Configure container DNS

Configure container DNS

The information in this section explains configuring container DNS within the Docker default bridge. This is a bridge network named bridge created automatically when you install Docker.

Note: The Docker networks feature allows you to create user-defined networks in addition to the default bridge network. Please refer to the Docker Embedded DNS section for more information on DNS configurations in user-defined networks.

How can Docker supply each container with a hostname and DNS configuration, without having to build a custom image with the hostname written inside? Its trick is to overlay three crucial /etc files inside the container with virtual files where it can write fresh information. You can see this by running mount inside a container:

$$ mount
...
/dev/disk/by-uuid/1fec...ebdf on /etc/hostname type ext4 ...
/dev/disk/by-uuid/1fec...ebdf on /etc/hosts type ext4 ...
/dev/disk/by-uuid/1fec...ebdf on /etc/resolv.conf type ext4 ...
...

This arrangement allows Docker to do clever things like keep resolv.conf up to date across all containers when the host machine receives new configuration over DHCP later. The exact details of how Docker maintains these files inside the container can change from one Docker version to the next, so you should leave the files themselves alone and use the following Docker options instead.

Four different options affect container domain name services.

Regarding DNS settings, in the absence of the --dns=IP_ADDRESS..., --dns-search=DOMAIN..., or --dns-opt=OPTION... options, Docker makes each container’s /etc/resolv.conf look like the /etc/resolv.conf of the host machine (where the docker daemon runs). When creating the container’s /etc/resolv.conf, the daemon filters out all localhost IP address nameserver entries from the host’s original file.

Filtering is necessary because all localhost addresses on the host are unreachable from the container’s network. After this filtering, if there are no more nameserver entries left in the container’s /etc/resolv.conf file, the daemon adds public Google DNS nameservers (8.8.8.8 and 8.8.4.4) to the container’s DNS configuration. If IPv6 is enabled on the daemon, the public IPv6 Google DNS nameservers will also be added (2001:4860:4860::8888 and 2001:4860:4860::8844).

Note: If you need access to a host’s localhost resolver, you must modify your DNS service on the host to listen on a non-localhost address that is reachable from within the container.

You might wonder what happens when the host machine’s /etc/resolv.conf file changes. The docker daemon has a file change notifier active which will watch for changes to the host DNS configuration.

Note: The file change notifier relies on the Linux kernel’s inotify feature. Because this feature is currently incompatible with the overlay filesystem driver, a Docker daemon using “overlay” will not be able to take advantage of the /etc/resolv.conf auto-update feature.

When the host file changes, all stopped containers which have a matching resolv.conf to the host will be updated immediately to this newest host configuration. Containers which are running when the host configuration changes will need to stop and start to pick up the host changes due to lack of a facility to ensure atomic writes of the resolv.conf file while the container is running. If the container’s resolv.conf has been edited since it was started with the default configuration, no replacement will be attempted as it would overwrite the changes performed by the container. If the options (--dns, --dns-search, or --dns-opt) have been used to modify the default host configuration, then the replacement with an updated host’s /etc/resolv.conf will not happen as well.

Note: For containers which were created prior to the implementation of the /etc/resolv.conf update feature in Docker 1.5.0: those containers will not receive updates when the host resolv.conf file changes. Only containers created with Docker 1.5.0 and above will utilize this auto-update feature.