Docker DNS-gen

dns-gen sets up a container running Dnsmasq and docker-gen. docker-gen generates a configuration for Dnsmasq and reloads it when containers are started and stopped.

Usage

First, you have to know the IP of your docker0 interface. It may be 172.17.42.1 but it could be something else. To known your IP, run the following command:

$ /sbin/ifconfig docker0 | grep "inet addr" | awk '{ print $2}' | cut -d: -f2

Now, you can start the dns-gen container:

$ docker run --detach \
  --name dns-gen \
  --publish 172.17.42.1:53:53/udp \
  --volume /var/run/docker.sock:/var/run/docker.sock \
  jderusse/dns-gen

Last thing: Register you new DnsServer in you resolv.conf

$ echo "nameserver 172.17.42.1" | sudo tee --append /etc/resolvconf/resolv.conf.d/head
$ sudo resolvconf -u

This is it. You can now start your containers and retrieve their IP:

$ docker run --name my_app --detach nginx
$ dig my_app.docker
$ dig sub.my_app.docker

You can customize the DNS name by providing an environment variable, like this: DOMAIN_NAME=subdomain.youdomain.com

$ docker run --env DOMAIN_NAME=foo.com --detach nginx
$ dig foo.com
$ dig sub.foo.com
$ docker run --env DOMAIN_NAME=bar.com,baz.com --detach nginx
$ dig bar.com
$ dig baz.com

Start the container automatically after reboot

You can tell docker (version >= 1.2) to automatically start the DNS container after booting, by passing the option --restart always to your run command.

$ docker run --daemon --name dns-gen \
  --restart always \
  --publish 172.17.42.1:53:53/udp \
  --volume /var/run/docker.sock:/var/run/docker.sock \
  jderusse/dns-gen

beware! When your host will restart, it may change the IP address of the docker0 interface. This small change will prevent docker to start your dns-gen container. Indeed, remember our container is configured to forward port 53 to the previous docker0 interface which may not exist after reboot. Your container just will not start, you will have to re-create it. To solve this drawback, force docker to always use the same IP range by editing the default configuration of the docker daemon (sometimes located in /etc/default/docker but may change regarding your distribution). You have to restart the docker service to take the changes into account. Sometimes the interface is not updated, you will have to restart your host.

$ vim /etc/default/docker
DOCKER_OPTS="--bip=172.17.42.1/24"

$ sudo service docker restart

One more thing When you start your host, the docker service is not fully loaded. Until this daemon is loaded, the dns container will not be automatically started and you will notice bad performance when your host will try to resolve DNS. The service is not fully loaded, because it uses a feature of systemd called socket activation: The first access to the docker socket will trigger the start of the true service. To skip this feature, you simply have to activate the docker service.

$ sudo update-rc.d docker enable

Et voila, now, docker will really start with your host, it will always use the same range of IP addresses and will always start/restart the container dns-gen.

Bonus

You can automatically update the resolv.conf of the container when your host changes its DNS (ie: network switching) by using the container dns-sync.

$ docker run --detach --name dns-sync \
    --restart always \
    --volume /var/run/docker.sock:/var/run/docker.sock \
    --volume /etc:/data/dns/etc \
    --volume /run:/data/dns/run \
    jderusse/dns-sync

When coupled with dns-sync, you can force all containers to use this DNS by updating the docker's default options

$ vim /etc/default/docker
DOCKER_OPTS="--bip=172.17.42.1/24 --dns=172.17.42.1"

$ sudo service docker restart