Squid (caching proxy)
See: http://www.squid-cache.org/
Custom Logging
Custom logging formats are easy to set up, and can be helpful in diagnosing traffic details (for instance for checking HTTP headers):
The following log has a human-readable datestamp, the apparent IP (which could be an upstream proxy or a load balancer for a reverse proxy configuration) and then three HTTP header fields to show some X-Forwarded-* header details:
logformat bjdeanlog %tl %>a %{x-forwarded-for}>h %{x-forwarded-proto}>h %{x-forwarded-port}>h
access_log /var/log/squid3/bjdean.log bjdeanlog
See: http://www.squid-cache.org/Doc/config/logformat/
Trusting X-Forwarded-* headers
If you trust up-stream providers of X-Forwarded-* headers (for instance where an X-Forwarded-For header was set by a load-balancer in front of squid) use follow_x_forwarded_for:
acl load_balancer src 10.1.2. follow_x_forwarded_for allow load_balancer
See: http://www.squid-cache.org/Doc/config/follow_x_forwarded_for/