What's the easiest way to sniff TCP traffic data on Linux?

I want a simple way to show all the TCP data (not the TCP headers or anything else) going over any interface on my Linux box.

For instance, I want a magical command that if I do:

magic_commmand_I_want port=1234

then if there was a server listening on port 1234 on my machine, and someone did:

echo hello | nc localhost 1234# Note: "nc" (aka "netcat") is a simple tool that sends data to a host/port

Then the magical command would just print out:

hello

I've tried "tcpdump", "ethereal", "tethereal", "tshark", and others, but it isn't obvious how you get them to:

• not show IP addresses or other metadata
• only show the "data" being sent, not individual packets and their headers
• print the data as-is, not in hex, and not with packet-offset markers
• sniff all network traffic (whether it's on eth0 or eth1 or lo, etc...)

Yes, you could probably string together a piped set of unix commands to do this, but that isn't very easy to remember for next time :)

If you have a simple example of an exact command-line that does this, that's what I'd like.