Simple multicast testing application for UNIX
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Signed-off-by: Joachim Nilsson <>
Latest commit 9ae3eb6 Apr 18, 2017

m c j o i n - tiny multicast testing tool

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mcjoin is a very simple and easy-to-use tool to test IPv4 multicast. Simply start a multicast generator (server) on one end and one or more data sinks (clients).

By default the group and the UDP port 1234 is used, you may want to use the MCAST_TEST_NET from RFC5771,, or the ompoing(8) test group, defined in this IETF draft and UDP port 4321. At the moment max 250 groups can be joined.


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


the multicast producer, mcjoin -s, can send without a default route, but the sink need a default route to be able to receive the UDP stream.

in particular, this issue will arise if you run mcjoin in isolated network namespaces in Linux. e.g.

ip netns add sink
ip link set eth2 netns sink
ip netns exec sink /bin/bash
ip address add dev lo
ip link set lo up
ip link set eth2 name eth0
ip address add dev eth0
ip link set eth0 up
ip route add default via


    $ mcjoin -h
    Usage: mcjoin [dhjqsv] [-c COUNT] [-i IFNAME] [-p PORT] [-r SEC] [-t TTL]
    	      [GROUP0 .. GROUPN | GROUP+NUM]
      -c COUNT     Exit after COUNT number of received and/or sent packets
      -d           Debug output
      -h           This help text
      -i IFNAME    Interface to use for multicast groups, default eth0
      -j           Join groups, default unless acting as sender
      -p PORT      UDP port number to listen to, default: 1234
      -q           Quiet mode
      -r SEC       Do a join/leave every SEC seconds
      -s           Act as sender, sends packets to select groups
      -t TTL       TTL to use when sending multicast packets, default 1
      -v           Display program version
    Bug report address:
    Project homepage:


usually there is a limit of 20 group joins per socket in UNIX, this is the IP_MAX_MEMBERSHIPTS define. on Linux this can be tweaked using a /proc setting:

echo 40 > /proc/sys/net/ipv4/igmp_max_memberships

mcjoin has a different approach, it opens a unique socket per each group to join and for each socket disables the odd IP_MULTICAST_ALL socket option, which is enabled by default. Citing the Linux ip(7) man page, emphasis added:

IP_MULTICAST_ALL (since Linux 2.6.31)

This option can be used to modify the delivery policy of multicast messages to sockets bound to the wildcard INADDR_ANY address. The argument is a boolean integer (defaults to 1). If set to 1, the socket will receive messages from all the groups that have been joined globally on the whole system. Otherwise, it will deliver messages only from the groups that have been explicitly joined (for example via the IP_ADD_MEMBERSHIP option) on this particular socket.

hence, by default all multicast applications in UNIX will receive all multicast frames from all groups joined by all other applications on the same system ...

... which IMO is a weird default since multicast by default is opt-in, not opt-out, which is what POSIX makes it. OK, may it's not mandated by POSIX, and (unregulated) multicast is akin to broadcast, but still! I bet most developer's don't know about this.