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Name

kubernetes - enables reading zone data from a Kubernetes cluster.

Description

This plugin implements the Kubernetes DNS-Based Service Discovery Specification.

CoreDNS running the kubernetes plugin can be used as a replacement for kube-dns in a kubernetes cluster. See the deployment repository for details on how to deploy CoreDNS in Kubernetes.

stubDomains and upstreamNameservers are implemented via the forward plugin. See the examples below.

This plugin can only be used once per Server Block.

Syntax

kubernetes [ZONES...]

With only the plugin specified, the kubernetes plugin will default to the zone specified in the server's block. It will handle all queries in that zone and connect to Kubernetes in-cluster. It will not provide PTR records for services or A records for pods. If ZONES is used it specifies all the zones the plugin should be authoritative for.

kubernetes [ZONES...] {
    endpoint URL
    tls CERT KEY CACERT
    kubeconfig KUBECONFIG [CONTEXT]
    apiserver_qps QPS
    apiserver_burst BURST
    apiserver_max_inflight MAX
    namespaces NAMESPACE...
    labels EXPRESSION
    pods POD-MODE
    endpoint_pod_names
    ttl TTL
    noendpoints
    fallthrough [ZONES...]
    ignore empty_service
    multicluster [ZONES...]
    zonal
    startup_timeout DURATION
}

  • endpoint specifies the URL for a remote k8s API endpoint. If omitted, it will connect to k8s in-cluster using the cluster service account. Needs tls for clusters with authentication. This option is ignored if kubeconfig is set.

  • tls CERT KEY CACERT are the TLS cert, key and the CA cert file names for remote k8s connection. This option is ignored if connecting in-cluster (i.e. endpoint is not specified).

  • kubeconfig KUBECONFIG [CONTEXT] authenticates the connection to a remote k8s cluster using a kubeconfig file. [CONTEXT] is optional, if not set, then the current context specified in kubeconfig will be used. It supports TLS, username and password, or token-based authentication. This option is ignored if omitted. The cluster address in the kubeconfig is given preference.

  • apiserver_qps QPS sets the maximum queries per second (QPS) rate limit for requests. This allows you to control the rate at which the plugin sends requests to the API server to prevent overwhelming it.

  • apiserver_burst BURST sets the maximum burst size for requests. This allows temporary spikes in request rate up to this value, even if it exceeds the QPS limit.

  • apiserver_max_inflight MAX sets the maximum number of concurrent in-flight requests. This caps the total number of simultaneous requests the plugin can make to the API server.

  • namespaces NAMESPACE [NAMESPACE...] only exposes the k8s namespaces listed. If this option is omitted all namespaces are exposed

  • namespace_labels EXPRESSION only expose the records for Kubernetes namespaces that match this label selector. The label selector syntax is described in the Kubernetes Documentation - Labels and Selectors. An example that only exposes namespaces labeled as "istio-injection=enabled", would use: labels istio-injection=enabled.

  • labels EXPRESSION only exposes the records for Kubernetes objects that match this label selector. The label selector syntax is described in the Kubernetes Documentation - Labels and Selectors. An example that only exposes objects labeled as "application=nginx" in the "staging" or "qa" environments, would use: labels environment in (staging, qa),application=nginx.

  • pods POD-MODE sets the mode for handling IP-based pod A records, e.g. 1-2-3-4.ns.pod.cluster.local. in A 1.2.3.4. This option is provided to facilitate use of SSL certs when connecting directly to pods. Valid values for POD-MODE:

    • disabled: Default. Do not process pod requests, always returning NXDOMAIN
    • insecure: Always return an A record with IP from request (without checking k8s). This option is vulnerable to abuse if used maliciously in conjunction with wildcard SSL certs. This option is provided for backward compatibility with kube-dns.
    • verified: Return an A record if there exists a pod in same namespace with matching IP. This option maintains a watch on all pods in the cluster, which requires additional memory in CoreDNS (it keeps the IP, name, namespace and labels of every pod) and adds load to the Kubernetes API server, since every pod state change in the cluster is streamed to CoreDNS.

  • endpoint_pod_names uses the pod name of the pod targeted by the endpoint as the endpoint name in A records, e.g., endpoint-name.my-service.namespace.svc.cluster.local. in A 1.2.3.4 By default, the endpoint-name name selection is as follows: Use the hostname of the endpoint, or if hostname is not set, use the dashed form of the endpoint IP address (e.g., 1-2-3-4.my-service.namespace.svc.cluster.local.) If this directive is included, then name selection for endpoints changes as follows: Use the hostname of the endpoint, or if hostname is not set, use the pod name of the pod targeted by the endpoint. If there is no pod targeted by the endpoint or pod name is longer than 63, use the dashed IP address form.

  • ttl allows you to set a custom TTL for responses. The default is 5 seconds. The minimum TTL allowed is 0 seconds, and the maximum is capped at 3600 seconds. Setting TTL to 0 will prevent records from being cached.

