kubernetes

Source

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]
    namespaces NAMESPACE...
    labels EXPRESSION
    pods POD-MODE
    endpoint_pod_names
    ttl TTL
    noendpoints
    fallthrough [ZONES...]
    ignore empty_service
}

endpoint specifies the URL for a remote k8s API endpoint. If omitted, it will connect to k8s in-cluster using the cluster service account.
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 connecting in-cluster (i.e., the endpoint is not specified).
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 User Guide - Labels. 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 User Guide - Labels. 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 requires substantially more memory than in insecure mode, since it will maintain a watch on all pods.
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.

Enabling zone transfer is done by using the transfer plugin.

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.

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.

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
}

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)

The kubernetes/client-namespace and kubernetes/client-pod-name 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 only supports the “headless_with_selector” service currently.