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Docker resolving dns issues on ovh

Docker: Resolving DNS issues on OVH

The Issue

One of the many lower-priority issues discussed in the annals of Docker’s Github issue pages has to do with an apparent issue of being unable to resolve docker.io repositories from within Ubuntu on OVH servers.

What’s causing it?

I’m not really sure anymore. I worked through the issue about a month ago, but can tell you this:

It’s not necessarily due to the custom Kernel that OVH loads into their servers. Well, at least if they actually do give you the distribution kernel when you deploy a custom build and uncheck the option to use their kernel.

However, you should never use the OVH custom kernel if you plan on using linux containers. Just install lxc and run

                
        lxc-checkconfig

to see a list of incompatibilities [0]. You can switch out your kernel or ensure that you deselect the option to use a custom kernel when you’re going through the set-up process for your host machine. 2. I had issues with containers running on a host which was running Ubuntu 14.04 being unable to resolve any DNS.< 3. If you’re not on OVH but landed here because you’re having issues with your firewall and containers, you should note that adding IPTables rules after your docker daemon has started is a bad idea. You can see why just by glancing at the Network Driver

Fix it fix it fix it fix it fix it fix it

There’s no need to uninstall bind9 or resolvconf. Simply use Google’s public DNS servers instead by adding them to your box’s network config. Don’t add the definitions to /etc/resolv.conf, as they’ll be removed on reboot. Add them to /etc/network/interfaces on your host like so:

                
# /etc/network/interfaces
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet static
      address xxx.xxx.xxx.xxx
      netmask xxx.xxx.xxx.xxx
      network xxx.xxx.xxx.xxx
      broadcast xxx.xxx.xxx.xxx
      gateway xxx.xxx.xxx.xxx
      dns-nameservers 127.0.0.1 8.8.8.8 8.8.4.4
iface eth0 inet6 static
...

Now, if you also have DNS issues within your containers, it’s easy to tell the Docker daemon to provide Google’s public DNS servers to your containers as well. There’s more than one way to set these options, but, let’s just be clear that editing the upstart conf file found in /etc/init/docker.io.conf is not the most ideal place to make these types of changes.

Let’s make these changes in the idiomatic place: your host’s /etc/default/docker.io file:

                
  # /etc/default/docker.io
  DOCKER_OPTS="-H unix:///var/run/docker.sock --dns 8.8.8.8 --dns 8.8.4.4"

Then, run these commands at the terminal to ensure the changes are set, replacing eth0 with your network interface:

                
  ifdown eth0 && ifup eth0 && service docker.io restart

La voila, you’re all set. If you had been screwing around with your network prior to reading this article, it might be a good idea to reboot before and after these changes.

Hope that helps some poor soul - sorry for the quick and dirty post; send me a note if you want clarification on any piece!

[0] http://lxc.sourceforge.net/man/lxc.html