. The current fh is for /this/is/the and
is within the pseudo-fs.
o LOOKUP "path" --> NFS_OK. The current fh is for /this/is/the/path
and is within a new, absent file system, but ... the client will
never see the value of that fh.
o GETFH --> NFS4ERR_MOVED. Fails, because the current fh is in an
absent file system at the start of the operation and the
specification makes no exception for GETFH.
o GETATTR(fsid, fileid, size, time_modify). Not executed, because
the failure of the GETFH stops the processing of the COMPOUND.
Given the failure of the GETFH, the client has the job of determining
the root of the absent file system and where to find that file
system, i.e., the server and path relative to that server's root fh.
Note here that in this example, the client did not obtain filehandles
and attribute information (e.g., fsid) for the intermediate
directories, so that it would not be sure where the absent file
system starts. It could be the case, for example, that /this/is/the
is the root of the moved file system and that the reason that the
lookup of "path" succeeded is that the file system was not absent on
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that operation but was moved between the last LOOKUP and the GETFH
(since COMPOUND is not atomic). Even if we had the fsids for all of
the intermediate directories, we could have no way of knowing that
/this/is/the/path was the root of a new file system, since we don't
yet have its fsid.
In order to get the necessary information, let us re-send the chain
of LOOKUPs with GETFHs and GETATTRs to at least get the fsids so we
can be sure where the appropriate file system boundaries are. The
client could choose to get fs_locations at the same time, but in most
cases the client will have a good guess as to where the file system
boundaries are (because of where NFS4ERR_MOVED was, and was not,
received), making the fetching of fs_locations unnecessary.
OP01: PUTROOTFH --> NFS_OK
- The current fh is at the root of the pseudo-fs.
OP02: GETATTR(fsid) --> NFS_OK
- Just for completeness. Normally, clients will know the fsid of
the pseudo-fs as soon as they establish communication with a
server.
OP03: LOOKUP "this" --> NFS_OK
OP04: GETATTR(fsid) --> NFS_OK
- Get the current fsid to see where the file system boundaries are.
The fsid will be that for the pseudo-fs in this example, so no
boundary.
OP05: GETFH --> NFS_OK
- The current fh is for /this and is within the pseudo-fs.
OP06: LOOKUP "is" --> NFS_OK
- The current fh is for /this/is and is within the pseudo-fs.
OP07: GETATTR(fsid) --> NFS_OK
- Get the current fsid to see where the file system boundaries are.
The fsid will be that for the pseudo-fs in this example, so no
boundary.
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OP08: GETFH --> NFS_OK
- The current fh is for /this/is and is within the pseudo-fs.
OP09: LOOKUP "the" --> NFS_OK
- The current fh is for /this/is/the and is within the pseudo-fs.
OP10: GETATTR(fsid) --> NFS_OK
- Get the current fsid to see where the file system boundaries are.
The fsid will be that for the pseudo-fs in this example, so no
boundary.
OP11: GETFH --> NFS_OK
- The current fh is for /this/is/the and is within the pseudo-fs.
OP12: LOOKUP "path" --> NFS_OK
- The current fh is for /this/is/the/path and is within a new,
absent file system, but ...
- The client will never see the value of that fh.
OP13: GETATTR(fsid, fs_locations) --> NFS_OK
- We are getting the fsid to know where the file system boundaries
are. In this operation, the fsid will be different than that of
the parent directory (which in turn was retrieved in OP10). Note
that the fsid we are given will not necessarily be preserved at
the new location. That fsid might be different, and in fact the
fsid we have for this file system might be a valid fsid of a
different file system on that new server.
- In this particular case, we are pretty sure anyway that what has
moved is /this/is/the/path rather than /this/is/the since we have
the fsid of the latter and it is that of the pseudo-fs, which
presumably cannot move. However, in other examples, we might not
have this kind of information to rely on (e.g., /this/is/the might
be a non-pseudo-file system separate from /this/is/the/path), so
we need to have other reliable source information on the boundary
of the file system that is moved. If, for example, the file
system /this/is had moved, we would have a case of migration
rather than referral, and once the boundaries of the migrated file
system were clear we could fetch fs_locations.
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- We are fetching fs_locations because the fact that we got an
NFS4ERR_MOVED at this point means that this is most likely a
referral and we need the destination. Even if it is the case that
/this/is/the is a file system that has migrated, we will still
need the location information for that file system.
OP14: GETFH --> NFS4ERR_MOVED
- Fails because current fh is in an absent file system at the start
of the operation, and the specification makes no exception for
GETFH. Note that this means the server will never send the client
a filehandle from within an absent file system.
Given the above, the client knows where the root of the absent file
system is (/this/is/the/path) by noting where the change of fsid
occurred (between "the" and "path"). The fs_locations attribute also
gives the client the actual location of the absent file system so
that the referral can proceed. The server gives the client the bare
minimum of information about the absent file system so that there
will be very little scope for problems of conflict between
information sent by the referring server and information of the file
system's home. No filehandles and very few attributes are present on
the referring server, and the client can treat those it receives as
transient information with the function of enabling the referral.
8.7.2. Referral Example (READDIR)
Another context in which a client may encounter referrals is when it
does a READDIR on a directory in which some of the subdirectories are
the roots of absent file systems.
Suppose such a directory is read as follows:
o PUTROOTFH
o LOOKUP "this"
o LOOKUP "is"
o LOOKUP "the"
o READDIR(fsid, size, time_modify, mounted_on_fileid)
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In this case, because rdattr_error is not requested, fs_locations is
not requested, and some of the attributes cannot be provided, the
result will be an NFS4ERR_MOVED error on the READDIR, with the
detailed results as follows:
o PUTROOTFH --> NFS_OK. The current fh is at the root of the
pseudo-fs.
o LOOKUP "this" --> NFS_OK. The current fh is for /this and is
within the pseudo-fs.
o LOOKUP "is" --> NFS_OK. The current fh is for /this/is and is
within the pseudo-fs.
o LOOKUP "the" --> NFS_OK. The current fh is for /this/is/the and
is within the pseudo-fs.
o READDIR(fsid, size, time_modify, mounted_on_fileid) -->
NFS4ERR_MOVED. Note that the same error would have been returned
if /this/is/the had migrated, but it is returned because the
directory contains the root of an absent file system.
So now suppose that we re-send with rdattr_error:
o PUTROOTFH
o LOOKUP "this"
o LOOKUP "is"
o LOOKUP "the"
o READDIR(rdattr_error, fsid, size, time_modify, mounted_on_fileid)
The results will be:
o PUTROOTFH --> NFS_OK. The current fh is at the root of the
pseudo-fs.
o LOOKUP "this" --> NFS_OK. The current fh is for /this and is
within the pseudo-fs.
o LOOKUP "is" --> NFS_OK. The current fh is for /this/is and is
within the pseudo-fs.
o LOOKUP "the" --> NFS_OK. The current fh is for /this/is/the and
is within the pseudo-fs.
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o READDIR(rdattr_error, fsid, size, time_modify, mounted_on_fileid)
--> NFS_OK. The attributes for the directory entry with the
component named "path" will only contain rdattr_error with the
value NFS4ERR_MOVED, together with an fsid value and a value for
mounted_on_fileid.
So suppose we do another READDIR to get fs_locations (although we
could have used a GETATTR directly, as in Section 8.7.1):
o PUTROOTFH
o LOOKUP "this"
o LOOKUP "is"
o LOOKUP "the"
o READDIR(rdattr_error, fs_locations, mounted_on_fileid, fsid, size,
time_modify)
The results would be:
o PUTROOTFH --> NFS_OK. The current fh is at the root of the
pseudo-fs.
o LOOKUP "this" --> NFS_OK. The current fh is for /this and is
within the pseudo-fs.
o LOOKUP "is" --> NFS_OK. The current fh is for /this/is and is
within the pseudo-fs.
o LOOKUP "the" --> NFS_OK. The current fh is for /this/is/the and
is within the pseudo-fs.
o READDIR(rdattr_error, fs_locations, mounted_on_fileid, fsid, size,
time_modify) --> NFS_OK. The attributes will be as shown below.
The attributes for the directory entry with the component named
"path" will only contain:
o rdattr_error (value: NFS_OK)
o fs_locations
o mounted_on_fileid (value: unique fileid within referring file
system)
o fsid (value: unique value within referring server)
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The attributes for entry "path" will not contain size or time_modify,
because these attributes are not available within an absent file
system.
8.8. The Attribute fs_locations
The fs_locations attribute is defined by both fs_location4
(Section 2.2.6) and fs_locations4 (Section 2.2.7). It is used to
represent the location of a file system by providing a server name
and the path to the root of the file system within that server's
namespace. When a set of servers have corresponding file systems at
the same path within their namespaces, an array of server names may
be provided. An entry in the server array is a UTF-8 string and
represents one of a traditional DNS host name, IPv4 address, IPv6
address, or a zero-length string. A zero-length string SHOULD be
used to indicate the current address being used for the RPC. It is
not a requirement that all servers that share the same rootpath be
listed in one fs_location4 instance. The array of server names is
provided for convenience. Servers that share the same rootpath may
also be listed in separate fs_location4 entries in the fs_locations
attribute.
