PROXY Protocol

A protocol created by HAProxy that prepends a small header to a proxied TCP connection containing the original client's IP address and port. When a load balancer or proxy forwards TCP traffic to a backend, the original client IP is normally lost because the backend sees only the proxy's address. PROXY Protocol v1 uses a human-readable ASCII header; v2 uses a binary format for efficiency. Backend applications must explicitly support PROXY Protocol to parse the header; it is supported by Nginx, HAProxy backends, and many cloud load balancers. Not to be confused with the HTTP X-Forwarded-For header, which works at Layer 7.