Manual:$wgCdnServersNoPurge/en

Details
This variable is about telling MediaWiki an array of  blocks or individual IP addresses where any IP address that is part of the range has to be considered as a Proxy server. This means that  headers will be respected for requests from IP addresses or ranges on this list.

Same as, except that MW does not send PURGE requests to clear cache. You can use this setting for proxy servers that do not cache content or are purged by a different means such as Purged.

All documentation in Manual:$wgCdnServers#Configuration example with Varnish is valid, but with difference described below.