The information in this section covers the embedded DNS server operation for containers in user-defined networks. DNS lookup for containers connected to user-defined networks works differently compared to the containers connected to default bridge network.
Note: In order to maintain backward compatibility, the DNS configuration in
default bridgenetwork is retained with no behavioral change. Please refer to the DNS in default bridge network for more information on DNS configuration in thedefault bridgenetwork.
As of Docker 1.10, the docker daemon implements an embedded DNS server which provides built-in service discovery for any container created with a valid name or net-alias or aliased by link. The exact details of how Docker manages the DNS configurations inside the container can change from one Docker version to the next. So you should not assume the way the files such as /etc/hosts, /etc/resolv.conf are managed inside the containers and leave the files alone and use the following Docker options instead.
Various container options that affect container domain name services.
In the absence of the --dns=IP_ADDRESS..., --dns-search=DOMAIN..., or --dns-opt=OPTION... options, Docker uses the /etc/resolv.conf of the host machine (where the docker daemon runs). While doing so 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.