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Re: DNS support added to OpenShift



+++ Luke Meyer [09/03/15 11:03 -0400]:
It probably seems obvious to you, but... in addition to the "what" and "how" could you talk a bit about "why"? What are the main use cases where this comes into play?

I'm not sure if this is the main motivation but I could see this
keeping pods from having to restart when services come and go.

eg, Today with the sample app
(https://github.com/openshift/origin/tree/master/examples/sample-app)
if you create the frontend first and then later create the database
you have to restart (or recreate) the frontend pod to pick up the
service addition.  That's because the only interface for injecting the
service is an environment variable ultimately set when Docker launches
the container.

--Brenton


----- Original Message -----
From: "Clayton Coleman" <ccoleman redhat com>
To: dev lists openshift redhat com
Sent: Sunday, March 8, 2015 9:17:05 PM
Subject: DNS support added to OpenShift

Pull https://github.com/openshift/origin/pull/1254 adds split-horizon DNS inside the cluster by running a SkyDNS server on each master that can answer DNS queries for services.

By default, when the master starts it will start listening on port 53.  It will bind to the same addresses the master is configured for, and in an all-in-one the Kubelet will use that address to query for requests.  When the node is started in a real cluster (openshift start node) the Kubelet will look up the "kubernetes" service and assume the first endpoint for that service can be used to talk DNS.  If you are running OpenShift on top of Kube you'll need to change the kubelet --cluster-dns parameters to point to the OpenShift master.

On startup of a node the presence of this message indicates the Kubelet correctly resolved the master:

0308 19:51:03.118430    4484 node.go:197] Started Kubelet for node openshiftdev.local, server at 0.0.0.0:10250
I0308 19:51:03.118459    4484 node.go:199]   Kubelet is setting 10.0.2.15 as a DNS nameserver for domain "local"

If you don't see the second message, the "kubernetes" service may not be available.

NOTE: If you are not running as root when you start OpenShift, you can't bind to port 53. The master will instead use 8053, but because libc name resolution doesn't have a way to use custom ports you can only query directly against the master with tools like dig, and cluster DNS won't work.  Unless you're on Mac, which allows custom ports in resolve.conf, but Macs can't run Docker anyway, so the rabbit hole stops there.

How does cluster DNS work?
--------------------------

When nodes are properly configured, each Docker container gets an additional nameserver (the master) added to the front of its nameserver list, and the default search domain for the container will be ".<pod_namespace>.local".  The container will then direct nameserver queries to the master first, before using any other nameservers configured on the node (the Docker default behavior).

The master will answer queries on the ".local" domain that have the following form:

   <namespace_name>.local
   <service_name>.<namespace_name>.local

SkyDNS will respond with a single A record for any service you ask for that contains the service Portal IP.  Since that IP is resolvable within your containers, you'll get automatic DNS for services.  For example, if you're in namespace "foo" and you have a service "bar", any pods in your namespace will be able to:

   $ dig foo.bar.local. A
   # ....
   ;; ANSWER SECTION:
   foo.bar.local.	30	IN	A	172.30.17.2

You can also get SRV, CNAME, and do wildcard lookups. Reverse lookups for a service portal IP will resolve to the CNAME of the service ("foo.bar.local"). Also, because the search domain is set, you can lookup "foo" or "foo.local" and see similar results.

SkyDNS is set up to forward any queries outside of .local to the nameservers set in your master (so configure /etc/resolv.conf normally).  You can populate the etcd schema for SkyDNS (/skydns/*) and it should also match those values, so you can set up custom queries with the full power of SkyDNS (weights, priority, etc).

Future work may be to implement headless services (service will respond with multiple A records, one for each pod), allow routes to resolve internally, allow an external delegated domain to resolve to OpenShift, and to enable DNSSEC (which SkyDNS fully supports but I have not yet enabled).

There's probably some rough edges still, so folks with deeper DNS experience than I please get involved and try out the support.

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