dns-gen sets up a container running Dnsmasq and docker-gen. docker-gen generates a configs for Dnsmasq and reloads it when containers are started and stopped.
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 -d --name dns-gen -p 172.17.42.1:53:53/udp -v /var/run/docker.sock:/var/run/docker.sock jderusse/dns-gen
Last things: 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
That is. You can now start yours containers and retreive there IP:
$ docker run --name my_app -d nginx
$ dig my_app.docker
$ dig sub.my_app.docker
You can customize the DNS name by providing an env var DOMAIN_NAME=subdomain.youdomain.com
$ docker run -e DOMAIN_NAME=foo.com -d nginx
$ dig foo.com
$ dig sub.foo.com
$ docker run -e DOMAIN_NAME=bar.com,baz.com -d nginx
$ dig bar.com
$ dig baz.com
You can tel docker (version >= 1.2) to start automaticly the containers after
booting, by passing the option --restart always to your run command.
$ docker run -d --name dns-gen --restart always -p 172.17.42.1:53:53/udp -v /var/run/docker.sock:/var/run/docker.sock jderusse/dns-gen
beware! When your host will restart, it may change the addresse IP of
the docker0 interface.
This small change will prevent docker to start your dns-gen container.
Indeed, remember our container is configurered to forward the port 53 to the
previous docker0 interface which may not exist after reboot.
Your container just wont start, you'll have to re-create it.
To solve this drawback, force docker to always use the same IP range by
editing the default configuration of docker daemon (sometime located in
/etc/default/docker but may change regarding your distribution).
You have to restart the docker service to take the changes it account.
Sometime the interface is not updated, you'll 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 automaticaly started and you'll notice "slowness" when your host will try to resolve DNS. The service is not fully loaded, because it use a feature of systemd called socket activation: The first access to the docker's 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.
You can automaticly update the resolv.conf of the container when your host changes his DNS (ie: network switching) by using the container dns-sync.
$ docker run -d --name dns-sync \
--restart always \
-v /var/run/docker.sock:/var/run/docker.sock \
-v /etc:/data/dns/etc \
-v /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