Netcat Relays

Netcat is a fantastic tool for tying networked devices together in the spirit of DucTape,  baling wire and twine...

How it Works

Netcat connects a network socket to stdin and stdout.  To initiate a connection, it can act either as a listener or a client.  Once connected, it will simply shovel data back and forth.  That is all.  

What you do with it is totally up to your imagination.

Here is a netcat based Double Listener Relay:

mkfifo /tmp/buf

nc –l 5566 <buf | nc -l 6655 >buf

It uses two instances of netcat, tied together through stdin and stdout, with the help of a fifo.  One could make such a relay on a public addressable server where anyone can connect to it.

The first netcat listens on port 5566 and gets its std input from the fifo buf and sends its std output to the std input of another netcat instance through a pipe operator.  The second netcat listens on port 6655, gets its std input from the previous netcat and sends its std output to the fifo buf, which effectively loops it back to the first netcat.

Two clients that may be behind firewalls can connect to each other via this relay:

nc relayipaddress 5566

nc relayipaddress 6655

and they can then communicate with each other despite the intervening firewalls.

Note that the latest and greatest version of netcat from the Nmap project called ncat has this kind of relay feature built in as the --broker option, but doing it as above with the original netcat is much more fun and illustrates stdio redirection and FIFOs.

In the Spirit of the NSA Scandal, here is a Spying Relay:

mkfifo /tmp/buf

nc –l 5566 <buf | nc -l 6655 | tee buf

The tee program will send the data to the fifo and also to the terminal, so that you can see what is going through the relay, in order to debug something.

Keeping netcat Running

When a connection is terminated, the netcat lister will usually also exit.  Some versions of netcat has a 'keep' or -k operand which can be used together with the -l option to keep it running and listen for the next user.

Another way to keep a listener going is to stuff netcat inside an endless loop:
while true; do nc -l 5566 </dev/ttyUSB0 >/dev/ttyUSB0; sleep 1; done &

Also put a sleep 1 command in the loop to slow it down in case of an error, so that it won't spin at high speed and consume all processing resources.

Perfect Insecurity

Of course, running a relay of any kind on a public machine is a HUGE security problem, so don't leave it running for too long or an angry IT Admin may want to hit you upside the head with a clue by four...