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Identification Protocol
Port 113 was historically used for the Identification Protocol (Ident), a service that identifies the user of a particular TCP connection. This service allowed remote servers to query a client system to determine which user initiated a connection request, facilitating logging and access controls. While once commonly enabled on servers and network devices, Ident's use has declined substantially due to privacy concerns, inherent security risks, and the development of more secure authentication methods..
The Identification Protocol (Ident), defined in RFC 1413, was designed to provide a means for servers to determine the identity of a user initiating a TCP connection. When a client connects to a server, the server may open a connection back to port 113 on the client to perform an Ident query. The client responds with the username associated with the connection's local endpoint. This was especially useful on multi-user systems, such as Unix servers and IRC clients, helping identify individual users on shared IP addresses.
The protocol operates in plain text, listening on TCP and UDP port 113. Requests are simple text queries containing the server- and client-side port numbers associated with the original connection. The responding host returns a response with system and user information. Due to its simplicity, Ident had minimal overhead but lacked any native encryption or authentication mechanisms.
While effective in early networking environments, the protocol gradually fell out of favor. Nowadays, most modern operating systems disable the Ident service by default, and firewalls typically block inbound connections on port 113. Alternative authentication frameworks, including more secure identity management and logging solutions, have better addressed the needs once fulfilled by Ident.