I was recently involved in setting up a complex load balanced Auto-scaling multi server setup and to make life easy I wanted to set a header that contained the servers hostname so it was clear which server behind the load balancer satisfied each request.

I thought this would be easy... Not so much! But I managed it and here's how...

The Options

There are 2 options and the one you want depends on which version of Apache you're using. You're more than likely using 2.4.7 as that's the version that ships with Ubuntu 14.04 LTS.

If you are using <2.4.10 you will need to setup an environment variable as described next, other wise you can use the more elegant expression method, see even further below for that one.

Environment Variables

There is an envvars file, found on Ubuntu in /etc/apache2/envvars, that defines a number of environment variables Apache uses for various things. We can define our own variable in here and set it to the servers hostname by adding the following line

export HOSTNAME=$(hostname);

You will need to restart Apache, yes restart, reload isn't enough, to get this to stick; once you do you should be able to access the hostname by using ${HOSTNAME} in your vhosts, making it easy to set the header:

Header set X-Host "%{HOSTNAME}"

Header Expressions

As mentioned above if you are using Apache 2.4.10 or later then life becomes a lot easier as you can pass an expression to the value portion of the Header directive.

The expressions we can pass include a bunch of functions (the full list can be found here). We're interested in the file function as we can use this to read the contents of the /etc/hostname file that contains our hostname.

So you will end up with something that looks like this:

Header set X-Host "expr=%{file:/etc/hostname}"

After a reload you should see your X-Host header appear.

One thing about this method, it seems the file is read on every request which would be really useful if the file was changing but I don't know about you but I don't tend to change my hostname all that often so it may add a little unnecessary overhead especially for high traffic sites.