isabella232 / multicluster

CoreDNS plugin implementing K8s multi-cluster services DNS spec.

multicluster

Name

multicluster - implementation of Multicluster DNS

Description

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

Syntax

multicluster [ZONES...] {
    kubeconfig KUBECONFIG [CONTEXT]
    noendpoints
    fallthrough [ZONES...]
}

• 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).
• 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.

Startup

When CoreDNS starts with the multicluster 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 multicluster 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.

Examples

Handle all queries in the clusterset.local zone. Connect to Kubernetes in-cluster.

.:53 {
    multicluster clusterset.local
}

Installation

See CoreDNS documentation about Compile Time Enabling or Disabling Plugins.

Recompile coredns

Add the plugin to plugins.cfg file. The ordering of plugins matters, add it just below kubernetes plugin that has very similar functionality:

...
kubernetes:kubernetes
multicluster:github.com/coredns/multicluster
...

Follow the coredns README file to build it.

Modify cluster's corefile

To enable the plugin for clusterset.local zone, add multicluster configuration to the corefile. Resulting corefile may look like this:

.:53 {
    errors
    health
    multicluster clusterset.local
    kubernetes cluster.local in-addr.arpa ip6.arpa {
      pods insecure
      fallthrough in-addr.arpa ip6.arpa
    }
    prometheus :9153
    forward . /etc/resolv.conf
    cache 30
    loop
    reload
    loadbalance
}