A libnss plugin for Docker

So we’ve been starting to use Docker at Infoxchange. A little while ago my Infoxchange colleague Ricky Cook wrote a Twisted service to resolve Docker container names into their private IP addresses.

I thought this was a pretty neat idea, although you had to do some things like return SERVFAIL to round-robin to another DNS server if it wasn’t a Docker host. However it got problematic when something like dhclient/NetworkManager was in charge of your resolv.conf.

I realised on Friday night the the Truely UNIX solution was to write an NSS plugin. This was not quite as trivial as I thought it would be. However I spent a bit of time staring at Lennart’s mdns plugin and started writing one of my own.

docker-nss should work well enough to resolve container IDs and temporary assigned names that you get from docker ps.

There’s a bit of work to do. It can’t reverse IPs back into container names yet. Nor does it return a full list of aliases for a container. Nor does it return a list of IPs for an image name. Patches welcome! It currently depends on GLib, which drags GLib into the memory space of every process on your system, however it wouldn’t be too hard to port away from it (I’m just lazy).

It has unit tests via a mocked docker client. You do have to have modified your nsswitch.conf in order to be able to run the tests. Although if this really bothers someone we could make the test suite itself a Docker container.

Author: Danielle

Danielle is an Australian software engineer, computer scientist and feminist. She doesn't really work on GNOME any more (sadly). Opinions and writing are solely her own and so not represent her employer, the GNOME Foundation, or anyone else but herself.