More Uses for Generic Apache Logging

In my last post, I showed how it was possible to fix Apache logging so that it would properly log client IPs accessing a back-end through a reverse proxy. It turns out that logging request headers in that fashion can be handy in other contexts as well. Last night, while debugging a vexing and time-sensitive software issue, we found the need to figure out how large the payload of a POST operation was. The normal combined┬álog format only shows the amount of data returned to the client, so I needed to start hacking again. It turns out that the Content-Length header that’s set during POSTs was just what we needed. I just added the following to our LogFormat directive, and we started getting the data we were looking for.

CL:%{Content-Length}i

Note that the “CL:” bit is just to make a distinction between the Content-Length size and other arbitrary numerical fields I’ve added to on to that logging format. It’s just a label, and not necessary to make things work.

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