ExtractIP calls ExtractIPFromHeader(r) to extract a valid IP address.

ExtractIPFromHeader extracts host information from `Forwarded`, `X-Real-IP`, `X-Forwarded-For` headers, in this order.

ParseIP returns the IP address parsed from a given input if a valid IP can be extracted.

ParseTCPAddr returns a net.Addr parsed from a given input, if it is a valid IP:port pair.

RemoteAddr returns the remote address for the HTTP request.

UserAgentHeader fetches all `user-agent` values from a given header and combines them into one string.