The fs_locations4 data type and fs_locations attribute contain an
array of such locations. Since the namespace of each server may be
constructed differently, the fs_root field is provided. The path
represented by the fs_root represents the location of the file system
in the current server's namespace, i.e., that of the server from
which the fs_locations attribute was obtained. The fs_root path is
meant to aid the client by clearly referencing the root of the file
system whose locations are being reported, no matter what object
within the current file system the current filehandle designates.
The fs_root is simply the pathname the client used to reach the
object on the current server (i.e., the object to which the
fs_locations attribute applies).
When the fs_locations attribute is interrogated and there are no
alternative file system locations, the server SHOULD return a
zero-length array of fs_location4 structures, together with a
valid fs_root.
As an example, suppose there is a replicated file system located at
two servers (servA and servB). At servA, the file system is located
at path /a/b/c. At servB, the file system is located at path /x/y/z.
If the client were to obtain the fs_locations value for the directory
at /a/b/c/d, it might not necessarily know that the file system's
root is located in servA's namespace at /a/b/c. When the client
switches to servB, it will need to determine that the directory it
first referenced at servA is now represented by the path /x/y/z/d
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on servB. To facilitate this, the fs_locations attribute provided by
servA would have an fs_root value of /a/b/c and two entries in
fs_locations. One entry in fs_locations will be for itself (servA),
and the other will be for servB with a path of /x/y/z. With this
information, the client is able to substitute /x/y/z for /a/b/c at
the beginning of its access path and construct /x/y/z/d to use for
the new server.
Note that there is no requirement that the number of components in
each rootpath be the same; there is no relation between the number of
components in the rootpath or fs_root, and none of the components in
each rootpath and fs_root have to be the same. In the above example,
we could have had a third element in the locations array, with server
equal to "servC" and rootpath equal to "/I/II", and a fourth element
in the locations array, with server equal to "servD" and rootpath
equal to "/aleph/beth/gimel/daleth/he".
The relationship between an fs_root and a rootpath is that the client
replaces the pathname indicated in the fs_root for the current server
for the substitute indicated in the rootpath for the new server.
For an example of a referred or migrated file system, suppose there
is a file system located at serv1. At serv1, the file system is
located at /az/buky/vedi/glagoli. The client finds that the object
at glagoli has migrated (or is a referral). The client gets the
fs_locations attribute, which contains an fs_root of /az/buky/vedi/
glagoli, and one element in the locations array, with server equal to
serv2, and rootpath equal to /izhitsa/fita. The client replaces
/az/buky/vedi/glagoli with /izhitsa/fita and uses the latter pathname
on serv2.
Thus, the server MUST return an fs_root that is equal to the path the
client used to reach the object to which the fs_locations attribute
applies. Otherwise, the client cannot determine the new path to use
on the new server.
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9. File Locking and Share Reservations
Integrating locking into the NFS protocol necessarily causes it to be
stateful. With the inclusion of share reservations, the protocol
becomes substantially more dependent on state than the traditional
combination of NFS and NLM (Network Lock Manager) [xnfs]. There are
three components to making this state manageable:
o clear division between client and server
o ability to reliably detect inconsistency in state between client
and server
o simple and robust recovery mechanisms
In this model, the server owns the state information. The client
requests changes in locks, and the server responds with the changes
made. Non-client-initiated changes in locking state are infrequent.
The client receives prompt notification of such changes and can
adjust its view of the locking state to reflect the server's changes.
Individual pieces of state created by the server and passed to the
client at its request are represented by 128-bit stateids. These
stateids may represent a particular open file, a set of byte-range
locks held by a particular owner, or a recallable delegation of
privileges to access a file in particular ways or at a particular
location.
In all cases, there is a transition from the most general information
that represents a client as a whole to the eventual lightweight
stateid used for most client and server locking interactions. The
details of this transition will vary with the type of object, but it
always starts with a client ID.
To support Win32 share reservations, it is necessary to atomically
OPEN or CREATE files and apply the appropriate locks in the same
operation. Having a separate share/unshare operation would not allow
correct implementation of the Win32 OpenFile API. In order to
correctly implement share semantics, the previous NFS protocol
mechanisms used when a file is opened or created (LOOKUP, CREATE,
ACCESS) need to be replaced. The NFSv4 protocol has an OPEN
operation that subsumes the NFSv3 methodology of LOOKUP, CREATE, and
ACCESS. However, because many operations require a filehandle, the
traditional LOOKUP is preserved to map a filename to a filehandle
without establishing state on the server. The policy of granting
access or modifying files is managed by the server based on the
client's state. These mechanisms can implement policy ranging from
advisory only locking to full mandatory locking.
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9.1. Opens and Byte-Range Locks
It is assumed that manipulating a byte-range lock is rare when
compared to READ and WRITE operations. It is also assumed that
server restarts and network partitions are relatively rare.
Therefore, it is important that the READ and WRITE operations have a
lightweight mechanism to indicate if they possess a held lock. A
byte-range lock request contains the heavyweight information required
to establish a lock and uniquely define the owner of the lock.
The following sections describe the transition from the heavyweight
information to the eventual stateid used for most client and server
locking and lease interactions.
9.1.1. Client ID
For each LOCK request, the client must identify itself to the server.
This is done in such a way as to allow for correct lock
identification and crash recovery. A sequence of a SETCLIENTID
operation followed by a SETCLIENTID_CONFIRM operation is required to
establish the identification onto the server. Establishment of
identification by a new incarnation of the client also has the effect
of immediately breaking any leased state that a previous incarnation
of the client might have had on the server, as opposed to forcing the
new client incarnation to wait for the leases to expire. Breaking
the lease state amounts to the server removing all lock, share
reservation, and, where the server is not supporting the
CLAIM_DELEGATE_PREV claim type, all delegation state associated with
the same client with the same identity. For a discussion of
delegation state recovery, see Section 10.2.1.
Owners of opens and owners of byte-range locks are separate entities
and remain separate even if the same opaque arrays are used to
designate owners of each. The protocol distinguishes between
open-owners (represented by open_owner4 structures) and lock-owners
(represented by lock_owner4 structures).
Both sorts of owners consist of a clientid and an opaque owner
string. For each client, the set of distinct owner values used with
that client constitutes the set of owners of that type, for the given
client.
Each open is associated with a specific open-owner, while each
byte-range lock is associated with a lock-owner and an open-owner,
the latter being the open-owner associated with the open file under
which the LOCK operation was done.
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Client identification is encapsulated in the following structure:
struct nfs_client_id4 {
verifier4 verifier;
opaque id<NFS4_OPAQUE_LIMIT>;
};
The first field, verifier, is a client incarnation verifier that is
used to detect client reboots. Only if the verifier is different
from that which the server has previously recorded for the client (as
identified by the second field of the structure, id) does the server
start the process of canceling the client's leased state.
The second field, id, is a variable-length string that uniquely
defines the client.
There are several considerations for how the client generates the id
string:
o The string should be unique so that multiple clients do not
present the same string. The consequences of two clients
presenting the same string range from one client getting an error
to one client having its leased state abruptly and unexpectedly
canceled.
o The string should be selected so the subsequent incarnations
(e.g., reboots) of the same client cause the client to present the
same string. The implementer is cautioned against an approach
that requires the string to be recorded in a local file because
this precludes the use of the implementation in an environment
where there is no local disk and all file access is from an NFSv4
server.
o The string should be different for each server network address
that the client accesses, rather than common to all server network
addresses. The reason is that it may not be possible for the
client to tell if the same server is listening on multiple network
addresses. If the client issues SETCLIENTID with the same id
string to each network address of such a server, the server will
think it is the same client, and each successive SETCLIENTID will
cause the server to begin the process of removing the client's
previous leased state.
o The algorithm for generating the string should not assume that the
client's network address won't change. This includes changes
between client incarnations and even changes while the client is
still running in its current incarnation. This means that if the
client includes just the client's and server's network address in
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the id string, there is a real risk, after the client gives up the
network address, that another client, using a similar algorithm
for generating the id string, will generate a conflicting id
string.
Given the above considerations, an example of a well-generated id
string is one that includes:
o The server's network address.
o The client's network address.
o For a user-level NFSv4 client, it should contain additional
information to distinguish the client from other user-level
clients running on the same host, such as a universally unique
identifier (UUID).
o Additional information that tends to be unique, such as one or
more of:
* The client machine's serial number (for privacy reasons, it is
best to perform some one-way function on the serial number).
* A MAC address (for privacy reasons, it is best to perform some
one-way function on the MAC address).
