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:ro 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 the configured fallback DNS (default 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:ro ruudud/devdns
$ docker run -d --name redis redis:alpine
$ docker run -it --rm \
  --dns=`docker inspect -f "{{ range.NetworkSettings.Networks }}{{ .IPAddress }}{{ end }}" devdns | head -n1` alpine \
  ping redis.test

Please note that the --dns flag will prepend the given DNS server to the Docker default, 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 as per the dockerd reference.

The exact process to set these options varies by the way you launch the Docker daemon and/or the underlying OS:

• systemd (Ubuntu, Debian, RHEL 7, CentOS 7, Fedora, Archlinux) — sudo systemctl edit docker.service, change the ExecStart line
• Ubuntu 12.04 — set DOCKER_OPTS in /etc/default/docker
• OS/X — select Preferences -> Daemon -> Advanced

The extra flags you'll have to add are

--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.

Note: There are some caveats with Docker and how it manages a container's /etc/resolv.conf file. Unless you do something exotic, like parsing this file, you should be fine. See Docker DNS docs for more information.

Host Machine → Containers

You will need to add some configuration to your OS DNS resolving mechanism to make it query devdns.

NOTE: This is only practical if you added -p 53:53/udp when starting devdns.

Linux

Nowadays, direct edits of /etc/resolv.conf will often 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

Managed resolv.conf

Another solution is mounting the host machine's /etc/resolv.conf at /mnt/resolv.conf and have devdns automatically add configuration on startup:

docker run -d -v /var/run/docker.sock:/var/run/docker.sock:ro \
      -v /etc/resolv.conf:/mnt/resolv.conf \
      ruudud/devdns

Example config prepended to /etc/resolv.conf:

nameserver 192.168.16.2 # added by devdns

The configuration will be automatically removed when container is stopped or killed.

⚠️ It's common that /etc/resolv.conf becomes overwritten as many operating systems now manage the creation of that file, and in some cases not even rely on it at all.

OSX

Create a file /etc/resolver/test containing

nameserver 127.0.0.1

In OSX and Docker for Mac, port binding should work directly on the host machine. Please note that the name of the file created in /etc/resolver has to match the value of the DNS_DOMAIN setting (default "test").

Configuration

• DNS_DOMAIN: set the local domain used. (default: test)
• FALLBACK_DNS: set the DNS used for unknown hosts. (default: 8.8.8.8)
• 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: '')
• NAMING: set to "full" to convert _ to - (default: up to first _ of container name)
• NETWORK: set the network to use. Set to "auto" to automatically use the first network interface (e.g. when using docker-compose) (default: bridge)

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 NAMING=full \
  -e NETWORK=mynetwork \
  -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:ro \
    -p 53:53/udp ruudud/devdns