DNS And Docker Containers

Following our previous post about container migration, I wanted to explain a simple trick to get containers to talk with each another using their names that we’ve used a few times while building devo.ps.

The problem at hand is that with Docker, containers have different IP addresses on start. Even after a successful data migration or simply a restart of your container, you need to propagate that new information. For example let your app container know that the database changed IP address.

Sure, you can use etcd or related types of project, but it sometime is overkill - or you simply don’t have the time or resources.

Let’s go instead for the poor’s man approach: simply set a DNS. We’re gonna keep it simple and use dnsmasq on the current host since it’s also used by docker for DHCP purpose.

Let’s get staretd:

1. The dnsmasq service loads extra config files

    $ > cat /etc/dnsmasq.conf
   
    listen-address=0.0.0.0
    interface=lo
    interface=eth0
    interface=docker0
    resolv-file=/etc/resolv.dnsmasq.conf
    conf-dir=/opt/docker/dnsmasq.d         # <== Here !
   

2. We get the containers to use the dnsmasq as their resolver; usually 172.17.42.1 is the IP of your host

    $ > docker run -d -dns 172.17.42.1 -name db -h db db_image
    $ > docker run -d -dns 172.17.42.1 -name app -h app app_image
   

3. Whenever we change a container, we update the host config and get the DNS service updated

    $ > container='db'
    $ > new_ip=$(docker inspect $container | grep IPAddress | cut -f4 -d'"')
    $ > echo "host-record=$container,$new_ip" > /opt/docker/dnsmasq.d/0host_$container
    $ > service dnsmasq restart
   

Et voila! Your app can now simply refer to its database by using db as host in its config. Script the above in a better way to handle edge cases and you’re good to go.