The gem5 simulator has two different Network Interface Cards (NICs) devices that can be used to connect together two simulation instances over a simulated ethernet link.
You can get a list of the packet on the ethernet link by creating a Etherdump object, setting it's file parameter, and setting the dump parameter on the EtherLink to it. This is easily accomplished with our fs.py example configuration by adding the command line option --etherdump=<filename>. The resulting file will be named <file> and be in a standard pcap format. This file can be read with wireshark or anything else that understands the pcap format.