DBCLIENT(1)

General Commands Manual

DBCLIENT(1)

SYNOPSIS

dbclient [flag arguments] [-p port] [-i id] [-L l:h:p] [-R l:h:p] [-l user] host [more flags] [command]

dbclient [args] [user1]@host1[^port1],[user2]@host2[^port2],...

DESCRIPTION

dbclient is the client part of Dropbear SSH

A command to run on the remote host. This will normally be run by the remote host using the user's shell. The command begins at the first hyphen argument after the host argument. If no command is specified an interactive terminal will be opened (see -t and -T).

-p port

Connect to port on the remote host. Alternatively a port can be specified as hostname^port. Default is 22.

-i idfile

Identity file. Read the identity key from file idfile (multiple allowed). This file is created with dropbearkey(1) or converted from OpenSSH with dropbearconvert(1). The default path ~/.ssh/id_dropbear is used.

-L [listenaddress]:listenport:host:port

Local port forwarding. Forward listenport on the local host through the SSH connection to port on host.

-R [listenaddress]:listenport:host:port

Remote port forwarding. Forward listenport on the remote host through the SSH connection to port on host.

-l user

Username. Login as user on the remote host. An alternative is to specify user@host.

-t

Allocate a PTY. This is the default when no command is given, it gives a full interactive remote session. The main effect is that keystrokes are sent remotely immediately as opposed to local line-based editing.

-T

Don't allocate a PTY. This is the default when a command is given. See -t.

-N

Don't request a remote shell or run any commands. Any command arguments are ignored.

-f

Fork into the background after authentication. A command argument (or -N) is required. This is useful when using password authentication.

-g

Allow non-local hosts to connect to forwarded ports. Applies to -L and -R forwarded ports, though remote connections to -R forwarded ports may be limited by the ssh server.

-y

Always accept hostkeys if they are unknown. If a hostkey mismatch occurs the connection will abort as normal. If specified a second time no host key checking is performed at all, this is usually undesirable.

-A

Forward agent connections to the remote host. dbclient will use any OpenSSH-style agent program if available (SSH_AUTH_SOCK will be set) for public key authentication. Forwarding is only enabled if -A is specified.

Beware that a forwarded agent connection will allow the remote server to have the same authentication credentials as you have used locally. A compromised remote server could use that to log in to other servers.

In many situations Dropbear's multi-hop mode is a better and more secure alternative to agent forwarding, avoiding having to trust the intermediate server.

If the SSH agent program is set to prompt when a key is used, the -o DisableTrivialAuth option can prevent UI confusion.

-W windowsize

Specify the per-channel receive window buffer size. Increasing this may improve network performance at the expense of memory use. Use -h to see the default buffer size.

-K timeout_seconds

Ensure that traffic is transmitted at a certain interval in seconds. This is useful for working around firewalls or routers that drop connections after a certain period of inactivity. The trade-off is that a session may be closed if there is a temporary lapse of network connectivity. A setting if 0 disables keepalives. If no response is received for 3 consecutive keepalives the connection will be closed.

-I idle_timeout

Disconnect the session if no traffic is transmitted or received for idle_timeout seconds.

-z

By default Dropbear will send network traffic with the AF21 setting for QoS, letting network devices give it higher priority. Some devices may have problems with that, -z can be used to disable it.

-J proxy_command | -J &fd

Use the standard input/output of the program proxy_command rather than using a normal TCP connection. A hostname should be still be provided, as this is used for comparing saved hostkeys. This command will be executed as ‘exec proxy_command ...’ with the default shell.

The second form &fd will make dbclient use the numeric file descriptor as a socket. This can be used for more complex tunnelling scenarios. Example usage with socat is

socat EXEC:'dbclient -J &38 ev',fdin=38,fdout=38 TCP4:host.example.com:22

-B endhost:endport

“Netcat-alike” mode, where Dropbear will connect to the given host, then create a forwarded connection to endhost. This will then be presented as dbclient's standard input/output.

-c cipherlist

Specify a comma separated list of ciphers to enable. Use -c help to list possibilities.

-m MAClist

Specify a comma separated list of authentication MACs to enable. Use -m help to list possibilities.

-o option

Can be used to give options in the format used by OpenSSH config file. This is useful for specifying options for which there is no separate command-line flag. For full details of the options listed below, and their possible values, see ssh_config(5). yes/no arguments also accept y/n.

The following options have currently been implemented:

BatchMode

Disable interactive prompts e.g. password prompts and host key confirmation. The argument must be yes or no (the default).

BindAddress

Specify address and port on the local machine as the source address of the connection.

Compression

Can be set to yes or no. Default is no compression (with default build options). Enabling compression can be a security weakness in some circumstances, as the size of network traffic may leak information about the encrypted data.

