devdns

Make docker containers discoverable via DNS for development environments, like when running a bunch of containers on your laptop. Useful for container to container communication, or just an easy way of reaching containers from the host machine.

Running

docker run -d --name devdns -p 53:53/udp \
  -v /var/run/docker.sock:/var/run/docker.sock ruudud/devdns

devdns requires access to the Docker socket to be able to query for container names and IP addresses, in addition to listen to start/stop events.

Binding port 53 on the host machine is optional, but will make it easier when configuring local resolving.

The DNS server running in devdns is set to proxy requests for unknown hosts to Google's DNS server 8.8.8.8. It also adds a wildcard record (normally *.test, see DNS_DOMAIN below) pointing back at the host machine (bridge IP in Linux), to facilitate communication when running a combination of services "inside" and "outside" of Docker.

Using

Container ↔ Container

When running other containers, specify the devdns container IP as the DNS to use:

$ docker run -d --name devdns -p 53:53/udp \
  -v /var/run/docker.sock:/var/run/docker.sock ruudud/devdns
$ docker run -d --name redis redis
$ docker run -it --rm \
  --dns=`docker inspect -f "{{ .NetworkSettings.IPAddress }}" devdns` debian \
  ping redis.test

Please note that the --dns flag will prepend the given DNS server to the Docker default (normally 8.8.8.8), so lookups for external addresses will still work.

Docker Daemon Configuration

If you want devdns to be added by default to all new containers, you need to add some custom Docker daemon options. The place to put this config varies:

• Ubuntu <= 14.10: /etc/default/docker, see the Docker configuring docs
• Ubuntu >= 15.04: /etc/systemd/system/docker.service, see the Docker systemd docs
• OSX boot2docker: /var/lib/boot2docker/profile, see the boot2docker faq

The extra options you'll have to add is

--dns 172.17.0.1 --dns-search test

Replace test with whatever you set as config for DNS_DOMAIN.

172.17.0.1 is the default IP of the Docker bridge, and port 53 on this host should be reachable from within all started containers given that you've included -p 53:53/udp when starting the devdns container.

Host Machine → Containers

You will need to add some configuration to your OS resolving mechanism.
NOTE: This is only practical if you added -p 53:53/udp when starting devdns.

OSX

Create a file /etc/resolver/test containing

nameserver <listen address of devdns>

In OSX, there's a good chance you're using boot2docker, so the listen address will probably be the output of boot2docker ip. Please note that the name of the file created in /etc/resolver has to match the value of the DNS_DOMAIN setting (default "test").

Linux / Ubuntu

Nowadays, direct edits of /etc/resolv.conf will be removed at reboot. Thus, the best place to add extra resolvers in Linux, is to use your network configurator. YMMV. This means NetworkManager (see section below), WICD, or manually using /etc/network/interfaces:

auto p3p1
iface p3p1 inet dhcp
dns-search test
dns-nameservers 127.0.0.1

Alternatively, edit /etc/dhcp/dhclient.conf instead. Uncomment or add the following line:

supersede domain-name "test";
prepend domain-name-servers 127.0.0.1;

Configuration

• DNS_DOMAIN: set the local domain used. (default: test)
• HOSTMACHINE_IP: IP address of non-matching queries (default: 172.17.0.1)
• EXTRA_HOSTS: list of extra records to create, space-separated string of host=ip pairs. (default: '')

Example:

docker run -d -v /var/run/docker.sock:/var/run/docker.sock \
  -e DNS_DOMAIN=docker \
  -e HOSTMACHINE_IP=192.168.1.1 \
  -e EXTRA_HOSTS="dockerhost=172.17.0.1 doubleclick.net=127.0.0.1" \
  ruudud/devdns

Caveats

Container name to DNS record conversion

RFC 1123 states that _ are not allowed in DNS records, but Docker allows it in container names. devdns ignores _ and whatever follows, allowing multiple simultaneous containers with matching names to run at the same time.

The DNS will resolve to the lastly added container, and try to re-toggle the previous matching container when stopping the currently active one.

Example:

# (devdns already running)
$ docker run -d --name redis_local-V1 redis
$ dig redis.test     # resolves to the IP of redis_local-V1

$ docker run -d --name redis_test redis
$ dig redis.test     # resolves to the IP of redis_test

$ docker stop redis_test
$ dig redis.test     # resolves to the IP of redis_local-V1

$ docker stop redis_local-V1
$ dig redis.test     # resolves to the IP of the host machine (default)

NetworkManager on Ubuntu

If you're using NetworkManager, you should disable the built-in DNSMasq to get the port binding of port 53 to work.

Edit /etc/NetworkManager/NetworkManager.conf and comment out the line dns=dnsmasq so it looks like this:

# dns=dnsmasq

Restart using sudo service network-manager restart.

Now you should be able to do

docker run -d -v /var/run/docker.sock:/var/run/docker.sock \
    -p 53:53/udp ruudud/devdns