COREDNS-K8S_EXTERNAL(7)

NAME

k8s_external - resolve load balancer and external IPs from outside kubernetes clusters.

DESCRIPTION

This plugin allows an additional zone to resolve the external IP address(es) of a Kubernetes service. This plugin is only useful if the kubernetes plugin is also loaded.

The plugin uses an external zone to resolve in-cluster IP addresses. It only handles queries for A, AAAA and SRV records, all others result in NODATA responses. To make it a proper DNS zone it handles SOA and NS queries for the apex of the zone.

By default the apex of the zone will look like (assuming the zone used is example.org):

    example.org.	5 IN	SOA ns1.dns.example.org. hostmaster.example.org. (
    				12345      ; serial
    				14400      ; refresh (4 hours)
    				3600       ; retry (1 hour)
    				604800     ; expire (1 week)
    				5          ; minimum (4 hours)
    				)
    example.org		5 IN	NS ns1.dns.example.org.
    ns1.dns.example.org.  5 IN  A    ....
    ns1.dns.example.org.  5 IN  AAAA ....

Note we use the dns subdomain to place the records the DNS needs (see the apex directive). Also note the SOA's serial number is static. The IP addresses of the nameserver records are those of the CoreDNS service.

The k8s_external plugin handles the subdomain dns and the apex of the zone by itself, all other queries are resolved to addresses in the cluster.

SYNTAX

    k8s_external [ZONE...]

•   ZONES zones k8s_external should be authoritative for.

If you want to change the apex domain or use a different TTL for the return records you can use this extended syntax.

    k8s_external [ZONE...] {
        apex APEX
        ttl TTL
    }

•   APEX is the name (DNS label) to use the apex records, defaults to dns.

•   ttl allows you to set a custom TTL for responses. The default is 5 (seconds).

Enable names under example.org to be resolved to in cluster DNS addresses.

    . {
       kubernetes cluster.local
       k8s_external example.org
    }

With the Corefile above, the following Service will get an A record for test.default.example.org with IP address 192.168.200.123.

    apiVersion: v1
    kind: Service
    metadata:
     name: test
     namespace: default
    spec:
     clusterIP: None
     externalIPs:
     - 192.168.200.123
     type: ClusterIP

For some background see resolve external IP address ⟨https://github.com/kubernetes/dns/issues/242⟩. And A records for services with Load Balancer IP ⟨https://github.com/coredns/coredns/issues/1851⟩.

PTR queries for the reverse zone is not supported.