Tracking data flow between apache and a reverse proxy?

Question

I have a web app running as a reverse-proxy behind Apache httpd. Every now and then, my web requests get the wrong thing returned. In particular, a request for the main page, an image, a css file, or a javascript file will return instead a previously generated version of the main page, as text/plain, missing the newline character after the doctype.

I can't figure out where the problem lies -- I've turned off all caching that I know about in httpd. When the problem occurs, the web-app's log functionality doesn't show a request, while httpd's does.

Is there some way that I can watch what is going on between httpd and the web-app so that I can see if the error is with the web-app or with httpd? I'm not sure I know how to capture this…

If it helps, the web-app is a Plack-based app, running under Starman.

Answer 1

Of course, the act of posting the question, and phrasing it properly, makes it obvious that what I was looking for was tcpdump.

Although I don't know if I've solved the actual problem yet, the answer to the question I was asking (how to look at what is going on between Apache and the web-app) is to use tcpdump (or another libpcap-or-similar based tool) to capture the data.