The TCP Port Service Multiplexer (TCPMUX) is a little-used
Internet
The Internet (or internet) is the Global network, global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a internetworking, network of networks ...
protocol defined in . The specification describes a
multiplexing
In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource� ...
service that may be accessed with a
network protocol
A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any variation of a physical quantity. The protocol defines the rules, syntax, semantics, and synchronization of ...
to contact any one of a number of available
TCP services of a host on a single, well-known
port number
In computer networking, a port is a communication endpoint. At the software level within an operating system, a port is a logical construct that identifies a specific process or a type of network service. A port is uniquely identified by a numbe ...
.
The specification of TCPMUX, RFC 1078, was deprecated in 2016 by for technical reasons and lack of use in the Internet.
[RFC 7805, ''Moving Outdated TCP Extensions and TCP-Related Documents to Historic or Informational Status'' (April 2016)]
Description
A host may connect to a server that supports the TCPMUX protocol on TCP port 1. The host then sends a name of the service required, followed by a pair of carriage return and line feed characters (
CRLF
A newline (frequently called line ending, end of line (EOL), next line (NEL) or line break) is a control character or sequence of control characters in character encoding specifications such as ASCII, EBCDIC, Unicode, etc. This character, or ...
). The server replies with a '+' or '-' character and an optional message, followed by CRLF. In case of a positive reply ('+'), the protocol or service requested is started, otherwise the connection is closed.
This service also features a reserved name, "HELP". If the remote server receives this message it will output a multi-line message listing the names of all supported services, one service name per line.
Security risks
Enabling TCPMUX on a server enables an attacker to easily find out the services running on the host, either by using the "HELP" command or by requesting a large number of services. This has the same effect as
port scanning the host for available services iteratively. Because TCPMUX allows someone to use any service only by accessing port number 1, the protocol makes it difficult to apply traditional port-based firewall rules that block access from certain or all hosts to specific services.
See also
*
List of TCP and UDP port numbers
This is a list of TCP and UDP port numbers used by protocols for operation of network applications. The Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) only need one Port (computer networking), port for Duplex (telecommuni ...
References
Internet protocols
Transmission Control Protocol
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