Thread: Sniffing Only Packets that don't belong to my machine

libpcap and a few rules perhaps?
Programming with pcap

For what purpose? Gbps of traffic isn't a problem - if your network ports can supply it, you can sniff it on any modern machine, but it very much depends on what you're doing. Writing every packet to disk? That needs storage capable of keeping up with Gbps of writes. Analysing each packet? Shouldn't be a problem on a fast-enough computer if you're just acting as a passthrough. Picking out only certain streams - again, shouldn't be a problem, much less system resources required but you need to know how to extract just the streams you wanted. Logging only basic statistics and endpoints? Existing proxy software can do just that with no programming required. Intending to modify the packets in-flight? That needs whole new swathes of code.

I'd be very surprised if what you want doesn't already exist out there, to be honest. Hell, I've set up dozens of machines (sometimes nothing more than abandoned desktops) to be an intermediate transparent DNS and Squid proxy/cache for 100's of client machines on their default gateway - and they do a lot more hard work than just looking at the packets.

Hell, set up Wireshark on a linux PC with bridged network cards and set up rules to ignore the local MAC. Takes about ten minutes and does exactly what you describe.

The really, really nagging question at the back of my mind, though, is: Why would you need to do this programmatically and not use existing software that's designed for the purpose? Some kind of nefarious interference?