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Port 11 Details

known port assignments and vulnerabilities
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11 tcp,udp systat system / active users information.

On some UNIX machines, creating a TCP connection to this port will dump the active processes and who launched them. The original intent for this was to make remote management of UNIX easier. However, intruders will query the systat information in order to map out the system.
This service is rarely available anymore because of these security concerns.
On UNIX, there are also local commands that show this information, such as systat or ps.

Skun trojan also uses this port.

See also: [RFC866]
11 tcp,udp Active Users (official) Wikipedia
11 tcp trojan Skun Trojans
11 tcp,udp systat Active Users IANA
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Related ports: 15  

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External Resources
SANS Internet Storm Center: port 11

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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