How to configure Apache web server to redirect the requests it receives without modifying the source IP

I always wrestle with a problem for hours before giving up and posting a question - only to solve my own problem minutes after posting ...

For those interested, the solution is not to use ProxyPassReverse, but rather to use the Header directive - it' let's you mess with outbound headers. In this case, I can capture the Location response header and run a regex on it to fix the URL's protocol:

ProxyPreserveHost       on
RewriteEngine           on
RewriteRule ^/app$ /app/ [PT]
ProxyPass /app ajp://localhost:8009/app
Header edit Location ^http(\:\/\/proxy.*)$ https$1

Voila!

If apache complains, it might be that mod_headers is not yet enabled: a2enmod headers

I would use tcpdump or ngrep, it would be nice to have the port on the switch connected to the web server mirrored, but in absence of that, you can run ngrep or tcpdump on the server itself.

You will need superuser access to run either of those programs.

You're going to want to read a bit, as you obviously know what you are looking for in the traffic, ngrep does allow you to select traffic by regex, which might allow you to pick out packets with better accuracy.

ngrep -l -q -d eth0 "^POST " tcp and port 80 -O dump.file

This would get you any HTTP data POSTed to port 80 on eth0. You might be able to pick out something much more specific. If you are going to be reading the traffic directly from the file, you might want to add -W byline as it makes the packets much more readable as it respects the line breaks, so you can see the packet written out more logically (for humans). -O dump.file will write the output of your packet capture to a file. The output can be as detailed as you'd like, to replay the packets, have a look at tcpreplay