/etc/squid/squid.conf:
# TAG: forwarded_for on|off
# If set, Squid will include your system's IP address or name
# in the HTTP requests it forwards. By default it looks like
# this:
#
# X-Forwarded-For: 192.1.2.3
#
# If you disable this, it will appear as
#
# X-Forwarded-For: unknown
#
#Default:
# forwarded_for on