Ident

The Ident Protocol, specified in RFC 1413, is an old internet server system that helps identify the user of a particular TCP connection. One popular ident daemon is identd.

When a user or program at computer A makes an ident request of computer B, it may only ask for the identity of users of connections between A and B. The ident server on B listens for connections on TCP port 113. The client at A establishes a connection, then specifies which connection it wants identification for by sending the numbers of the ports on A and B that the connection is using. The server at B determines what user is using that connection, and replies to A with a string that names that user.

Though it is still used on most IRC networks, today many people consider ident useless and unsafe to keep running. While the ident port (113) can be hard to set to "stealth" or "filtered" mode with some hardware firewalls, this can be circumvented by forwarding all requests to an arbitrary, non-existent IP address within the network.