  • noendpoints will turn off the serving of endpoint records by disabling the watch on endpoints. All endpoint queries and headless service queries will result in an NXDOMAIN.

  • fallthrough [ZONES...] If a query for a record in the zones for which the plugin is authoritative results in NXDOMAIN, normally that is what the response will be. However, if you specify this option, the query will instead be passed on down the plugin chain, which can include another plugin to handle the query. If [ZONES...] is omitted, then fallthrough happens for all zones for which the plugin is authoritative. If specific zones are listed (for example in-addr.arpa and ip6.arpa), then only queries for those zones will be subject to fallthrough.

  • ignore empty_service returns NXDOMAIN for services without any ready endpoint addresses (e.g., ready pods). This allows the querying pod to continue searching for the service in the search path. The search path could, for example, include another Kubernetes cluster.

  • multicluster defines the multicluster zones as defined by Multi-Cluster Services API (MCS-API). Specifying this option is generally paired with the installation of an MCS-API implementation and the ServiceImport and ServiceExport CRDs. The plugin MUST be authoritative for the zones listed here.

  • zonal enables zone-scoped names for headless services (see the Zonal Names section below). It also publishes the kubernetes/zone metadata label (the requested topology zone, empty for non-zonal queries) when the metadata plugin is enabled.

  • startup_timeout specifies the DURATION value that limits the time to wait for informer cache synced when the kubernetes plugin starts. If not specified, the default timeout will be 5s.

Enabling zone transfer is done by using the transfer plugin.

Zonal Names

With the zonal option, headless services additionally answer zone-scoped forms of their name:

topology-zone.pin._zone.service.namespace.svc.zone
topology-zone.prefer._zone.service.namespace.svc.zone

e.g. us-west-2a.pin._zone.db.prod.svc.cluster.local returns only the db endpoints whose EndpointSlice zone field is us-west-2a. The zone value is every label left of the directive, joined, since Kubernetes zone label values may themselves contain dots (corp.example.com.pin._zone.db.prod.svc.cluster.local selects the zone corp.example.com). Headless services have no ClusterIP for kube-proxy's trafficDistribution to act on — every client receives every address — so the zone selector in the query name lets a client scope an answer to its own zone. Plain service names are not affected in any way, and short relative names still work from pods (us-west-2a.pin._zone.db completes via the first search list entry in the same namespace).

The directive label chooses the fallback semantics, so a client states in the name whether an empty zone is an error or a shrug:

  • pin — zone-local endpoints only. A zone label no endpoint of the service carries (a drained zone and a mistyped one alike) answers NODATA: "no endpoints carry that zone" is true either way, the answer is identical on every replica, and resolution still fails visibly.
  • prefer — zone-local endpoints if there are any, otherwise every endpoint of the service. One query, no client-side fallback logic; the widening is chosen in the name, never applied silently to a pin.

Both directives answer A/AAAA and SRV (filtering happens at endpoint selection, so SRV records and their glue are zone-filtered too), answer NODATA for other query types, and are answered identically by every replica. A nonexistent service is NXDOMAIN as ever; ClusterIP and ExternalName services are NXDOMAIN — zone-scoped names are defined for headless services only; use trafficDistribution for VIP topology. Unknown directives keep the stock too-long NXDOMAIN, as does the entire shape when the option is off. Zonal names are not defined inside multicluster zones. Endpoints whose EndpointSlices carry no zone are never matched by any zone selector.

Only names of existing headless services answer at all, so the grammar adds no capture surface beyond the one service creation itself has always had: a relative name shaped x.pin._zone.<existing-headless-service> stops a resolver search walk with NODATA, exactly as creating a service captures colliding relative names today.

Relationship to Topology Aware Routing: pin and prefer are a topology addressing primitive, not an extension of trafficDistribution. They select on the endpoint's physical topology zone (Endpoint.Zone), which the EndpointSlice controller publishes without any Service-side opt-in — not on the routing hints (Endpoint.Hints.ForZones), which exist only when a Service opts in via trafficDistribution or the legacy service.kubernetes.io/topology-mode: Auto annotation, and which encode the zone tier of a routing decision rather than placement (under Auto an endpoint can be hinted for a zone it is not in, and the controller withdraws hints entirely when its safeguards trip). A client naming a zone under these directives gets the endpoints that are actually there. A hints-consuming selector is a distinct primitive with distinct semantics (kube-proxy ignores hints entirely for unhinted, partially-hinted, and safeguard-withdrawn services); if one is added, it takes its own directive label in this grammar. Unknown directives answer the stock too-long NXDOMAIN today, so that addition is compatible and nothing here forecloses it.

The option requires the endpoint cache: combining zonal with noendpoints is a configuration error, since zone-scoped answers come from endpoint data and the noendpoints contract (NXDOMAIN for all headless queries) could not hold for them.

