I need a memory hook for this. All the time I search for things like proxy, forward – but never for „rpaf“ (from the description: „rpaf is short for reverse proxy add forward“).
So, purely for me, this is the $!%/%%%$$$$ module:
aptitude install libapache2-mod-rpaf
** Update Day 295/2016: For newer Apaches >= 2.4 the module mod_remoteip is recommended.