Manual:$wgApplyIpBlocksToXff

Details
When it's set to true, IP address 1>Special:MyLanguage/Help:Blocking users|blocks apply also to users having a blocked IP in their XFF header (X-Forwarded-For).

Useful when 1>Special:MyLanguage/Manual:Combating vandalism|dealing with vandals making use of well-behaving proxies.