75 lines
3.4 KiB
Markdown
75 lines
3.4 KiB
Markdown
# DNS in Kubernetes
|
|
|
|
Kubernetes offers a DNS cluster addon, which most of the supported environments
|
|
enable by default. We use [SkyDNS](https://github.com/skynetservices/skydns)
|
|
as the DNS server, with some custom logic to slave it to the kubernetes API
|
|
server.
|
|
|
|
## 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 (as compared to Pod IPs which can change over time due to
|
|
crashes or scheduling changes). This maps well to DNS, which has a long
|
|
history of clients that, on purpose or on accident, do not respect DNS TTLs
|
|
(see previous remark about Pod IPs changing).
|
|
|
|
## 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 want to assign a static IP to use for
|
|
the Service. For example, if you assign the DNS Service IP as `10.0.0.10`, you
|
|
can configure your kubelet to pass that on to each container as a DNS server.
|
|
|
|
Of course, giving services a name is just half of the problem - DNS names need a
|
|
domain also. This implementation uses a configurable local domain, which can
|
|
also be passed to containers by kubelet as a DNS search suffix.
|
|
|
|
## How do I configure it?
|
|
The easiest way to use DNS is to use a supported kubernetes cluster setup,
|
|
which should have the required logic to read some config variables and plumb
|
|
them all the way down to kubelet.
|
|
|
|
Supported environments offer the following config flags, which are used at
|
|
cluster turn-up to create the SkyDNS pods and configure the kubelets. For
|
|
example, see `cluster/gce/config-default.sh`.
|
|
|
|
```shell
|
|
ENABLE_CLUSTER_DNS=true
|
|
DNS_SERVER_IP="10.0.0.10"
|
|
DNS_DOMAIN="kubernetes.local"
|
|
DNS_REPLICAS=1
|
|
```
|
|
|
|
This enables DNS with a DNS Service IP of `10.0.0.10` and a local domain of
|
|
`kubernetes.local`, served by a single copy of SkyDNS.
|
|
|
|
If you are not using a supported cluster setup, you will have to replicate some
|
|
of this yourself. First, each kubelet needs to run with the following flags
|
|
set:
|
|
|
|
```
|
|
--cluster_dns=<DNS service ip>
|
|
--cluster_domain=<default local domain>
|
|
```
|
|
|
|
Second, you need to start the DNS server ReplicationController and Service. See
|
|
the example files ([ReplicationController](skydns-rc.yaml.in) and
|
|
[Service](skydns-svc.yaml.in)), but keep in mind that these are templated for
|
|
Salt. You will need to replace the `{{ <param> }}` blocks with your own values
|
|
for the config variables mentioned above. Other than the templating, these are
|
|
normal kubernetes objects, and can be instantiated with `kubectl create`.
|
|
|
|
## How does it work?
|
|
SkyDNS depends on etcd for what to serve, but it doesn't really need all of
|
|
what etcd offers (at least not 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](kube2sky/) also runs in
|
|
the pod and acts a bridge between Kubernetes and SkyDNS. It finds the
|
|
Kubernetes master through the `kubernetes-ro` service (via environment
|
|
variables), pulls service info from the master, and writes that to etcd for
|
|
SkyDNS to find.
|
|
|
|
## Known issues
|
|
Kubernetes installs do not configure the nodes' resolv.conf files to use the
|
|
cluster DNS by default, because that process is inherently distro-specific.
|
|
This should probably be implemented eventually.
|