socat SOcket CAT
Connects two arbitrary data channels and pipes everything arriving on one out through the other. Netcat with ten times more address types — and ten times more awkward syntax.
The channels can be TCP, UDP, Unix sockets, files, pipes, pseudo-terminals, serial ports, a subprocess or a TLS connection, in any combination. That's why socat solves things no other tool does in one command: expose a Unix socket over the network, terminate TLS in front of a service that only speaks plaintext, bridge a serial port to a TCP port, or give a program a real terminal when it refuses to run without one. A typical invocation: socat -d -d TCP-LISTEN:8080,fork,reuseaddr TCP:internal:80. Read the man page — it's enormous and full of examples, which is the only sensible way to learn the tool.