Internet-Security.com TCP port 113 details

TCP port 113 is used by the Ident (AUTH) service defined in RFC 1413.
IRC servers such as UnrealIRCd and InspIRCd (used by networks like Libera Chat and EFnet) routinely query port 113 to verify a client’s ident/username.
Many IRC clients and bouncers provide an ident daemon to answer these queries, e.g., mIRC and HexChat (built-in identd), irssi with oidentd/pidentd, and ZNC via its ident module.
Mail transfer agents have supported RFC1413 lookups for logging/policy; for example, Sendmail and Exim can perform ident queries on incoming SMTP connections.
FTP servers such as ProFTPD support optional ident lookups (IdentLookups directive) for connection logging.
Classic TCP Wrappers (tcpd) and xinetd can perform RFC1413/IDENT user lookups on incoming TCP connections to enhance logs.
Security note: identd can disclose local usernames, so attackers often probe port 113 for reconnaissance, and historical identd vulnerabilities led many admins to block it by default.

TCP 113