Manual:$wgSquidServers

Details
A list of proxy servers (IPs if possible) to purge on changes. Don't specify ports here (80 is default). eg:

Any IPs listed in this array will be treated as trusted surrogates (reverse proxies). IP addresses displayed for users connected via these Squid servers therefore will match individual user IP's, not the Squid's IP.

''Note: In versions prior to 1.4.0 this setting was not defined in DefaultSettings.php (although its usage was commented there). This may have been because it wasn't implemented properly or may simply have been an oversight.''