mirror of https://github.com/k3s-io/k3s
54 lines
2.3 KiB
Markdown
54 lines
2.3 KiB
Markdown
|
# DNS in Kubernetes
|
||
|
|
This directory holds an example of how to run
|
||
|
|
[SkyDNS](https://github.com/skynetservices/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`.
|
||
|
|
|
||
|
|
```shell
|
||
|
|
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" \
|
||
|
|
./cluster/addons/dns/skydns-rc.yaml.in \
|
||
|
|
| ./cluster/kubectl.sh create -f -
|
||
|
|
|
||
|
|
sed -e "s/{DNS_SERVER_IP}/$DNS_SERVER_IP/g" \
|
||
|
|
./cluster/addons/dns/skydns-svc.yaml.in \
|
||
|
|
| ./cluster/kubectl.sh create -f -
|
||
|
|
```
|
||
|
|
|
||
|
|
## How does it work?
|
||
|
|
SkyDNS depends on etcd for what to serve, but it doesn't really need all of
|
||
|
|
what etcd offers in the way we use it. For simplicty, we run etcd and SkyDNS
|
||
|
|
together in a pod, and we do not try to link etcd instances across replicas. A
|
||
|
|
helper container called `kube2sky` also runs in the pod and acts a bridge
|
||
|
|
between Kubernetes and SkyDNS. It finds the Kubernetes master through the
|
||
|
|
`kubernetes-ro` service, it pulls service info from the master, and it writes
|
||
|
|
that to etcd for SkyDNS to find.
|
||
|
|
|
||
|
|
## Known issues
|
||
|
|
DNS resolution does not work from nodes directly, but it DOES work for
|
||
|
|
containers. As best I (thockin) 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).
|