coredns/middleware/kubernetes

kubernetes

The kubernetes middleware enables the reading zone data from a Kubernetes cluster. It implements the Kubernetes DNS-Based Service Discovery Specification.

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

Syntax

kubernetes ZONE [ZONE...] [
	resyncperiod DURATION
	endpoint URL
	tls CERT KEY CACERT]
	namespaces NAMESPACE [NAMESPACE...]
	labels EXPRESSION
	pods POD-MODE]
	upstream ADDRESS [ADDRESS...]
	federation NAME DOMAIN
	fallthrough
}

• resyncperiod specifies the Kubernetes data API DURATION period.

• endpoint specifies the URL for a remove k8s API endpoint. If omitted, it will connect to k8s in-cluster using the cluster service account. Multiple k8s API endpoints could be specified, separated by ,s, e.g. endpoint http://k8s-endpoint1:8080,http://k8s-endpoint2:8080. CoreDNS will automatically perform a healthcheck and proxy to the healthy k8s API endpoint.

• 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).

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

• 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 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.

• upstream ADDRESS [ADDRESS...] defines the upstream resolvers used for resolving services that point to external hosts (External Services). ADDRESS can be an ip, an ip:port, or a path to a file structured like resolv.conf.

• federation NAME DOMAIN defines federation membership. One line for each federation membership. Each line consists of the name of the federation, and the domain.

• fallthrough If a query for a record in the cluster zone 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 middleware chain, which can include another middleware to handle the query.

Examples

Example 1: This is a minimal configuration with no options other than zone. It will handle all queries in the cluster.local zone and connect to Kubernetes in-cluster, but it will not provide PTR records for services, or A records for pods.

kubernetes cluster.local

Example 2: Handle all queries in the cluster.local zone. Connect to Kubernetes in-cluster. Handle all PTR requests for 10.0.0.0/16 . Verify the existence of pods when answering pod requests. Resolve upstream records against 10.102.3.10.

10.0.0.0/16 cluster.local {
    kubernetes {
        pods verified
        upstream 10.102.3.10:53
    }
}

Selective Exposure Example: Handle all queries in the cluster.local zone. Connect to Kubernetes in-cluster. Only expose objects in the test and staging namespaces. Resolve upstream records using the servers configured in /etc/resolv.conf.

kubernetes cluster.local {
	namespaces test staging

Federation Example: Handle all queries in the cluster.local zone. Connect to Kubernetes in-cluster. Handle federated service requests in the prod and stage federations. Resolve upstream records using the servers configured in /etc/resolv.conf.

cluster.local {
    kubernetes {
	    federation prod prod.feddomain.com
	    federation stage stage.feddomain.com
	    upstream /etc/resolv.conf
	}
}

Out-Of-Cluster Example: Handle all queries in the cluster.local zone. Connect to Kubernetes from outside the cluster. Verify the existence of pods when answering pod requests. Resolve upstream records against 10.102.3.10.

kubernetes cluster.local {
	endpoint https://k8s-endpoint:8443
	tls cert key cacert
	pods verified
	upstream 10.102.3.10:53
}

Wildcard

Some query labels accept a wildcard value to match any value. If a label is a valid wildcard (*, or the word "any"), then that label will match all values. The labels that accept wildcards are:

• service in an A record request: service.namespace.svc.zone.
  • e.g. *.ns.svc.myzone.local
• namespace in an A record request: service.namespace.svc.zone.
  • e.g. nginx.*.svc.myzone.local
• port and/or protocol in an SRV request: _port._protocol.service.namespace.svc.zone.
  • e.g. _http.*.service.ns.svc.
• multiple wild cards are allowed in a single query.
  • e.g. A Request *.*.svc.zone. or SRV request *.*.*.*.svc.zone.