For the better part of the last ten years I have been working with multicast in one way or another. I’ve used many different tools for testing, but on most systems I usually resort to ping(1) and tcpdump(1), which are quite sufficient. However, you often need to tell bridges (switches) to open up multicast in your general direction for your pings to get through, so you need to send an IGMP “join” first.

Way back in 2006 I stumbled upon a neat tool called mcjoin, written by David Stevens and announced in this posting to LKML. I started improving and adding features to it over the years.

Now, my interest and fascination with multicast only grew with time, and despite elegant tools like mgen(1) and omping(1) I was never quite happy.

When releasing SMCRoute v2.1.0 recently, and currently working on the pimd v2.3.2 release, I was so tired of having to do so many manual steps just to verify correct operation of a routing daemon. Therefore I spent the better part of the past weekend fixing up my old mcjoin tool.

I wanted a reliable, simple, and UNIX-y tool to just test things for me. So I cleaned up the old mcjoin project, first by migrating it from the toolbox repo, removing confusing command line options, improving and simplifying the syntax, and then I added send/receive capabilities.

I’ve been meaning to get around to this for ages, and now it seems I had finally had enough. So here it is, v2.0:


Most of the time I simply want to see a resulting IGMP join message in Wireshark, watch a group entry pop up in a switch’s FDB or a routing daemon’s forwarding table. So, join is the default operation, and also continues to be the name of the tool. My favourite testing group is set as the default,, so you only need to start the tool and you’re off. To send to the same default group (, simply add -s to the sender side.

sender$ mcjoin -s

receiver$ mcjoin
joined group on eth0 ...
Received total: 66 packets

If you ever need anything else, e.g. routing multicast, there’s even a man page. It mentions setting the TTL and other such nastiness :)