What is a TCP/IP relay?

[Manu Zacharia (-M-)]

I was just googling for netcat, tcpxd etc and came across this line that made me write this:

txpxd is a general purpose TCP/IP relay program....

and

Jonama is a piece of software acting as a relay between a client over the Net and your internal systems

Can we have a definition or explanation for "what is a tcp/ip relay"? Is it simply another jargon for a daemon service?

[jimbob]

Sounds like a TCP forwarding. Programs like datapipe allow you to start a service on a port which simple forwards traffic to another port on the same system or a remote one. The host of the services effectively bridges the TCP traffic between the source and destination.

Jim

[pseud0]

jimbob got it.  You can use this little widget a couple of different ways, the most popular being having it accept traffic on port X and send it out on port Y therefore acting as a relay/redirect.  The traffic doesn't have to come from a different system, meaning you can redirect traffic that never actually leaves your system.  This is useful if case you needed to maybe reroute your winamp traffic to an another port so that it can pass through the corporate firewall because the Technology Services guys are a bunch of Nazi Commies that can't seem to understand why you need your techno trance while you're getting your pen test on...

[dean]

Manu, you mentioned netcat in your post. here's a quick way to create a relay using netcat with named pipes:

Run 'mknod backpipe p' on the box you want for your relay.

then run:

nc –l -p 4321 0<backpipe | nc <target> 2222 1>backpipe

Set up a netcat listener that pipes the incoming data to a netcat client that will forward the data to the target machine. Any data that is received by the netcat client is sent to the named pipe and from there is redirected back to the netcat listener. The listener will then send the data back to the Attacker.

Attacker <--> Relay <--> Target


dean