coredns/plugin/k8s_external

k8s_external

Name

k8s_external - resolves load balancer, external IPs from outside Kubernetes clusters and if enabled headless services.

Description

This plugin allows an additional zone to resolve the external IP address(es) of a Kubernetes service and headless services. 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, SRV, and PTR records; 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 the following (assuming the zone used is example.org):

example.org.	5 IN	SOA ns1.dns.example.org. hostmaster.example.org. (
				12345      ; serial
				7200       ; refresh (2 hours)
				1800       ; retry (30 minutes)
				86400      ; expire (1 day)
				5          ; minimum (5 seconds)
				)
example.org		5 IN	NS ns1.dns.example.org.

ns1.dns.example.org.  5 IN  A    ....
ns1.dns.example.org.  5 IN  AAAA ....

Note that we use the dns subdomain for the records 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 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 returned records you can use this extended syntax.

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

• APEX is the name (DNS label) to use for the apex records; it defaults to dns.
• ttl allows you to set a custom TTL for responses. The default is 5 (seconds).

If you want to enable headless service resolution, you can do so by adding headless option.

k8s_external [ZONE...] {
    headless
}

• if there is a headless service with external IPs set, external IPs will be resolved

If the queried domain does not exist, you can fall through to next plugin by adding the fallthrough option.

k8s_external [ZONE...] {
    fallthrough [ZONE...]
}

Examples

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 the 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

The k8s_external plugin can be used in conjunction with the transfer plugin to enable zone transfers. Notifies are not supported.

    . {
        transfer example.org {
            to *
        }
        kubernetes cluster.local
        k8s_external example.org
    }

With the fallthrough option, if the queried domain does not exist, it will be passed to the next plugin that matches the zone.

. {
   kubernetes cluster.local
   k8s_external example.org {
     fallthrough
   }
   forward . 8.8.8.8
}

See Also

For some background see resolve external IP address. And A records for services with Load Balancer IP.