Netkit-NG includes support for uml_dump.
uml_dump is a program for UML tools (various tools to connect several User-mode Linux instances together) developed by Julien Iguchi-Cartigny and two students (Jean-Baptiste Machemie and Benoit Bitarelle) during Spring 2009. It dumps packets from the virtual switch (uml_switch) using CAP format to the standard output. Thus, it is possible to see packets exchanged between UML machines in any CAP viewer (for instance wireshark or tcpdump) or save packets in a file.
It was already possible to dump packet using the original Netkit but it requires root access and TUN/TAP tunnels (not very useful using shared Linux computers).
Usage
$ vdump switch_name
Where switch_name is the network id shared between machines using the same collision domain in Netkit. For instance, to dump all packet from the network A and print them using wireshark:
$ vdump A | wireshark -i - -k
Behind the scene, the vdump command calls uml_dump:
$ uml_dump fifo
where fifo is the named pipe to connect to the uml_switch instance. If omitted /tmp/uml.ctl is used.