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Port 113 Details

known port assignments and vulnerabilities
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Port(s) Protocol Service Details Source
113 tcp,udp IDENT Port 113 used for Identification/Authorization service. When a client program on your end contacts a remote server for services such as POP, IMAP, SMTP, IRC, FTP, etc. that remote server sends back a query to the IDENT port 113 asking for identification from your system...

Port 113 can be probed by attackers and it poses some security concerns, but the problem with filtering/stealthing port 113 is that if legitimate requests get no response at all from port 113 queries, the connection to them (which initiated their query in the first place) will be delayed or perhaps even completely abandoned.

The simplest solution is to close, rather than filter port 113.

Some trojans also use this port: ADM worm, Alicia, Cyn, DataSpy Network X, Dosh, Gibbon, Invisible Identd Deamon, Kazimas, Taskman,W32.Korgo.F
W32.Bofra.C@mm (11.11.2004) - It opens ports 1639/tcp and 1640/tcp for listening, opens an ident daemon on port 113/tcp, connects to IRC servers on port 6667/tcp.
W32.Linkbot.A (11.05.2004) - worm that exploits the MS Windows LSASS Buffer Overrun Vulnerability. It also creates an IRC backdoor and attempts to install adware on the infected machine. It can affect all current Windows versions. Listens on port 113/tcp for remote commands.
W32.Spybot.LZI (04.06.2005) - worm that attempts to exploit the MS DCOM RPC vulnerability on ports 135, 445 & 1025. Opens a backdoor on port 113.
W32.Linkbot.M (05.24.2005) - opens a backdoor on port 6667/tcp. Also listens on port 113/tcp.

Stack-based buffer overflow in TinyIdentD 2.2 and earlier allows remote attackers to execute arbitrary code via a long string to TCP port 113.
References: [CVE-2007-2711] [BID-23981] [SECUNIA-25248] [OSVDB-36053]
113 udp ident - old user identification system, still used by IRC servers to identify users (official) Wikipedia
113 tcp,udp Authentication Service (auth) (official) Wikipedia
113 tcp trojan ADM worm, Alicia, Cyn, DataSpy Network X, Dosh, Gibbon, Taskman Trojans
113 tcp ident ident IANA
113 tcp,udp auth Authentication Service IANA
113 tcp,udp auth ident tap Authentication Service SANS
113 tcp,udp auth ident, tap, Authentication Service Nmap
113 tcp,udp auth ident Authentication Service Neophasis
113 tcp InvisibleIdentdDaemon [trojan] Invisible Identd Daemon Neophasis
113 tcp InvisibleIdentdDeamon [trojan] Invisible Identd Deamon Neophasis
113 tcp Kazimas [trojan] Kazimas Neophasis
113 tcp threat Invisible Identd Deamon Bekkoame
113 tcp threat Kazimas Bekkoame
113 tcp threat Randex Bekkoame
113 tcp threat W32.Bofra Bekkoame
113 tcp threat W32.Korgo Bekkoame
113 tcp threat W32.Linkbot Bekkoame
113 tcp threat W32.Mydoom Bekkoame
113 tcp threat W32.Spybot Bekkoame
20 records found
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External Resources
SANS Internet Storm Center: port 113

Well Known Ports: 0 through 1023.
Registered Ports: 1024 through 49151.
Dynamic/Private : 49152 through 65535.

TCP ports use the Transmission Control Protocol. TCP is the most commonly used protocol on the Internet and any TCP/IP network. Whereas the IP protocol deals only with packets, TCP enables two hosts to establish a connection and exchange streams of data. TCP guarantees delivery of data and that packets will be delivered in the same order in which they were sent. Guaranteed communication/delivery is the key difference between TCP and UDP.

UDP ports use the Datagram Protocol, a communications protocol for the Internet network, transport, and session layers. Like TCP (Transmission Control Protocol), UDP is used with IP (the Internet Protocol) and makes possible the transmission of datagrams from one computer to applications on another computer, but unlike TCP, UDP is connectionless and does not guarantee reliable communication; it's up to the application that received the message to process any errors and verify correct delivery. UDP is often used with time-sensitive applications, such as audio/video streaming, where dropping some packets is preferable to waiting for delayed data.

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