[Bender]

load balancer / proxy, since you need a way to get the real ip

If it is at all helpful most layer-7 load balancers have an option to enable a header that contains the real IP address, then one need only reconfigure their web daemon to log that header. It's very similar to how CDN's send the real IP in a header.

[inigyou]

And it's almost certainly exploitable by a client sending that same header.

[Bender]

Most load balancers will silently drop the real IP address header on the ingress side to avoid exactly what you are describing. That used to be a thing around 2000 - 2001 era. If you find a load balancer with that issue today open a case with their vendor.

[inigyou]

Not reliably enough. Look for parser differentials.

[63stack]

Please enlighten us

[inigyou]

Differences in parsing between two systems can allow data to sneak through. For example you might put a space before the header name. The reverse proxy might treat it as a header name starting with a space, so not matching the one it wants to remove, and not removed, while the origin server trims the space.

This is a ubiquitous security concern, like IDOR and memory safety. Every time you have a separate validation step for anything, you have the possibility of a parser differential vulnerability.

If you don't know it yet, look up "HTTP request smuggling" - pretty interesting.