There are some good articles out there about using docker containers as Jenkins slaves.  There are many good reasons to do this. My use case was, we have some special snowflake test setups that didn’t play well together, but didn’t require all of a dedicated machine, so creating docker images for them, and sharing a machine, made more sense.

Mostly It’s Straight Forward

The posts out there cover the topic well.  Basically there’s a Jenkins Plugin needed, the Docker image setup, and then some configuration of the Docker engine on the host machine.

But, There are Always Challeges

I hit three bumps:

1. Setting up you Docker engine to accept remote requests. This is mentioned in most of the articles, and usually covered to one degree or another. The snag is, pretty much every OS’s installation of Docker is a little different. So while I knew I had to add the “-H tcp://0.0.0.0:2375” argument to my dockerd, finding out how on our RedHat install took a bit of doing.
2. Dealing with AWS’ security groups.  If you’re talking to port 2375, well obviously that port needs to be open. Duh. But that only got me so far, containers fired up but Jenkins’ builds hung.  What wasn’t immediately apparent was, that the ssh communication to the slaves wasn’t going to happen on the traditional port 22. Yes, inside the container it would listen on port 22, but that would be mapped externally to a port in a range of numbers.  So my AWS security group needed to have that range open for inbound connections too.  Using docker inspect on the containers that resulted allowed me to see what they were exposing 22 as. I’m not sure I got “the range”, but I got “a range” that’s worked so far.
3. The image I built had a banner and messages up login. That confused Jenkins apparently. Once I had it so no messages were displayed when I ssh’d in that resolved that issue.

Working

With those two issues addressed, I’ve now got the special snowflake setup as docker images, and Jenkins spins those up and tears them down as needed.  I’m not entirely happy with the “port range” business, and may revisit it, but for now, like most things Jenkins, its working even if there’s a bit of a code smell.