I want to write a program that intercepts in real-time all network comms of whatever protocol that goes through eth0 (in and out) on my box. What do I need to take into account and where can I get necessary info? Pointers to samples & algorithms would be most welcome. There must be a standard way to access/use eth0, but I can't find/(ask wrong question) with google.

You could look at the Wireshark source code.

Welcome to LQ.

There is a way to do it. Wireshark does, though I don't know how. If Wireshark doesn't do what you need, then you could check its source. (I know. It's a pain, but it is a place to start. Squid's transparent proxy might also be a relevant source sample.)

You may be able to do something spiffy using iptables. I heard once that it could be tapped into somehow, but I don't see the rule off-hand. It might not be useful, but I thought I'd mention it.

Monitoring all interface traffic isn't something frequently done. The people who can provide good answers are probably not checking the Newbie forum. You might get a better response from the Networking or Programming forums. (Don't start a duplicate thread. Click "Report" and ask them to move it.)

Dang it. I got beat to the punch while trying to find a better answer!

Ain't that just always the way! but you suggested some other options too.

iptables has a LOG option. Depends if you want to go any lower (more detail), in which case Wireshark (tcpdump) as above.
Note that you can rapidly fill your disk with data doing this...

Thanks for the pointers ..

I once used the mysql facility to log all SQL calls it executes & forgot to remove it after solving the problem - it was on a production box, it was also an interesting experience..