Port 1
TCPMUX
The TCP Port Service Multiplexer (TCPMUX) is a protocol used on port 1 to allow clients to query a server for a list of active services and connect to specific daemon processes dynamically. It acts as a service directory or initial contact point, facilitating connection establishment to various services hosted on a server..
Technical Details
Overview:
TCPMUX, operating on port 1, is designed to act as a central point for accessing multiple services on a host using a common entry port. When a client connects, it sends the name of the desired service, and the multiplexer either forwards the connection or responds with a service list. This eliminates the need for each service to have a separate port, simplifying initial client connections.
Operation:
After establishing a TCP or UDP connection to port 1, the client transmits the service name followed by a carriage return and line feed sequence (CRLF). The multiplexer will then either forward this connection to the appropriate daemon/service or respond with a ‘+’ or ‘-’ indicating acceptance or denial. Services can be discovered by sending a special ‘help’ request, which prompts a list of hosted services.
Usage History and Prevalence:
Despite its design as a universal access point, TCPMUX has seen little adoption in modern systems due to security risks, ease of port-based service management, and the preference for dedicated well-known ports. Most contemporary servers disable this port by default to limit exposure. Some legacy or embedded systems may still implement it, yet it is largely obsolete.