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Tim Hockin 270f1fd153 Add SkyDNS example This is not a complete solution, but a piece of one. More coming.		2014-11-06 17:27:14 -08:00
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README.md	Add SkyDNS example	2014-11-06 17:27:14 -08:00
skydns-rc.yaml.in	Add SkyDNS example	2014-11-06 17:27:14 -08:00
skydns-svc.yaml.in	Add SkyDNS example	2014-11-06 17:27:14 -08:00

README.md

DNS in Kubernetes

This directory holds an example of how to run SkyDNS in a Kubernetes cluster.

What things get DNS names?

The only objects to which we are assigning DNS names are Services. Every Kubernetes Service is assigned a virtual IP address which is stable as long as the Service exists. This maps well to DNS, which has a long history of clients that, on purpose or on accident, do not respect DNS TTLs.

How do I find the DNS server?

The DNS server itself runs as a Kubernetes Service. This gives it a stable IP address. When you run the SkyDNS service, you can assign a static IP to use for the Service. For example, if you assign DNS_SERVER_IP (see below) as 10.0.0.10, you can configure your docker daemon with the flag --dns 10.0.0.10.

Of course, giving services a name is just half of the problem - DNS names need a domain also. This implementation uses the variable DNS_DOMAIN (see below). You can configure your docker daemon with the flag --dns-search.

How do I run it?

The first thing you have to do is substitute the variables into the configuration. You can then feed the result into kubectl.

DNS_SERVER_IP=10.0.0.10
DNS_DOMAIN=kubernetes.local
DNS_REPLICAS=2

sed -e "s/{DNS_DOMAIN}/$DNS_DOMAIN/g" \
    -e "s/{DNS_REPLICAS}/$DNS_REPLICAS/g" \
    ./contrib/dns/skydns-rc.yaml.in \
    | ./cluster/kubectl.sh create -f -

sed -e "s/{DNS_SERVER_IP}/$DNS_SERVER_IP/g" \
    ./contrib/dns/skydns-svc.yaml.in \
    | ./cluster/kubectl.sh create -f -

How does it work?

SkyDNS depends on etcd, but it doesn't really need what etcd offers when in Kubernetes mode. SkyDNS finds the Kubernetes master through the kubernetes-ro service, and pulls service info from it, essentially using etcd as a cache. For simplicity, we run etcd and SkyDNS together in a pod, without linking the etcd instances into a cluster.

Known issues

DNS resolution does not work from nodes directly, but it DOES work for containers. As best I can figure out, this is some oddity around DNAT and localhost in the kernel. I think I have a workaround, but it's not quite baked as of the this writing (11/6/2014).