* The timestamp of when the NFSv4 software was first installed on
the client (though this is subject to the previously mentioned
caution about using information that is stored in a file,
because the file might only be accessible over NFSv4).
* A true random number. However, since this number ought to be
the same between client incarnations, this shares the same
problem as that of using the timestamp of the software
installation.
As a security measure, the server MUST NOT cancel a client's leased
state if the principal that established the state for a given id
string is not the same as the principal issuing the SETCLIENTID.
Note that SETCLIENTID (Section 16.33) and SETCLIENTID_CONFIRM
(Section 16.34) have a secondary purpose of establishing the
information the server needs to make callbacks to the client for the
purpose of supporting delegations. It is permitted to change this
information via SETCLIENTID and SETCLIENTID_CONFIRM within the same
incarnation of the client without removing the client's leased state.
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Once a SETCLIENTID and SETCLIENTID_CONFIRM sequence has successfully
completed, the client uses the shorthand client identifier, of type
clientid4, instead of the longer and less compact nfs_client_id4
structure. This shorthand client identifier (a client ID) is
assigned by the server and should be chosen so that it will not
conflict with a client ID previously assigned by the server. This
applies across server restarts or reboots. When a client ID is
presented to a server and that client ID is not recognized, as would
happen after a server reboot, the server will reject the request with
the error NFS4ERR_STALE_CLIENTID. When this happens, the client must
obtain a new client ID by use of the SETCLIENTID operation and then
proceed to any other necessary recovery for the server reboot case
(see Section 9.6.2).
The client must also employ the SETCLIENTID operation when it
receives an NFS4ERR_STALE_STATEID error using a stateid derived from
its current client ID, since this also indicates a server reboot,
which has invalidated the existing client ID (see Section 9.6.2 for
details).
See the detailed descriptions of SETCLIENTID (Section 16.33.4) and
SETCLIENTID_CONFIRM (Section 16.34.4) for a complete specification of
the operations.
9.1.2. Server Release of Client ID
If the server determines that the client holds no associated state
for its client ID, the server may choose to release the client ID.
The server may make this choice for an inactive client so that
resources are not consumed by those intermittently active clients.
If the client contacts the server after this release, the server must
ensure that the client receives the appropriate error so that it will
use the SETCLIENTID/SETCLIENTID_CONFIRM sequence to establish a new
identity. It should be clear that the server must be very hesitant
to release a client ID since the resulting work on the client to
recover from such an event will be the same burden as if the server
had failed and restarted. Typically, a server would not release a
client ID unless there had been no activity from that client for many
minutes.
Note that if the id string in a SETCLIENTID request is properly
constructed, and if the client takes care to use the same principal
for each successive use of SETCLIENTID, then, barring an active
denial-of-service attack, NFS4ERR_CLID_INUSE should never be
returned.
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However, client bugs, server bugs, or perhaps a deliberate change of
the principal owner of the id string (such as the case of a client
that changes security flavors, and under the new flavor there is no
mapping to the previous owner) will in rare cases result in
NFS4ERR_CLID_INUSE.
In that event, when the server gets a SETCLIENTID for a client ID
that currently has no state, or it has state but the lease has
expired, rather than returning NFS4ERR_CLID_INUSE, the server MUST
allow the SETCLIENTID and confirm the new client ID if followed by
the appropriate SETCLIENTID_CONFIRM.
9.1.3. Use of Seqids
In several contexts, 32-bit sequence values called "seqids" are used
as part of managing locking state. Such values are used:
o To provide an ordering of locking-related operations associated
with a particular lock-owner or open-owner. See Section 9.1.7 for
a detailed explanation.
o To define an ordered set of instances of a set of locks sharing a
particular set of ownership characteristics. See Section 9.1.4.2
for a detailed explanation.
Successive seqid values for the same object are normally arrived at
by incrementing the current value by one. This pattern continues
until the seqid is incremented past NFS4_UINT32_MAX, in which case
one (rather than zero) is to be the next seqid value.
When two seqid values are to be compared to determine which of the
two is later, the possibility of wraparound needs to be considered.
In many cases, the values are such that simple numeric comparisons
can be used. For example, if the seqid values to be compared are
both less than one million, the higher value can be considered the
later. On the other hand, if one of the values is at or near
NFS_UINT32_MAX and the other is less than one million, then
implementations can reasonably decide that the lower value has had
one more wraparound and is thus, while numerically lower, actually
later.
Implementations can compare seqids in the presence of potential
wraparound by adopting the reasonable assumption that the chain of
increments from one to the other is shorter than 2**31. So, if the
difference between the two seqids is less than 2**31, then the lower
seqid is to be treated as earlier. If, however, the difference
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between the two seqids is greater than or equal to 2**31, then it can
be assumed that the lower seqid has encountered one more wraparound
and can be treated as later.
9.1.4. Stateid Definition
When the server grants a lock of any type (including opens,
byte-range locks, and delegations), it responds with a unique stateid
that represents a set of locks (often a single lock) for the same
file, of the same type, and sharing the same ownership
characteristics. Thus, opens of the same file by different
open-owners each have an identifying stateid. Similarly, each set of
byte-range locks on a file owned by a specific lock-owner has its own
identifying stateid. Delegations also have associated stateids by
which they may be referenced. The stateid is used as a shorthand
reference to a lock or set of locks, and given a stateid, the server
can determine the associated state-owner or state-owners (in the case
of an open-owner/lock-owner pair) and the associated filehandle.
When stateids are used, the current filehandle must be the one
associated with that stateid.
All stateids associated with a given client ID are associated with a
common lease that represents the claim of those stateids and the
objects they represent to be maintained by the server. See
Section 9.5 for a discussion of the lease.
Each stateid must be unique to the server. Many operations take a
stateid as an argument but not a clientid, so the server must be able
to infer the client from the stateid.
9.1.4.1. Stateid Types
With the exception of special stateids (see Section 9.1.4.3), each
stateid represents locking objects of one of a set of types defined
by the NFSv4 protocol. Note that in all these cases, where we speak
of a guarantee, it is understood there are situations such as a
client restart, or lock revocation, that allow the guarantee to be
voided.
o Stateids may represent opens of files.
Each stateid in this case represents the OPEN state for a given
client ID/open-owner/filehandle triple. Such stateids are subject
to change (with consequent incrementing of the stateid's seqid) in
response to OPENs that result in upgrade and OPEN_DOWNGRADE
operations.
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o Stateids may represent sets of byte-range locks.
All locks held on a particular file by a particular owner and all
gotten under the aegis of a particular open file are associated
with a single stateid, with the seqid being incremented whenever
LOCK and LOCKU operations affect that set of locks.
o Stateids may represent file delegations, which are recallable
guarantees by the server to the client that other clients will not
reference, or will not modify, a particular file until the
delegation is returned.
A stateid represents a single delegation held by a client for a
particular filehandle.
9.1.4.2. Stateid Structure
Stateids are divided into two fields: a 96-bit "other" field
identifying the specific set of locks and a 32-bit "seqid" sequence
value. Except in the case of special stateids (see Section 9.1.4.3),
a particular value of the "other" field denotes a set of locks of the
same type (for example, byte-range locks, opens, or delegations), for
a specific file or directory, and sharing the same ownership
characteristics. The seqid designates a specific instance of such a
set of locks, and is incremented to indicate changes in such a set of
locks, by either the addition or deletion of locks from the set, a
change in the byte-range they apply to, or an upgrade or downgrade in
the type of one or more locks.
When such a set of locks is first created, the server returns a
stateid with a seqid value of one. On subsequent operations that
modify the set of locks, the server is required to advance the
seqid field by one whenever it returns a stateid for the same
state-owner/file/type combination and the operation is one that might
make some change in the set of locks actually designated. In this
case, the server will return a stateid with an "other" field the same
as previously used for that state-owner/file/type combination, with
an incremented seqid field.
Seqids will be compared, by both the client and the server. The
client uses such comparisons to determine the order of operations,
while the server uses them to determine whether the
NFS4ERR_OLD_STATEID error is to be returned. In all cases, the
possibility of seqid wraparound needs to be taken into account, as
discussed in Section 9.1.3.
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9.1.4.3. Special Stateids
Stateid values whose "other" field is either all zeros or all ones
are reserved. They MUST NOT be assigned by the server but have
special meanings defined by the protocol. The particular meaning
depends on whether the "other" field is all zeros or all ones and the
specific value of the seqid field.
The following combinations of "other" and seqid are defined in NFSv4:
Anonymous Stateid: When "other" and seqid are both zero, the stateid
is treated as a special anonymous stateid, which can be used in
READ, WRITE, and SETATTR requests to indicate the absence of any
open state associated with the request. When an anonymous stateid
value is used, and an existing open denies the form of access
requested, then access will be denied to the request.
READ Bypass Stateid: When "other" and seqid are both all ones, the
stateid is a special READ bypass stateid. When this value is used
in WRITE or SETATTR, it is treated like the anonymous value. When
used in READ, the server MAY grant access, even if access would
normally be denied to READ requests.