Deployment notes: enable the option on every replica behind a shared Service before pointing clients at _zone names — replicas without the option answer NXDOMAIN for them, which clients negative-cache per name for the SOA minttl (this follows the ttl option). Zonal names are answered at query time only; they are not included in zone transfers.

Startup

When CoreDNS starts with the kubernetes plugin enabled, it will delay serving DNS for up to 5 seconds until it can connect to the Kubernetes API and synchronize all object watches. If this cannot happen within 5 seconds, then CoreDNS will start serving DNS while the kubernetes plugin continues to try to connect and synchronize all object watches. CoreDNS will answer SERVFAIL to any request made for a Kubernetes record that has not yet been synchronized. You can also determine how long to wait by specifying startup_timeout.

Monitoring Kubernetes Endpoints

The kubernetes plugin watches Endpoints via the discovery.EndpointSlices API.

Ready

This plugin reports readiness to the ready plugin. This will happen after it has synced to the Kubernetes API.

PTR Records

This plugin creates PTR records for every Pod selected by a Service. If a given Pod is selected by more than one Service a separate PTR record will exist for each Service selecting it.

Examples

Handle all queries in the cluster.local zone. Connect to Kubernetes in-cluster. Also handle all in-addr.arpa PTR requests for 10.0.0.0/17 . Verify the existence of pods when answering pod requests.

10.0.0.0/17 cluster.local {
    kubernetes {
        pods verified
    }
}

Or you can selectively expose some namespaces:

kubernetes cluster.local {
    namespaces test staging
}

Connect to Kubernetes with CoreDNS running outside the cluster:

kubernetes cluster.local {
    endpoint https://k8s-endpoint:8443
    tls cert key cacert
}

Configure multicluster

kubernetes cluster.local clusterset.local {
    multicluster clusterset.local
}

stubDomains and upstreamNameservers

Here we use the forward plugin to implement a stubDomain that forwards example.local to the nameserver 10.100.0.10:53. Also configured is an upstreamNameserver 8.8.8.8:53 that will be used for resolving names that do not fall in cluster.local or example.local.

cluster.local:53 {
    kubernetes cluster.local
}
example.local {
    forward . 10.100.0.10:53
}
. {
    forward . 8.8.8.8:53
}

The configuration above represents the following Kube-DNS stubDomains and upstreamNameservers configuration.

stubDomains: |
   {“example.local”: [“10.100.0.10:53”]}
upstreamNameservers: |
   [“8.8.8.8:53”]

AutoPath

The kubernetes plugin can be used in conjunction with the autopath plugin. Using this feature enables server-side domain search path completion in Kubernetes clusters. Note: pods must be set to verified for this to function properly. Furthermore, the remote IP address in the DNS packet received by CoreDNS must be the IP address of the Pod that sent the request.

cluster.local {
    autopath @kubernetes
    kubernetes {
        pods verified
    }
}

Metadata

The kubernetes plugin will publish the following metadata, if the metadata plugin is also enabled:

  • kubernetes/endpoint: the endpoint name in the query
  • kubernetes/kind: the resource kind (pod or svc) in the query
  • kubernetes/namespace: the namespace in the query
  • kubernetes/port-name: the port name in an SRV query
  • kubernetes/protocol: the protocol in an SRV query
  • kubernetes/service: the service name in the query
  • kubernetes/client-namespace: the client pod's namespace (see requirements below)
  • kubernetes/client-pod-name: the client pod's name (see requirements below)
  • kubernetes/client-label/<label key>: a label on the client pod (see requirements below)

The kubernetes/client-namespace, kubernetes/client-pod-name, and kubernetes/client-label/<label key> metadata work by reconciling the client IP address in the DNS request packet to a known pod IP address. Therefore the following is required:

  • pods verified mode must be enabled
  • the remote IP address in the DNS packet received by CoreDNS must be the IP address of the Pod that sent the request.

Metrics

If monitoring is enabled (via the prometheus plugin) then the following metrics are exported:

  • coredns_kubernetes_dns_programming_duration_seconds{service_kind} - Exports the DNS programming latency SLI. The metrics has the service_kind label that identifies the kind of the kubernetes service. It may take one of the three values:
    • cluster_ip
    • headless_with_selector
    • headless_without_selector

The following are client level metrics to monitor apiserver request latency & status codes. verb identifies the apiserver request type and host denotes the apiserver endpoint.

  • coredns_kubernetes_rest_client_request_duration_seconds{verb, host} - captures apiserver request latency perceived by client grouped by verb and host.
  • coredns_kubernetes_rest_client_rate_limiter_duration_seconds{verb, host} - captures apiserver request latency contributed by client side rate limiter grouped by verb & host.
  • coredns_kubernetes_rest_client_requests_total{method, code, host} - captures total apiserver requests grouped by method, status_code & host.

Bugs

The duration metric does not yet support the headless_without_selector service kind.

See Also

See the autopath plugin to enable search path optimizations. And use the transfer plugin to enable outgoing zone transfers.

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