In an effort to debug issues on a cluster, I was trying to determine which machine on the cluster was causing the issue. My idea was that I could insert a header token identifying the server.

My first idea was to add the directive Header add X-Server-Ip "%{SERVER\_ADDR}e in my httpd.conf. However, due to the nature of our load balancer, Apache was somehow resolving this to the load balancer IP address on all machines of the cluster — which was really, really not useful.

I finally stumbled on a good solution, however: you can set environment variables in apachectl, and then pass them into the Apache environment using the PassEnv directive from mod_env; once that's done, you can use the environment variable anywhere.

In my apachectl, I added the line export HOSTNAME=\hostname``. Then, in my httpd.conf, I added first the line PassEnv HOSTNAME, followed by the directive Header add X-Server-Name "%{HOSTNAME}e". Voilá! I now had the hostname in the header, which gave me the information I needed for debugging.