If a stateid value is used that has all zeros or all ones in the
"other" field but does not match one of the cases above, the server
MUST return the error NFS4ERR_BAD_STATEID.
Special stateids, unlike other stateids, are not associated with
individual client IDs or filehandles and can be used with all valid
client IDs and filehandles.
9.1.4.4. Stateid Lifetime and Validation
Stateids must remain valid until either a client restart or a server
restart, or until the client returns all of the locks associated with
the stateid by means of an operation such as CLOSE or DELEGRETURN.
If the locks are lost due to revocation, as long as the client ID is
valid, the stateid remains a valid designation of that revoked state.
Stateids associated with byte-range locks are an exception. They
remain valid even if a LOCKU frees all remaining locks, so long as
the open file with which they are associated remains open.
It should be noted that there are situations in which the client's
locks become invalid, without the client requesting they be returned.
These include lease expiration and a number of forms of lock
revocation within the lease period. It is important to note that in
these situations, the stateid remains valid and the client can use it
to determine the disposition of the associated lost locks.
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An "other" value must never be reused for a different purpose (i.e.,
different filehandle, owner, or type of locks) within the context of
a single client ID. A server may retain the "other" value for the
same purpose beyond the point where it may otherwise be freed, but if
it does so, it must maintain seqid continuity with previous values.
One mechanism that may be used to satisfy the requirement that the
server recognize invalid and out-of-date stateids is for the server
to divide the "other" field of the stateid into two fields:
o An index into a table of locking-state structures.
o A generation number that is incremented on each allocation of a
table entry for a particular use.
And then store the following in each table entry:
o The client ID with which the stateid is associated.
o The current generation number for the (at most one) valid stateid
sharing this index value.
o The filehandle of the file on which the locks are taken.
o An indication of the type of stateid (open, byte-range lock, file
delegation).
o The last seqid value returned corresponding to the current "other"
value.
o An indication of the current status of the locks associated with
this stateid -- in particular, whether these have been revoked
and, if so, for what reason.
With this information, an incoming stateid can be validated and the
appropriate error returned when necessary. Special and non-special
stateids are handled separately. (See Section 9.1.4.3 for a
discussion of special stateids.)
When a stateid is being tested, and the "other" field is all zeros or
all ones, a check that the "other" and seqid fields match a defined
combination for a special stateid is done and the results determined
as follows:
o If the "other" and seqid fields do not match a defined combination
associated with a special stateid, the error NFS4ERR_BAD_STATEID
is returned.
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o If the combination is valid in general but is not appropriate to
the context in which the stateid is used (e.g., an all-zero
stateid is used when an open stateid is required in a LOCK
operation), the error NFS4ERR_BAD_STATEID is also returned.
o Otherwise, the check is completed and the special stateid is
accepted as valid.
When a stateid is being tested, and the "other" field is neither all
zeros nor all ones, the following procedure could be used to validate
an incoming stateid and return an appropriate error, when necessary,
assuming that the "other" field would be divided into a table index
and an entry generation. Note that the terms "earlier" and "later"
used in connection with seqid comparison are to be understood as
explained in Section 9.1.3.
o If the table index field is outside the range of the associated
table, return NFS4ERR_BAD_STATEID.
o If the selected table entry is of a different generation than that
specified in the incoming stateid, return NFS4ERR_BAD_STATEID.
o If the selected table entry does not match the current filehandle,
return NFS4ERR_BAD_STATEID.
o If the stateid represents revoked state or state lost as a result
of lease expiration, then return NFS4ERR_EXPIRED,
NFS4ERR_BAD_STATEID, or NFS4ERR_ADMIN_REVOKED, as appropriate.
o If the stateid type is not valid for the context in which the
stateid appears, return NFS4ERR_BAD_STATEID. Note that a stateid
may be valid in general but invalid for a particular operation,
as, for example, when a stateid that doesn't represent byte-range
locks is passed to the non-from_open case of LOCK or to LOCKU, or
when a stateid that does not represent an open is passed to CLOSE
or OPEN_DOWNGRADE. In such cases, the server MUST return
NFS4ERR_BAD_STATEID.
o If the seqid field is not zero and it is later than the current
sequence value corresponding to the current "other" field, return
NFS4ERR_BAD_STATEID.
o If the seqid field is earlier than the current sequence value
corresponding to the current "other" field, return
NFS4ERR_OLD_STATEID.
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o Otherwise, the stateid is valid, and the table entry should
contain any additional information about the type of stateid and
information associated with that particular type of stateid, such
as the associated set of locks (e.g., open-owner and lock-owner
information), as well as information on the specific locks
themselves, such as open modes and byte ranges.
9.1.4.5. Stateid Use for I/O Operations
Clients performing Input/Output (I/O) operations need to select an
appropriate stateid based on the locks (including opens and
delegations) held by the client and the various types of state-owners
sending the I/O requests. SETATTR operations that change the file
size are treated like I/O operations in this regard.
The following rules, applied in order of decreasing priority, govern
the selection of the appropriate stateid. In following these rules,
the client will only consider locks of which it has actually received
notification by an appropriate operation response or callback.
o If the client holds a delegation for the file in question, the
delegation stateid SHOULD be used.
o Otherwise, if the entity corresponding to the lock-owner (e.g., a
process) sending the I/O has a byte-range lock stateid for the
associated open file, then the byte-range lock stateid for that
lock-owner and open file SHOULD be used.
o If there is no byte-range lock stateid, then the OPEN stateid for
the current open-owner, i.e., the OPEN stateid for the open file
in question, SHOULD be used.
o Finally, if none of the above apply, then a special stateid SHOULD
be used.
Ignoring these rules may result in situations in which the server
does not have information necessary to properly process the request.
For example, when mandatory byte-range locks are in effect, if the
stateid does not indicate the proper lock-owner, via a lock stateid,
a request might be avoidably rejected.
The server, however, should not try to enforce these ordering rules
and should use whatever information is available to properly process
I/O requests. In particular, when a client has a delegation for a
given file, it SHOULD take note of this fact in processing a request,
even if it is sent with a special stateid.
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9.1.4.6. Stateid Use for SETATTR Operations
In the case of SETATTR operations, a stateid is present. In cases
other than those that set the file size, the client may send either a
special stateid or, when a delegation is held for the file in
question, a delegation stateid. While the server SHOULD validate the
stateid and may use the stateid to optimize the determination as to
whether a delegation is held, it SHOULD note the presence of a
delegation even when a special stateid is sent, and MUST accept a
valid delegation stateid when sent.
9.1.5. Lock-Owner
When requesting a lock, the client must present to the server the
client ID and an identifier for the owner of the requested lock.
These two fields comprise the lock-owner and are defined as follows:
o A client ID returned by the server as part of the client's use of
the SETCLIENTID operation.
o A variable-length opaque array used to uniquely define the owner
of a lock managed by the client.
This may be a thread id, process id, or other unique value.
When the server grants the lock, it responds with a unique stateid.
The stateid is used as a shorthand reference to the lock-owner, since
the server will be maintaining the correspondence between them.
9.1.6. Use of the Stateid and Locking
All READ, WRITE, and SETATTR operations contain a stateid. For the
purposes of this section, SETATTR operations that change the size
attribute of a file are treated as if they are writing the area
between the old and new size (i.e., the range truncated or added to
the file by means of the SETATTR), even where SETATTR is not
explicitly mentioned in the text. The stateid passed to one of these
operations must be one that represents an OPEN (e.g., via the
open-owner), a set of byte-range locks, or a delegation, or it may be
a special stateid representing anonymous access or the READ bypass
stateid.
If the state-owner performs a READ or WRITE in a situation in which
it has established a lock or share reservation on the server (any
OPEN constitutes a share reservation), the stateid (previously
returned by the server) must be used to indicate what locks,
including both byte-range locks and share reservations, are held by
the state-owner. If no state is established by the client -- either
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byte-range lock or share reservation -- the anonymous stateid is
used. Regardless of whether an anonymous stateid or a stateid
returned by the server is used, if there is a conflicting share
reservation or mandatory byte-range lock held on the file, the server
MUST refuse to service the READ or WRITE operation.
Share reservations are established by OPEN operations and by their
nature are mandatory in that when the OPEN denies READ or WRITE
operations, that denial results in such operations being rejected
with error NFS4ERR_LOCKED. Byte-range locks may be implemented by
the server as either mandatory or advisory, or the choice of
mandatory or advisory behavior may be determined by the server on the
basis of the file being accessed (for example, some UNIX-based
servers support a "mandatory lock bit" on the mode attribute such
that if set, byte-range locks are required on the file before I/O is
possible). When byte-range locks are advisory, they only prevent the
granting of conflicting lock requests and have no effect on READs or
WRITEs. Mandatory byte-range locks, however, prevent conflicting I/O
operations. When they are attempted, they are rejected with
NFS4ERR_LOCKED. When the client gets NFS4ERR_LOCKED on a file it
knows it has the proper share reservation for, it will need to issue
a LOCK request on the region of the file that includes the region the
I/O was to be performed on, with an appropriate locktype (i.e.,
READ*_LT for a READ operation, WRITE*_LT for a WRITE operation).