Compression will only be used when the server also supports it.

DisableTrivialAuth

Disallow a server immediately giving successful authentication (without presenting any password/pubkey prompt). This avoids a UI confusion issue where it may appear that the user is accepting a SSH agent prompt from their local machine, but are actually accepting a prompt sent immediately by the remote server.

ExitOnForwardFailure

Specifies whether dbclient should terminate the connection if it cannot set up all requested local and remote port forwardings. The argument must be yes or no (the default).

ForwardAgent

Forward the authentication agent to the remote machine. The argument must be yes or no (the default).

GatewayPorts

Allow to remote host to connect to local forwarded ports. The argument must be yes or no (the default).

IdentityFile

Specify an authentication identity file path.

PasswordAuthentication

Allow to prompt a user for a password. If the DROPBEAR_PASSWORD env is specified then it still will be used. The argument must be yes (the default) or no.

Port

Specify a listening port, like the -p argument.

ProxyCommand

Specify the proxy command to use to connect to the server.

ServerAliveInterval

Sets a timeout interval in seconds between keep-alive messages through the encrypted channel. The default is 0 e.g. disabled.

StrictHostKeyChecking

Use yes to refuse connection to hosts where the host key is not already correct in known_hosts. Entries must be added to known_hosts manually.

Use no to skip the known_hosts key checking.

Use accept-new to add new host keys to the known_hosts and refuse to connect if the host key has changed.

ask is the default.

UseSyslog

Send dbclient log messages to syslog in addition to stderr.

-s

The specified command will be requested as a subsystem, used for sftp. Dropbear doesn't implement sftp itself but the OpenSSH sftp client can be used eg

sftp -S dbclient user@host

-b [address][:port]

Bind to a specific local address when connecting to the remote host. This can be used to choose from multiple outgoing interfaces. Either address or port (or both) can be given.

-Q algo

Query supported algorithms. Options are kex, sig, cipher, mac, compress.

-V

Print the version

MULTI-HOP

Dropbear will also allow multiple "hops" to be specified, separated by commas. In this case a connection will be made to the first host, then a TCP forwarded connection will be made through that to the second host, and so on. Hosts other than the final destination will not see anything other than the encrypted SSH stream. A port for a host can be specified with a caret (eg matt@martello^44). This syntax can also be used with scp or rsync (specifying dbclient as the ssh/rsh command). A file can be "bounced" through multiple SSH hops, eg

scp -S dbclient matt@martello,root@wrt,canyons:/tmp/dump .

Note that hostnames are resolved by the prior hop (so "canyons" would be resolved by the host "wrt") in the example above, the same way as other -L TCP forwarded hosts are. Host keys are checked locally based on the given hostname.

ESCAPE CHARACTERS

Typing a newline followed by the key sequence ‘~.’ (tilde, dot) will terminate a connection. The sequence ‘~^Z’ (tilde, ctrl-z) will background the connection. This behaviour only applies when a PTY is used.

‘~R’ will perform a key re-exchange of ephemeral session keys.

ENVIRONMENT

DROPBEAR_PASSWORD: A password to use for remote authentication can be specified in the environment variable DROPBEAR_PASSWORD. Care should be taken that the password is not exposed to other users on a multi-user system, or stored in accessible files.
SSH_ASKPASS: dbclient can use an external program to request a password from a user. SSH_ASKPASS should be set to the path of a program that will return a password on standard output. This program will only be used if either DISPLAY is set and standard input is not a TTY, or the environment variable SSH_ASKPASS_ALWAYS is set.

FILES

~/.ssh/dropbear_config

This is the per user configuration file. A very limited subset of the keywords for ssh_config(5) is supported, and none of the advanced features. The file contains key value pairs on a single line separated with space or ‘=’. Empty lines are ignored. Text starting with ‘#’ is a comment, and also ignored.

The file is not considered if multi-hop connection is used. Values on the command line override the respective values in the file.

The recognized keywords are as follows. Keywords are case insensitive and values are case insensitive.

Host: Defines the options that would be applied if this value matches the host specified on the command line. The next Host entry or EOF determine the list of applicable options.
HostName: Specifies the actual host name to connect to. Can be DNS name or IP address.
Port: Specifies the port number to use to connect to the remote host.
User: Specifies the user name to login in as.
IdentityFile: Specifies the file with the private key used for public key authentication with the remote host. The file must be in the Dropbear format. See dropbearkey(1) to generate one. ~/ at the start of the path will expanded to the executing user's home directory. A path that does not start with / will be treated relative to this configuration file's directory. Otherwise the path will be used as is.

Because this file contains a secret it must have strict permissions to prevent abuse attempts - read/write for the executing user, and no access to anyone else.

NOTES

If compiled with zlib support and if the server supports it, dbclient will always use compression.

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