A common way to access a Docker container from the outside is to use the publish ports options (-P or -p). However if you are running your containers with the default networking setup you can access the container ports directly by using the container IP address you can find using docker inspect on a running container. Although convenient this is hardly useable since the IP address is dynamically (read: randomly) assigned on the container startup. But there is still hope if you have a name server supporting dynamic updates.
As a Docker administrator…
As a Docker administrator I want a simple way to access the containers so that I won’t have to use docker inspect and update gazillions of configuration files when I need to access unpublished container ports. The answer to this story will be to leverage the bind9 DNS server dynamic update feature to assign a fixed hostname to a Docker container every time it get started.
Prerequisites
In order to had this working you first need to have a working bind9 server with the dynamic updates enabled. I found two comprehensive articles explaining the setup required have dynamic dns updates working.
Implementation
I shamelessly based my implementation on a very interesting post from Kelly Becker. I a nutshell the script will run as a daemon and will use docker events to get notified of starting containers and then:
- inspect the starting container to get its IP address, hostname and container name
- use
nsupdateto add: - an
Arecord associating the container IP address to the container hostname - a
CNAMErecord to defind the container name as an alias to the hostname
And voilà. The script as well as some files to use it with upstart are available on github.
Differences from Kelly’s script
With the exception of the implementation language the difference between Kelly’s script and mine is the use of namespaces. The original script was using ip netns to make the DNS updates from inside the container. I removed this part as it was adding a great deal of complexity to the script and was not needed in my case. The other difference is the addition of the CNAME to be able to reference a container by its name in addition to the hostname.