With NFSv3, there was no notion of a stateid, so there was no way to
tell if the application process of the client sending the READ or
WRITE operation had also acquired the appropriate byte-range lock on
the file. Thus, there was no way to implement mandatory locking.
With the stateid construct, this barrier has been removed.
Note that for UNIX environments that support mandatory file locking,
the distinction between advisory and mandatory locking is subtle. In
fact, advisory and mandatory byte-range locks are exactly the same
insofar as the APIs and requirements on implementation are concerned.
If the mandatory lock attribute is set on the file, the server checks
to see if the lock-owner has an appropriate shared (read) or
exclusive (write) byte-range lock on the region it wishes to read or
write to. If there is no appropriate lock, the server checks if
there is a conflicting lock (which can be done by attempting to
acquire the conflicting lock on behalf of the lock-owner and, if
successful, release the lock after the READ or WRITE is done), and if
there is, the server returns NFS4ERR_LOCKED.
For Windows environments, there are no advisory byte-range locks, so
the server always checks for byte-range locks during I/O requests.
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Thus, the NFSv4 LOCK operation does not need to distinguish between
advisory and mandatory byte-range locks. It is the NFSv4 server's
processing of the READ and WRITE operations that introduces the
distinction.
Every stateid other than the special stateid values noted in this
section, whether returned by an OPEN-type operation (i.e., OPEN,
OPEN_DOWNGRADE) or by a LOCK-type operation (i.e., LOCK or LOCKU),
defines an access mode for the file (i.e., READ, WRITE, or
READ-WRITE) as established by the original OPEN that began the
stateid sequence, and as modified by subsequent OPENs and
OPEN_DOWNGRADEs within that stateid sequence. When a READ, WRITE, or
SETATTR that specifies the size attribute is done, the operation is
subject to checking against the access mode to verify that the
operation is appropriate given the OPEN with which the operation is
associated.
In the case of WRITE-type operations (i.e., WRITEs and SETATTRs that
set size), the server must verify that the access mode allows writing
and return an NFS4ERR_OPENMODE error if it does not. In the case of
READ, the server may perform the corresponding check on the access
mode, or it may choose to allow READ on opens for WRITE only, to
accommodate clients whose write implementation may unavoidably do
reads (e.g., due to buffer cache constraints). However, even if
READs are allowed in these circumstances, the server MUST still check
for locks that conflict with the READ (e.g., another open specifying
denial of READs). Note that a server that does enforce the access
mode check on READs need not explicitly check for conflicting share
reservations since the existence of OPEN for read access guarantees
that no conflicting share reservation can exist.
A READ bypass stateid MAY allow READ operations to bypass locking
checks at the server. However, WRITE operations with a READ bypass
stateid MUST NOT bypass locking checks and are treated exactly the
same as if an anonymous stateid were used.
A lock may not be granted while a READ or WRITE operation using one
of the special stateids is being performed and the range of the lock
request conflicts with the range of the READ or WRITE operation. For
the purposes of this paragraph, a conflict occurs when a shared lock
is requested and a WRITE operation is being performed, or an
exclusive lock is requested and either a READ or a WRITE operation is
being performed. A SETATTR that sets size is treated similarly to a
WRITE as discussed above.
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9.1.7. Sequencing of Lock Requests
Locking is different than most NFS operations as it requires
"at-most-one" semantics that are not provided by ONC RPC. ONC RPC
over a reliable transport is not sufficient because a sequence of
locking requests may span multiple TCP connections. In the face of
retransmission or reordering, lock or unlock requests must have a
well-defined and consistent behavior. To accomplish this, each lock
request contains a sequence number that is a consecutively increasing
integer. Different state-owners have different sequences. The
server maintains the last sequence number (L) received and the
response that was returned. The server SHOULD assign a seqid value
of one for the first request issued for any given state-owner.
Subsequent values are arrived at by incrementing the seqid value,
subject to wraparound as described in Section 9.1.3.
Note that for requests that contain a sequence number, for each
state-owner, there should be no more than one outstanding request.
When a request is received, its sequence number (r) is compared to
that of the last one received (L). Only if it has the correct next
sequence, normally L + 1, is the request processed beyond the point
of seqid checking. Given a properly functioning client, the response
to (r) must have been received before the last request (L) was sent.
If a duplicate of last request (r == L) is received, the stored
response is returned. If the sequence value received is any other
value, it is rejected with the return of error NFS4ERR_BAD_SEQID.
Sequence history is reinitialized whenever the SETCLIENTID/
SETCLIENTID_CONFIRM sequence changes the client verifier.
It is critical that the server maintain the last response sent to the
client to provide a more reliable cache of duplicate non-idempotent
requests than that of the traditional cache described in [Chet]. The
traditional duplicate request cache uses a least recently used
algorithm for removing unneeded requests. However, the last lock
request and response on a given state-owner must be cached as long as
the lock state exists on the server.
The client MUST advance the sequence number for the CLOSE, LOCK,
LOCKU, OPEN, OPEN_CONFIRM, and OPEN_DOWNGRADE operations. This is
true even in the event that the previous operation that used the
sequence number received an error. The only exception to this rule
is if the previous operation received one of the following errors:
NFS4ERR_STALE_CLIENTID, NFS4ERR_STALE_STATEID, NFS4ERR_BAD_STATEID,
NFS4ERR_BAD_SEQID, NFS4ERR_BADXDR, NFS4ERR_RESOURCE,
NFS4ERR_NOFILEHANDLE, or NFS4ERR_MOVED.
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9.1.8. Recovery from Replayed Requests
As described above, the sequence number is per state-owner. As long
as the server maintains the last sequence number received and follows
the methods described above, there are no risks of a Byzantine router
re-sending old requests. The server need only maintain the
(state-owner, sequence number) state as long as there are open files
or closed files with locks outstanding.
LOCK, LOCKU, OPEN, OPEN_DOWNGRADE, and CLOSE each contain a sequence
number, and therefore the risk of the replay of these operations
resulting in undesired effects is non-existent while the server
maintains the state-owner state.
9.1.9. Interactions of Multiple Sequence Values
Some operations may have multiple sources of data for request
sequence checking and retransmission determination. Some operations
have multiple sequence values associated with multiple types of
state-owners. In addition, such operations may also have a stateid
with its own seqid value, that will be checked for validity.
As noted above, there may be multiple sequence values to check. The
following rules should be followed by the server in processing these
multiple sequence values within a single operation.
o When a sequence value associated with a state-owner is unavailable
for checking because the state-owner is unknown to the server, it
takes no part in the comparison.
o When any of the state-owner sequence values are invalid,
NFS4ERR_BAD_SEQID is returned. When a stateid sequence is
checked, NFS4ERR_BAD_STATEID or NFS4ERR_OLD_STATEID is returned as
appropriate, but NFS4ERR_BAD_SEQID has priority.
o When any one of the sequence values matches a previous request,
for a state-owner, it is treated as a retransmission and not
re-executed. When the type of the operation does not match that
originally used, NFS4ERR_BAD_SEQID is returned. When the server
can determine that the request differs from the original, it may
return NFS4ERR_BAD_SEQID.
o When multiple sequence values match previous operations but the
operations are not the same, NFS4ERR_BAD_SEQID is returned.
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o When there are no sequence values available for comparison and the
operation is an OPEN, the server indicates to the client that an
OPEN_CONFIRM is required, unless it can conclusively determine
that confirmation is not required (e.g., by knowing that no
open-owner state has ever been released for the current clientid).
9.1.10. Releasing State-Owner State
When a particular state-owner no longer holds open or file locking
state at the server, the server may choose to release the sequence
number state associated with the state-owner. The server may make
this choice based on lease expiration, the reclamation of server
memory, or other implementation-specific details. Note that when
this is done, a retransmitted request, normally identified by a
matching state-owner sequence, may not be correctly recognized, so
that the client will not receive the original response that it would
have if the state-owner state was not released.
If the server were able to be sure that a given state-owner would
never again be used by a client, such an issue could not arise. Even
when the state-owner state is released and the client subsequently
uses that state-owner, retransmitted requests will be detected as
invalid and the request not executed, although the client may have a
recovery path that is more complicated than simply getting the
original response back transparently.
In any event, the server is able to safely release state-owner state
(in the sense that retransmitted requests will not be erroneously
acted upon) when the state-owner is not currently being utilized by
the client (i.e., there are no open files associated with an
open-owner and no lock stateids associated with a lock-owner). The
server may choose to hold the state-owner state in order to simplify
the recovery path, in the case in which retransmissions of currently
active requests are received. However, the period for which it
chooses to hold this state is implementation specific.
