You might be interested in this one...
...as well (I forgot to add).
Even more, ShadowAce, our Linux listkeeper.
Apache:
1. Enable mod_headers (however done in the config)
2. RequestHeader unset Proxy early
https://www.apache.org/security/asf-httpoxy-response.txt
IIS:
appcmd set config /section:requestfiltering
/+requestlimits.headerLimits.[header='proxy',sizelimit='0']
https://support.microsoft.com/en-us/kb/3179800