Banning based on forwarded IP

[stashymane]

As noted in this discussion, fail2ban currently only uses the request's IP for banning. This makes it impossible to properly ban IPs when behind a proxy, like Cloudflare.
A configuration option to pull the IP from a request header or other source would be ideal.

[fliot]

100% agree on this very important feature.

[zhangjunjie6b]

I have to turn off the proxy function of Cloudflare for the same issue

[johnbizokk]

Indeed! Without support for blocking the IP address from the x-forwarded-for header, the plugin is non-functional when the traefik is behind a proxy (such as Cloudflare).

[tomMoulard]

Indeed this could be a very important feature. It would be nice to have a configuration like the source criterion of the InFlightReq middleware.
I'll be waiting for a PR until I'll do it myself in a while.