In the case that a LOCK, LOCKU, OPEN_DOWNGRADE, or CLOSE is
retransmitted after the server has previously released the
state-owner state, the server will find that the state-owner has no
files open and an error will be returned to the client. If the
state-owner does have a file open, the stateid will not match and
again an error is returned to the client.
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9.1.11. Use of Open Confirmation
In the case that an OPEN is retransmitted and the open-owner is being
used for the first time or the open-owner state has been previously
released by the server, the use of the OPEN_CONFIRM operation will
prevent incorrect behavior. When the server observes the use of the
open-owner for the first time, it will direct the client to perform
the OPEN_CONFIRM for the corresponding OPEN. This sequence
establishes the use of an open-owner and associated sequence number.
Since the OPEN_CONFIRM sequence connects a new open-owner on the
server with an existing open-owner on a client, the sequence number
may have any valid (i.e., non-zero) value. The OPEN_CONFIRM step
assures the server that the value received is the correct one. (See
Section 16.18 for further details.)
There are a number of situations in which the requirement to confirm
an OPEN would pose difficulties for the client and server, in that
they would be prevented from acting in a timely fashion on
information received, because that information would be provisional,
subject to deletion upon non-confirmation. Fortunately, these are
situations in which the server can avoid the need for confirmation
when responding to open requests. The two constraints are:
o The server must not bestow a delegation for any open that would
require confirmation.
o The server MUST NOT require confirmation on a reclaim-type open
(i.e., one specifying claim type CLAIM_PREVIOUS or
CLAIM_DELEGATE_PREV).
These constraints are related in that reclaim-type opens are the only
ones in which the server may be required to send a delegation. For
CLAIM_NULL, sending the delegation is optional, while for
CLAIM_DELEGATE_CUR, no delegation is sent.
Delegations being sent with an open requiring confirmation are
troublesome because recovering from non-confirmation adds undue
complexity to the protocol, while requiring confirmation on reclaim-
type opens poses difficulties in that the inability to resolve the
status of the reclaim until lease expiration may make it difficult to
have timely determination of the set of locks being reclaimed (since
the grace period may expire).
Requiring open confirmation on reclaim-type opens is avoidable
because of the nature of the environments in which such opens are
done. For CLAIM_PREVIOUS opens, this is immediately after server
reboot, so there should be no time for open-owners to be created,
found to be unused, and recycled. For CLAIM_DELEGATE_PREV opens,
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we are dealing with either a client reboot situation or a network
partition resulting in deletion of lease state (and returning
NFS4ERR_EXPIRED). A server that supports delegations can be sure
that no open-owners for that client have been recycled since client
initialization or deletion of lease state and thus can be confident
that confirmation will not be required.
9.2. Lock Ranges
The protocol allows a lock-owner to request a lock with a byte range
and then either upgrade or unlock a sub-range of the initial lock.
It is expected that this will be an uncommon type of request. In any
case, servers or server file systems may not be able to support
sub-range lock semantics. In the event that a server receives a
locking request that represents a sub-range of current locking state
for the lock-owner, the server is allowed to return the error
NFS4ERR_LOCK_RANGE to signify that it does not support sub-range lock
operations. Therefore, the client should be prepared to receive this
error and, if appropriate, report the error to the requesting
application.
The client is discouraged from combining multiple independent locking
ranges that happen to be adjacent into a single request, since the
server may not support sub-range requests, and for reasons related to
the recovery of file locking state in the event of server failure.
As discussed in Section 9.6.2 below, the server may employ certain
optimizations during recovery that work effectively only when the
client's behavior during lock recovery is similar to the client's
locking behavior prior to server failure.
9.3. Upgrading and Downgrading Locks
If a client has a write lock on a record, it can request an atomic
downgrade of the lock to a read lock via the LOCK request, by setting
the type to READ_LT. If the server supports atomic downgrade, the
request will succeed. If not, it will return NFS4ERR_LOCK_NOTSUPP.
The client should be prepared to receive this error and, if
appropriate, report the error to the requesting application.
If a client has a read lock on a record, it can request an atomic
upgrade of the lock to a write lock via the LOCK request by setting
the type to WRITE_LT or WRITEW_LT. If the server does not support
atomic upgrade, it will return NFS4ERR_LOCK_NOTSUPP. If the upgrade
can be achieved without an existing conflict, the request will
succeed. Otherwise, the server will return either NFS4ERR_DENIED or
NFS4ERR_DEADLOCK. The error NFS4ERR_DEADLOCK is returned if the
client issued the LOCK request with the type set to WRITEW_LT and the
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server has detected a deadlock. The client should be prepared to
receive such errors and, if appropriate, report them to the
requesting application.
9.4. Blocking Locks
Some clients require the support of blocking locks. The NFSv4
protocol must not rely on a callback mechanism and therefore is
unable to notify a client when a previously denied lock has been
granted. Clients have no choice but to continually poll for the
lock. This presents a fairness problem. Two new lock types are
added, READW and WRITEW, and are used to indicate to the server that
the client is requesting a blocking lock. The server should maintain
an ordered list of pending blocking locks. When the conflicting lock
is released, the server may wait the lease period for the first
waiting client to re-request the lock. After the lease period
expires, the next waiting client request is allowed the lock.
Clients are required to poll at an interval sufficiently small that
it is likely to acquire the lock in a timely manner. The server is
not required to maintain a list of pending blocked locks, as it is
not used to provide correct operation but only to increase fairness.
Because of the unordered nature of crash recovery, storing of lock
state to stable storage would be required to guarantee ordered
granting of blocking locks.
Servers may also note the lock types and delay returning denial of
the request to allow extra time for a conflicting lock to be
released, allowing a successful return. In this way, clients can
avoid the burden of needlessly frequent polling for blocking locks.
The server should take care with the length of delay in the event
that the client retransmits the request.
If a server receives a blocking lock request, denies it, and then
later receives a non-blocking request for the same lock, which is
also denied, then it should remove the lock in question from its list
of pending blocking locks. Clients should use such a non-blocking
request to indicate to the server that this is the last time they
intend to poll for the lock, as may happen when the process
requesting the lock is interrupted. This is a courtesy to the
server, to prevent it from unnecessarily waiting a lease period
before granting other lock requests. However, clients are not
required to perform this courtesy, and servers must not depend on
them doing so. Also, clients must be prepared for the possibility
that this final locking request will be accepted.
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9.5. Lease Renewal
The purpose of a lease is to allow a server to remove stale locks
that are held by a client that has crashed or is otherwise
unreachable. It is not a mechanism for cache consistency, and lease
renewals may not be denied if the lease interval has not expired.
The client can implicitly provide a positive indication that it is
still active and that the associated state held at the server, for
the client, is still valid. Any operation made with a valid clientid
(DELEGPURGE, LOCK, LOCKT, OPEN, RELEASE_LOCKOWNER, or RENEW) or a
valid stateid (CLOSE, DELEGRETURN, LOCK, LOCKU, OPEN, OPEN_CONFIRM,
OPEN_DOWNGRADE, READ, SETATTR, or WRITE) informs the server to renew
all of the leases for that client (i.e., all those sharing a given
client ID). In the latter case, the stateid must not be one of the
special stateids (anonymous stateid or READ bypass stateid).
Note that if the client had restarted or rebooted, the client would
not be making these requests without issuing the SETCLIENTID/
SETCLIENTID_CONFIRM sequence. The use of the SETCLIENTID/
SETCLIENTID_CONFIRM sequence (one that changes the client verifier)
notifies the server to drop the locking state associated with the
client. SETCLIENTID/SETCLIENTID_CONFIRM never renews a lease.
If the server has rebooted, the stateids (NFS4ERR_STALE_STATEID
error) or the client ID (NFS4ERR_STALE_CLIENTID error) will not be
valid, hence preventing spurious renewals.
This approach allows for low-overhead lease renewal, which scales
well. In the typical case, no extra RPCs are required for lease
renewal, and in the worst case, one RPC is required every lease
period (i.e., a RENEW operation). The number of locks held by the
client is not a factor since all state for the client is involved
with the lease renewal action.
Since all operations that create a new lease also renew existing
leases, the server must maintain a common lease expiration time for
all valid leases for a given client. This lease time can then be
easily updated upon implicit lease renewal actions.
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9.6. Crash Recovery
The important requirement in crash recovery is that both the client
and the server know when the other has failed. Additionally, it is
required that a client sees a consistent view of data across server
restarts or reboots. All READ and WRITE operations that may have
been queued within the client or network buffers must wait until the
client has successfully recovered the locks protecting the READ and
WRITE operations.
9.6.1. Client Failure and Recovery
In the event that a client fails, the server may recover the client's
locks when the associated leases have expired. Conflicting locks
from another client may only be granted after this lease expiration.
If the client is able to restart or reinitialize within the lease
period, the client may be forced to wait the remainder of the lease
period before obtaining new locks.
