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.

By default it will provide thoses domain:

• container_name.docker
• container_name.network_name.docker
• docker-composer_service.docker-composer_project.docker
• docker-composer_service.docker-composer_project.network_name.docker

easy install:

sudo sh -c "$(curl -fsSL https://raw.githubusercontent.com/jderusse/docker-dns-gen/master/bin/install)"

How it works

The container dns-gen expose a standard dnsmasq service. It returns ip of known container and fallback to host's resolv.conf for other domains.

Requirement

In order to ease the configuration of your host. We recommand to install resolvconf

apt install resolvconf

The container have to listen on port 53 on docker0 interface. You should assert that nothing else is listening to that port.

sudo netstat -ntlp --udp|grep ":53 "

If some service are listening to the same port, you should changes your setting to exclude the docker0 interface. For instance, in dnsmasq use :

except-interface=docker0

Q/A

Why mounting the entier host in the container (-v /:/host)

The dnsmasq embeded inside the container is configured to fallback to the default resolv.conf. But, given the host is configured to use the container to resolve DNS, and to avoid infinite loop, the container uses the hosts's /etc/resolv.conf without it own IP.

Mounting the -v /etc/resolv.conf:/etc/resolv.conf file is not possible as this file is often overriden by the host (changing network, connecting to wifi, connecting to a VPN) and docker wouldn't update the mounted file. And, in most of the cases, this file is a symlink to another location, and the the target should be accessible by the container too.

For instances, when using resolvconf on Debian 9:

/etc/resolv.conf -> /etc/resolvconf/run/resolv.conf
/etc/resolvconf/run -> /run/resolvconf

Why not using the container's /etc/resolv.conf file?

First of all, because the user could have configured the /etc/docker/daemon.json to use a specific name server inside the container, whereas we are trying to configure the host's DNS resolution.

Moreover, from my tests, the file is not updated when the host changes it name servers.

Why using the host network

Some distributions (like ubuntu) use a local dnsmasq instance to resolv DNS This instance, runing on the host local network, should be accessible by the container. Using the host network is the easiest way to be as close as possible to the host.

Previous version of docker-gen

If you already install and configured a previous version of dns-gen you should uninstall it.

• uninstall dnsmasq
• remove previous container docker rm -fv dns-gen
• remove custom configuration in /etc/docker/daemon.json
• remove /etc/NetworkManager/dnsmasq.d/01_docker
• restart the service NetworkManager (and in some case the machine)

Manual Install

Just run the all in one script ./bin/install

If you want to deep inside, here are the manuall steps to permform the same

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:

GATEWAY=$(ip -4 addr show docker0 | grep -oP '(?<=inet\s)\d+(\.\d+){3}')
echo ${GATEWAY}

Now, you can start the dns-gen container:

docker run -d --name dns-gen \
  --restart always \
  --net host \
  -e GATEWAY=$GATEWAY \
  --log-opt "max-size=10m" \
  --volume /:/host \
  --volume /var/run/docker.sock:/var/run/docker.sock \
  jderusse/dns-gen:2

You can test the container

docker run --name test nginx:alpine
dig test.docker @${GATEWAY}

Once OK, you can finally update your local resolver

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

beware! When your host will restart, it may change the IP address of the docker0 interface. You may have to run the command bin/install to fix your configuration.

Or you can force docker to always use the same IP by editing the /etc/docker/daemon.json file and adding:

{
  "bip": "172.17.42.1/24"
}

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

Troubleshooting

To see the list of register DNS, dump the content of the generated dnsmasq.conf

docker exec dns-gen cat /etc/dnsmasq.conf

On restart, if you loose the dns resolution, check the NetworkManager service status.

service NetworkManager status

Check the syntax of the /etc/resolv.conf file which should contain at the begging the IP of the docker0 interface:

nameserver 172.17.42.1

Check the containers logs

docker logs --tail 100 -f dns-gen