Facter, Docker and the public ip address

Using Docker on a Puppet managed host influences Facter when it tries to find the host ip address.

Which network interface is the best? #

Facter collects some so called facts about the system and provides them to Puppet modules. When using a fact like :ipaddress you’ll see that Facter only uses the output of the native ifconfig command, sorts all existing interfaces by name and takes the first non local interface as result. Facter then sets :ipaddress, :macaddress and :netmask as provided by that interface.

Most Linux based systems use eth0 as interface name for the public interface. This seems to be ok in most cases, but after installing Docker you’ll see an additional interface docker0. Facter now reads the network configuration from the Docker interface, which can lead to problems in your Puppet modules.

The same problem also arises without Docker when you have more than one interface installed. Which interface would you Facter expect to use, eth0 or eth1?

Telling Facter the truth #

Confronting your favourite search tool with the described behaviour (you’re never the first person having your problem, don’t ya?) should link you to the PuppetLabs Q&A page about the issue. The given answer provides three options to overcome the problem:

File a bugreport
Write a custom fact ipaddress_primary that returns your desired value
Use ipaddress_eth0 instead of ipaddress

Filing a bugreport is probably not a good idea, since Facter cannot know enough about which interface you expect. Using the dedicated ipaddress_eth0 should be valid, when you can be sure that the interface name is static, e.g. by setting it in your Puppet modules. Sadly, I couldn’t use the third option, because I had different interface names on different servers.

Just for the record: there’s at least a fourth option: manually set Docker’s interface name so that it appears after eth0 in the sorted list of interfaces. Well, sometimes you can’t simply change interface names, and I preferred to make Facter work as expected without changing the world around it.

Overriding facts #

So I tried to write a custom fact - the documentation helped a lot and it’s quite easy. I had the idea of overriding the built in :ipaddress fact, but that doesn’t seem to be possible by using custom facts. Luckily, PuppetLabs have provided a way of overriding facts by the use of environment variables: instead of adding custom facts, you can set a variable like FACTER_ipaddress="192.168.178.42".

Facter accepts any fact name after the prefix FACTER_, so you can override any fact you like. In my case the ip addresses are static, so this is a very convenient way of making Factor work together with Docker. When using dynamic ip addresses, you should check if any of the other suggestions above work for you.

You can manage /etc/environment with Puppet, so the environment variables are available without manually editing files on a server. My /etc/environment now looks like this:

FACTER_ipaddress="192.168.178.42"
FACTER_netmask="255.255.255.0"

Happy shipping!