Manual:$wgInternalServer

HTTP proxy (CDN) settings: $wgInternalServer
Internal server name as known to Squid, if different than `$wgServer` .
Introduced in version:	1.2.0
Removed in version:	Still in use
Allowed values:	(URL string)
Default value:	`false`, falling back to $wgServer
Other settings: Alphabetical \| By function

Details

Internal server name as known to Varnish (or other CDN servers), if different from $wgServer . e.g.:

$wgInternalServer = 'http://yourproxyserver.tld:8000';