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"'. Voila! I now had the
hostname in the header, which gave me the information I needed for
debugging.
