Configure container DNS

Estimated reading time: 5 minutes

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:

root@f38c87f2a42d:/# 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.

`-h HOSTNAME` or `--hostname=HOSTNAME`	Sets the hostname by which the container knows itself. This is written into `/etc/hostname`, into `/etc/hosts` as the name of the container's host-facing IP address, and is the name that `/bin/bash` inside the container will display inside its prompt. But the hostname is not easy to see from outside the container. It will not appear in `docker ps` nor in the `/etc/hosts` file of any other container.
`--link=CONTAINER_NAME` or `ID:ALIAS`	Using this option as you `run` a container gives the new container's `/etc/hosts` an extra entry named `ALIAS` that points to the IP address of the container identified by `CONTAINER_NAME_or_ID`. This lets processes inside the new container connect to the hostname `ALIAS` without having to know its IP. The `--link=` option is discussed in more detail below. Because Docker may assign a different IP address to the linked containers on restart, Docker updates the `ALIAS` entry in the `/etc/hosts` file of the recipient containers.
`--dns=IP_ADDRESS...`	Sets the IP addresses added as `nameserver` lines to the container's `/etc/resolv.conf` file. Processes in the container, when confronted with a hostname not in `/etc/hosts`, will connect to these IP addresses on port 53 looking for name resolution services.
`--dns-search=DOMAIN...`	Sets the domain names that are searched when a bare unqualified hostname is used inside of the container, by writing `search` lines into the container's `/etc/resolv.conf`. When a container process attempts to access `host` and the search domain `example.com` is set, for instance, the DNS logic will not only look up `host` but also `host.example.com`. Use `--dns-search=.` if you don't wish to set the search domain.
`--dns-opt=OPTION...`	Sets the options used by DNS resolvers by writing an `options` line into the container's `/etc/resolv.conf`. See documentation for `resolv.conf` for a list of valid options

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.

docker, bridge, docker0, network