autofs in docker containers · ./scriptthe.net

Today I started writing up a backupninja container for work. This container needs to be able to:

Login into some of our prod boxes
Store backup data on an NFS share

The logical choice for handling the back-end was to use autofs because of its ability to handle mounts that may drop out for whatever reason, and since we really need our storage available, doing a plain mount is just not going to cut it.

So in my dockerfile, apt-get install autofs

Additionally, autofs actually runs automounter in daemon mode. Since I’m using supervisor as my init, that needed to change:

BROWSE_MODE="no" /usr/sbin/automount -t 0 -f /etc/auto.master

This command runs with the same parameters as a basic autofs install on ubuntu (as defined by vars /etc/default/autofs), with one exception, -t. I don’t think a lot of people know this but autofs actually:

Mounts the storage when you enter/ask for the storage/mountpoint
Actually unmounts after a specified period of time, defined by the -t (or TIMEOUT variable)

Keeping that in mind, setting -t 0 disables unmounting. I like this as it decreases the time to mount when wanting to reuse the share at a later time. This kind of thing is useful in a fairly stable environment and especially nice when browsing to your mounts via a GUI.

With that out of the way, there is one more requirement before automounter will actually run correctly: loading the kernel module, autofs4 Unfortunately, this is not possible inside a docker container, yes, even a privileged one. I won’t get into why, but will say that:

On ubuntu 14 systems, you’ll have to run a modprobe autofs4 on the host before autofs in your container will work. Note that you do not need autofs installed on the host to do this.
On coreos systems, autofs4 is already loaded, presumably because of systemd automount

On all systems, you will need to run your container in --privileged mode!!! If you don’t, you’ll see the likes of:

/usr/sbin/automount: test mount forbidden or incorrect kernel protocol version, kernel protocol version 5.00 or above required.

Finally, I wanted to make note of how I handle defining my mounts to autofs. As you might know, the /etc/auto.master file defines locations and associated files that define how to mount to those locations. For my purposes, I decided to just define a single file that lists the mount points I want autofs to handle:

## Setting up our autofs mount(s)
RUN echo "/- /etc/auto.mounts" >> /etc/auto.master
ADD auto.mounts /etc/auto.mounts

In our auto.mounts, we merely define each mount, one per line:

/mnt -fstype=nfs,nolock,rw,bg,soft,intr,timeo=5,retrans=5,actimeo=10,retry=5 nas.example.net:/var/shared
/house -fstype=nfs,rw,bg,soft,intr,timeo=5,retrans=5,actimeo=10,retry=5 goliath:/mnt

For more information on setting up autofs, check out the ubuntu docs.