I think it should probably be 'Forbidden By Proxy', just like there are separate codes for 'Proxy Authentication Required' and 'Authentication Required'. Expli

I think it should probably be 'Forbidden By Proxy', just like there are separate codes for 'Proxy Authentication Required' and 'Authentication Required'. Explicitly mentioning a proxy is helpful because it discourages the use of this code in cases where there isn't already a proxy. The basic principle which has allowed the Internet to be so successful has been that IP packets don't need to be deeply inspected, and that the contents of a packet between two endpoints only needs to be interpreted by those endpoints. Ideally, ISPs should try to avoid trying to speak protocols like HTTP in response to a packet destined for an IP address they don't own; they could instead, for example, send an ICMP Destination Unreachable (Host Administratively Unreachable or Communication Administratively Prohibited) packet.