Coredns-nodecache

The Kubernetes Node-local dns add-on proposes running a DNS caching server on all of a Kubernetes cluster's nodes. The suggested caching server is node-cache, a thin wrapper around CoreDNS, that handles the setup & teardown of the dummy network interface & associated IPTables rules.

Coredns-nodecache is an attempt to implement node-cache as a CoreDNS plugin, rather than a wrapper. The motivations for this are:

the implementation relies only on CoreDNS Plugin API, which should be backward-compatible from version to version. This should make using the latest version of CoreDNS easier (see kubernetes/dns #306)
the configuration of nodecache would be done in the CoreDNS configuration file, instead of being split between the Corefile and command-line parameters.

Additionally, coredns-nodecache can provide a high-availability setup for Node-local.

Plugin configuration

An image is available on DockerHub: https://hub.docker.com/r/contentful/coredns-nodecache

Configuration is done by adding nodecache to configuration blocks in your CoreDNS configuration file.

nodecache [skipteardown]

skipteardown: skips removing the iptables and dummy network interface on shutdown. This is especially useful for high-availability setups.

As the following example shows, you can use the directive in several blocks. For each block, coredns-nodecache will add the "bind" address to the dummy interface, and create iptable rules for the IP:PORT.

.:5300 {
    bind 168.255.10.20
    nodecache
    forward . 1.1.1.1:53 {
        force_tcp
    }
}

.:5301 {
    bind 168.255.10.25
    nodecache
    forward . 1.1.1.1:53 {
        force_tcp
    }
}

High-availability setup

CoreDNS & coredns-nodecache can provide a high-availbility setup for Kubernetes Node-local, using two separate Daemonsets & SO_REUSEPORT. An example deployment is provided in k8s/node-local-ha.yaml.

Development

Checkout this repository & CoreDNS in your GOPATH. in the CoreDNS repository, in plugin.cfg, add the following line:

nodecache:github.com/contentful-labs/coredns-nodecache

Then at the end of go.mod:

replace github.com/contentful-labs/coredns-nodecache => ../../contentful-labs/coredns-nodecache

make should build CoreDNS and include the coredns-nodecache plugin.

./coredns -plugins
[...]
dns.metadata
dns.nodecache
dns.nsid
[...]