To minimize client delay upon restart, open and lock requests are
associated with an instance of the client by a client-supplied
verifier. This verifier is part of the initial SETCLIENTID call made
by the client. The server returns a client ID as a result of the
SETCLIENTID operation. The client then confirms the use of the
client ID with SETCLIENTID_CONFIRM. The client ID in combination
with an opaque owner field is then used by the client to identify the
open-owner for OPEN. This chain of associations is then used to
identify all locks for a particular client.
Since the verifier will be changed by the client upon each
initialization, the server can compare a new verifier to the verifier
associated with currently held locks and determine that they do not
match. This signifies the client's new instantiation and subsequent
loss of locking state. As a result, the server is free to release
all locks held that are associated with the old client ID that was
derived from the old verifier.
Note that the verifier must have the same uniqueness properties of
the verifier for the COMMIT operation.
9.6.2. Server Failure and Recovery
If the server loses locking state (usually as a result of a restart
or reboot), it must allow clients time to discover this fact and
re-establish the lost locking state. The client must be able to
re-establish the locking state without having the server deny valid
requests because the server has granted conflicting access to another
client. Likewise, if there is the possibility that clients have
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not yet re-established their locking state for a file, the server
must disallow READ and WRITE operations for that file. The duration
of this recovery period is equal to the duration of the lease period.
A client can determine that server failure (and thus loss of locking
state) has occurred, when it receives one of two errors. The
NFS4ERR_STALE_STATEID error indicates a stateid invalidated by a
reboot or restart. The NFS4ERR_STALE_CLIENTID error indicates a
client ID invalidated by reboot or restart. When either of these is
received, the client must establish a new client ID (see
Section 9.1.1) and re-establish the locking state as discussed below.
The period of special handling of locking and READs and WRITEs, equal
in duration to the lease period, is referred to as the "grace
period". During the grace period, clients recover locks and the
associated state by reclaim-type locking requests (i.e., LOCK
requests with reclaim set to TRUE and OPEN operations with a claim
type of either CLAIM_PREVIOUS or CLAIM_DELEGATE_PREV). During the
grace period, the server must reject READ and WRITE operations and
non-reclaim locking requests (i.e., other LOCK and OPEN operations)
with an error of NFS4ERR_GRACE.
If the server can reliably determine that granting a non-reclaim
request will not conflict with reclamation of locks by other clients,
the NFS4ERR_GRACE error does not have to be returned and the
non-reclaim client request can be serviced. For the server to be
able to service READ and WRITE operations during the grace period, it
must again be able to guarantee that no possible conflict could arise
between an impending reclaim locking request and the READ or WRITE
operation. If the server is unable to offer that guarantee, the
NFS4ERR_GRACE error must be returned to the client.
For a server to provide simple, valid handling during the grace
period, the easiest method is to simply reject all non-reclaim
locking requests and READ and WRITE operations by returning the
NFS4ERR_GRACE error. However, a server may keep information about
granted locks in stable storage. With this information, the server
could determine if a regular lock or READ or WRITE operation can be
safely processed.
For example, if a count of locks on a given file is available in
stable storage, the server can track reclaimed locks for the file,
and when all reclaims have been processed, non-reclaim locking
requests may be processed. This way, the server can ensure that
non-reclaim locking requests will not conflict with potential reclaim
requests. With respect to I/O requests, if the server is able to
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determine that there are no outstanding reclaim requests for a file
by information from stable storage or another similar mechanism, the
processing of I/O requests could proceed normally for the file.
To reiterate, for a server that allows non-reclaim lock and I/O
requests to be processed during the grace period, it MUST determine
that no lock subsequently reclaimed will be rejected and that no lock
subsequently reclaimed would have prevented any I/O operation
processed during the grace period.
Clients should be prepared for the return of NFS4ERR_GRACE errors for
non-reclaim lock and I/O requests. In this case, the client should
employ a retry mechanism for the request. A delay (on the order of
several seconds) between retries should be used to avoid overwhelming
the server. Further discussion of the general issue is included in
[Floyd]. The client must account for the server that is able to
perform I/O and non-reclaim locking requests within the grace period
as well as those that cannot do so.
A reclaim-type locking request outside the server's grace period can
only succeed if the server can guarantee that no conflicting lock or
I/O request has been granted since reboot or restart.
A server may, upon restart, establish a new value for the lease
period. Therefore, clients should, once a new client ID is
established, refetch the lease_time attribute and use it as the basis
for lease renewal for the lease associated with that server.
However, the server must establish, for this restart event, a grace
period at least as long as the lease period for the previous server
instantiation. This allows the client state obtained during the
previous server instance to be reliably re-established.
9.6.3. Network Partitions and Recovery
If the duration of a network partition is greater than the lease
period provided by the server, the server will have not received a
lease renewal from the client. If this occurs, the server may cancel
the lease and free all locks held for the client. As a result, all
stateids held by the client will become invalid or stale. Once the
client is able to reach the server after such a network partition,
all I/O submitted by the client with the now invalid stateids will
fail with the server returning the error NFS4ERR_EXPIRED. Once this
error is received, the client will suitably notify the application
that held the lock.
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9.6.3.1. Courtesy Locks
As a courtesy to the client or as an optimization, the server may
continue to hold locks, including delegations, on behalf of a client
for which recent communication has extended beyond the lease period,
delaying the cancellation of the lease. If the server receives a
lock or I/O request that conflicts with one of these courtesy locks
or if it runs out of resources, the server MAY cause lease
cancellation to occur at that time and henceforth return
NFS4ERR_EXPIRED when any of the stateids associated with the freed
locks is used. If lease cancellation has not occurred and the server
receives a lock or I/O request that conflicts with one of the
courtesy locks, the requirements are as follows:
o In the case of a courtesy lock that is not a delegation, it MUST
free the courtesy lock and grant the new request.
o In the case of a lock or an I/O request that conflicts with a
delegation that is being held as a courtesy lock, the server MAY
delay resolution of the request but MUST NOT reject the request
and MUST free the delegation and grant the new request eventually.
o In the case of a request for a delegation that conflicts with a
delegation that is being held as a courtesy lock, the server MAY
grant the new request or not as it chooses, but if it grants the
conflicting request, the delegation held as a courtesy lock MUST
be freed.
If the server does not reboot or cancel the lease before the network
partition is healed, when the original client tries to access a
courtesy lock that was freed, the server SHOULD send back an
NFS4ERR_BAD_STATEID to the client. If the client tries to access a
courtesy lock that was not freed, then the server SHOULD mark all of
the courtesy locks as implicitly being renewed.
9.6.3.2. Lease Cancellation
As a result of lease expiration, leases may be canceled, either
immediately upon expiration or subsequently, depending on the
occurrence of a conflicting lock or extension of the period of
partition beyond what the server will tolerate.
When a lease is canceled, all locking state associated with it is
freed, and the use of any of the associated stateids will result in
NFS4ERR_EXPIRED being returned. Similarly, the use of the associated
clientid will result in NFS4ERR_EXPIRED being returned.
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The client should recover from this situation by using SETCLIENTID
followed by SETCLIENTID_CONFIRM, in order to establish a new
clientid. Once a lock is obtained using this clientid, a lease will
be established.
9.6.3.3. Client's Reaction to a Freed Lock
There is no way for a client to predetermine how a given server is
going to behave during a network partition. When the partition
heals, the client still has either all of its locks, some of its
locks, or none of them. The client will be able to examine the
various error return values to determine its response.
NFS4ERR_EXPIRED:
All locks have been freed as a result of a lease cancellation that
occurred during the partition. The client should use a
SETCLIENTID to recover.
NFS4ERR_ADMIN_REVOKED:
The current lock has been revoked before, during, or after the
partition. The client SHOULD handle this error as it normally
would.
NFS4ERR_BAD_STATEID:
The current lock has been revoked/released during the partition,
and the server did not reboot. Other locks MAY still be renewed.
The client need not do a SETCLIENTID and instead SHOULD probe via
a RENEW call.
NFS4ERR_RECLAIM_BAD:
The current lock has been revoked during the partition, and the
server rebooted. The server might have no information on the
other locks. They may still be renewable.
NFS4ERR_NO_GRACE:
The client's locks have been revoked during the partition, and the
server rebooted. None of the client's locks will be renewable.
NFS4ERR_OLD_STATEID:
The server has not rebooted. The client SHOULD handle this error
as it normally would.
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9.6.3.4. Edge Conditions
When a network partition is combined with a server reboot, then both
the server and client have responsibilities to ensure that the client
does not reclaim a lock that it should no longer be able to access.
Briefly, those are:
o Client's responsibility: A client MUST NOT attempt to reclaim any
locks that it did not hold at the end of its most recent
successfully established client lease.
o Server's responsibility: A server MUST NOT allow a client to
reclaim a lock unless it knows that it could not have since
granted a conflicting lock. However, in deciding whether a
conflicting lock could have been granted, it is permitted to
assume that its clients are responsible, as above.
