ucspi-tcp (djb's TCP client-server command-line tools)

ucspi-tcp includes several programs. tcpserver waits for incoming
connections and, for each connection, runs a program of your choice,
it provides useful environment variables, resource limits, and access
control features. tcpclient makes a TCP connection and runs a program of
your choice. It sets up the same environment variables as tcpserver.

Also included are recordio, a tool that monitors all the input and output
of a server, and several sample clients built on top of tcpclient: who@,
date@, finger@, http@, tcpcat, and mconnect.
