I think they're referring to this protocol: http://www.haproxy.org/download/1.8/doc/proxy-protocol.txt http://www.haproxy.org/download/1.8/doc/proxy-protocol.tx

I think they're referring to this protocol: http://www.haproxy.org/download/1.8/doc/proxy-protocol.txt http://www.haproxy.org/download/1.8/doc/proxy-protocol.txt It wraps a TCP connection (not just HTTP) and forwards it, and preserves the originating IP information as a small header on the proxied connection. It's supported beyond HAProxy, too; e.g., AWS ELBs support it. I've used it when forwarding TLS connections, but not wanting to lose the remote IP information. (And not wanting to decrypt on the ELB, which is required to add, e.g., an HTTP Forwarded-For header.)