The PROXY protocol is supported by several proxy servers such as haproxy
and Amazon ELB. This protocol allows services running behind a proxy to
learn the remote address of the actual client connecting to the proxy,
by including a single textual line at the beginning of the TCP
connection.
http://www.haproxy.org/download/1.5/doc/proxy-protocol.txt

There are several Go libraries for this protocol (such as
https://github.com/armon/go-proxyproto), which operate by wrapping a
net.Conn with an implementation whose RemoteAddr method reads the
protocol line before returning. This means that RemoteAddr is a blocking
call.

(Blocking inside Accept is not an option either; the point is that we want srv.Serve to call Accept on the underlying socket as soon as the previous Accept call succeeded, without any blocking operations.)

This exists in current Go master (eg, 1.5.1) on all platforms (eg, Linux).