A server may consider a client's lease "successfully established"
once it has received an OPEN operation from that client.
The above are directed to CLAIM_PREVIOUS reclaims and not to
CLAIM_DELEGATE_PREV reclaims, which generally do not involve a server
reboot. However, when a server persistently stores delegation
information to support CLAIM_DELEGATE_PREV across a period in which
both client and server are down at the same time, similar strictures
apply.
The next sections give examples showing what can go wrong if these
responsibilities are neglected and also provide examples of server
implementation strategies that could meet a server's
responsibilities.
9.6.3.4.1. First Server Edge Condition
The first edge condition has the following scenario:
1. Client A acquires a lock.
2. Client A and the server experience mutual network partition, such
that client A is unable to renew its lease.
3. Client A's lease expires, so the server releases the lock.
4. Client B acquires a lock that would have conflicted with that of
client A.
5. Client B releases the lock.
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6. The server reboots.
7. The network partition between client A and the server heals.
8. Client A issues a RENEW operation and gets back an
NFS4ERR_STALE_CLIENTID.
9. Client A reclaims its lock within the server's grace period.
Thus, at the final step, the server has erroneously granted
client A's lock reclaim. If client B modified the object the lock
was protecting, client A will experience object corruption.
9.6.3.4.2. Second Server Edge Condition
The second known edge condition follows:
1. Client A acquires a lock.
2. The server reboots.
3. Client A and the server experience mutual network partition,
such that client A is unable to reclaim its lock within the
grace period.
4. The server's reclaim grace period ends. Client A has no locks
recorded on the server.
5. Client B acquires a lock that would have conflicted with that of
client A.
6. Client B releases the lock.
7. The server reboots a second time.
8. The network partition between client A and the server heals.
9. Client A issues a RENEW operation and gets back an
NFS4ERR_STALE_CLIENTID.
10. Client A reclaims its lock within the server's grace period.
As with the first edge condition, the final step of the scenario of
the second edge condition has the server erroneously granting
client A's lock reclaim.
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9.6.3.4.3. Handling Server Edge Conditions
In both of the above examples, the client attempts reclaim of a lock
that it held at the end of its most recent successfully established
lease; thus, it has fulfilled its responsibility.
The server, however, has failed, by granting a reclaim, despite
having granted a conflicting lock since the reclaimed lock was last
held.
Solving these edge conditions requires that the server either (1)
assume after it reboots that an edge condition occurs, and thus
return NFS4ERR_NO_GRACE for all reclaim attempts, or (2) record some
information in stable storage. The amount of information the server
records in stable storage is in inverse proportion to how harsh the
server wants to be whenever the edge conditions occur. The server
that is completely tolerant of all edge conditions will record in
stable storage every lock that is acquired, removing the lock record
from stable storage only when the lock is unlocked by the client and
the lock's owner advances the sequence number such that the lock
release is not the last stateful event for the owner's sequence. For
the two aforementioned edge conditions, the harshest a server can be,
and still support a grace period for reclaims, requires that the
server record in stable storage some minimal information. For
example, a server implementation could, for each client, save in
stable storage a record containing:
o the client's id string.
o a boolean that indicates if the client's lease expired or if there
was administrative intervention (see Section 9.8) to revoke a
byte-range lock, share reservation, or delegation.
o a timestamp that is updated the first time after a server boot or
reboot the client acquires byte-range locking, share reservation,
or delegation state on the server. The timestamp need not be
updated on subsequent lock requests until the server reboots.
The server implementation would also record in stable storage the
timestamps from the two most recent server reboots.
Assuming the above record keeping, for the first edge condition,
after the server reboots, the record that client A's lease expired
means that another client could have acquired a conflicting record
lock, share reservation, or delegation. Hence, the server must
reject a reclaim from client A with the error NFS4ERR_NO_GRACE or
NFS4ERR_RECLAIM_BAD.
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For the second edge condition, after the server reboots for a second
time, the record that the client had an unexpired record lock, share
reservation, or delegation established before the server's previous
incarnation means that the server must reject a reclaim from client A
with the error NFS4ERR_NO_GRACE or NFS4ERR_RECLAIM_BAD.
Regardless of the level and approach to record keeping, the server
MUST implement one of the following strategies (which apply to
reclaims of share reservations, byte-range locks, and delegations):
1. Reject all reclaims with NFS4ERR_NO_GRACE. This is extremely
harsh but is necessary if the server does not want to record lock
state in stable storage.
2. Record sufficient state in stable storage to meet its
responsibilities. In doubt, the server should err on the side of
being harsh.
In the event that, after a server reboot, the server determines
that there is unrecoverable damage or corruption to stable
storage, then for all clients and/or locks affected, the server
MUST return NFS4ERR_NO_GRACE.
9.6.3.4.4. Client Edge Condition
A third edge condition affects the client and not the server. If the
server reboots in the middle of the client reclaiming some locks and
then a network partition is established, the client might be in the
situation of having reclaimed some, but not all, locks. In that
case, a conservative client would assume that the non-reclaimed locks
were revoked.
The third known edge condition follows:
1. Client A acquires a lock 1.
2. Client A acquires a lock 2.
3. The server reboots.
4. Client A issues a RENEW operation and gets back an
NFS4ERR_STALE_CLIENTID.
5. Client A reclaims its lock 1 within the server's grace period.
6. Client A and the server experience mutual network partition,
such that client A is unable to reclaim its remaining locks
within the grace period.
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7. The server's reclaim grace period ends.
8. Client B acquires a lock that would have conflicted with
client A's lock 2.
9. Client B releases the lock.
10. The server reboots a second time.
11. The network partition between client A and the server heals.
12. Client A issues a RENEW operation and gets back an
NFS4ERR_STALE_CLIENTID.
13. Client A reclaims both lock 1 and lock 2 within the server's
grace period.
At the last step, the client reclaims lock 2 as if it had held that
lock continuously, when in fact a conflicting lock was granted to
client B.
This occurs because the client failed its responsibility, by
attempting to reclaim lock 2 even though it had not held that lock at
the end of the lease that was established by the SETCLIENTID after
the first server reboot. (The client did hold lock 2 on a previous
lease, but it is only the most recent lease that matters.)
A server could avoid this situation by rejecting the reclaim of
lock 2. However, to do so accurately, it would have to ensure that
additional information about individual locks held survives a reboot.
Server implementations are not required to do that, so the client
must not assume that the server will.
Instead, a client MUST reclaim only those locks that it successfully
acquired from the previous server instance, omitting any that it
failed to reclaim before a new reboot. Thus, in the last step above,
client A should reclaim only lock 1.
9.6.3.4.5. Client's Handling of Reclaim Errors
A mandate for the client's handling of the NFS4ERR_NO_GRACE and
NFS4ERR_RECLAIM_BAD errors is outside the scope of this
specification, since the strategies for such handling are very
dependent on the client's operating environment. However, one
potential approach is described below.
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When the client's reclaim fails, it could examine the change
attribute of the objects the client is trying to reclaim state for,
and use that to determine whether to re-establish the state via
normal OPEN or LOCK requests. This is acceptable, provided the
client's operating environment allows it. In other words, the client
implementer is advised to document the behavior for his users. The
client could also inform the application that its byte-range lock or
share reservations (whether they were delegated or not) have been
lost, such as via a UNIX signal, a GUI pop-up window, etc. See
Section 10.5 for a discussion of what the client should do for
dealing with unreclaimed delegations on client state.
For further discussion of revocation of locks, see Section 9.8.
9.7. Recovery from a Lock Request Timeout or Abort
In the event a lock request times out, a client may decide to not
retry the request. The client may also abort the request when the
process for which it was issued is terminated (e.g., in UNIX due to a
signal). It is possible, though, that the server received the
request and acted upon it. This would change the state on the server
without the client being aware of the change. It is paramount that
the client resynchronize state with the server before it attempts any
other operation that takes a seqid and/or a stateid with the same
state-owner. This is straightforward to do without a special
resynchronize operation.
Since the server maintains the last lock request and response
received on the state-owner, for each state-owner, the client should
cache the last lock request it sent such that the lock request did
not receive a response. From this, the next time the client does a
lock operation for the state-owner, it can send the cached request,
if there is one, and if the request was one that established state
(e.g., a LOCK or OPEN operation), the server will return the cached
result or, if it never saw the request, perform it. The client can
follow up with a request to remove the state (e.g., a LOCKU or CLOSE
operation). With this approach, the sequencing and stateid
information on the client and server for the given state-owner will
resynchronize, and in turn the lock state will resynchronize.
9.8. Server Revocation of Locks
At any point, the server can revoke locks held by a client and the
client must be prepared for this event. When the client detects that
its locks have been or may have been revoked, the client is
responsible for validating the state information between itself and
the server. Validating locking state for the client means that it
must verify or reclaim state for each lock currently held.
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