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Merge pull request #3510 from thockin/dns

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Update DNS doc
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Tim Hockin 2015-01-21 08:46:58 -08:00
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7
cluster/addons/dns/README.md
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@ -47,7 +47,6 @@ between Kubernetes and SkyDNS. It finds the Kubernetes master through the
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).
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.

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docs/dns.md
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# DNS Integration with SkyDNS
Since Kubernetes services changed to assign a single IP address to each service, it is
now possible to use DNS to resolve a DNS name directly to a Kubernetes service, which
would then use Kubernetes' proxy to connect to an appropriate pod running the application
pointed to by the service definition.
# DNS Integration with Kubernetes
As of kubernetes 0.8, DNS is offered as a cluster add-on. If enabled, a DNS
Pod and Service will be scheduled on the cluster, and the kubelets will be
configured to tell individual containers to use the DNS Service's IP.
Every Service defined in the cluster (including the DNS server itself) will be
assigned a DNS name. By default, a client Pod's DNS search list will
include the Pod's own namespace and the cluster's default domain. This is best
illustrated by example:
Assume a Service named `foo` in the kubernetes namespace `bar`. A Pod running
in namespace `bar` can look up this service by simply doing a DNS query for
`foo`. A Pod running in namespace `quux` can look up this service by doing a
DNS query for `foo.bar`.
The cluster DNS server ([SkyDNS](https://github.com/skynetservices/skydns))
supports forward lookups (A records) and service lookups (SRV records).
## How it Works
Version 2.0.1a of [SkyDNS](https://github.com/skynetservices/skydns) added a change that
allows it to poll the Kubernetes API looking for changes to the service definitions. Newly
added services are published in SkyDNS, and removed services are deleted from SkyDNS's
internal registry.
### Concrete Example
If you run the Guestbook example in the Kubernetes repository, you'll end up with a service
called `redismaster`. If you were also running SkyDNS with the `-kubernetes=true` flag and
`-master=http://my.kubernetes.master:8080` you would immediately be able to run queries against
the SkyDNS server for the `redismaster` service. By default, SkyDNS is authoratative for the
domain `skydns.local`, so a query to the SkyDNS server requesting redismaster.skydns.local will
return the IP Address of the `redismaster` service.
The DNS pod that runs holds 3 containers - skydns, etcd (which skydns uses),
and a kubernetes-to-skydns bridge called kube2sky. The kube2sky process
watches the kubernetes master for changes in Services, and then writes the
information to etcd, which skydns reads. This etcd instance is not linked to
any other etcd clusters that might exist, including the kubernetes master.
## Configuration
SkyDNS allows you to change the domain name that it will resolve by passing in a domain on the
command line using `-domain=mydomain.com` or by setting an environment variable `SKYDNS_DOMAIN`.
## Issues
If you change the Docker daemon on your Kubernetes minions to use SkyDNS for domain name resolution,
your pods will all be able to connect to services via DNS instead of using environment variables
or other configuration methods. To change Docker to use SkyDNS resolution, add `--dns=ip.of.skydns.server`
to the Docker startup command.
```
docker -d --dns=10.2.0.5 ...
```
The skydns service is reachable directly from kubernetes nodes (outside
of any container) and DNS resolution works if the skydns service is targetted
explicitly. However, nodes are not configured to use the cluster DNS service or
to search the cluster's DNS domain by default. This may be resolved at a later
time.
SkyDNS uses the etcd instance in Kubernetes as its storage backend, which means that you can run
multiple SkyDNS daemons if you wish to have more than one resolver on your cluster. You could run
a SkyDNS instance on each node in your Kubernetes cluster, and set Docker to use 127.0.0.1 as the
DNS resolver.
## For more information
## Starting SkyDNS in a Kubernetes Cluster
At a minimum, you need to provide the `-kubernetes` flag, and the `-master=http://my.kubernetes.master.ip:8080`
flag when you start SkyDNS. You may also wish to use `-domain=mydomain.com` to change the domain that
SkyDNS resolves.
SkyDNS can act as your external resolver, too. If you set your domain to use the external IP address of
the server running SkyDNS and bind SkyDNS to listen on all interfaces, SkyDNS will serve DNS for
your domain. You could then use a mixture of manually created hosts in SkyDNS and Kubernetes service
resolution to serve your various DNS endpoints. A simple example might be to run a Wordpress pod in Kubernetes
and create a service called `blog` in Kubernetes. Then external DNS requests to `blog.mydomain.com` will
automatically resolve to the service proxy and be forwarded to the pods running Wordpress.
Full documentation of the SkyDNS server is in the [SkyDNS repository](https://github.com/skynetservices/skydns)
and abbreviated information is available by typing `skydns --help`.
See [the docs for the cluster addon](cluster/addons/dns/README.md).