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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 and IPv6 multicast. it features:

  • an optional multicast generator (server)
  • an end device that can act as a data sink (client)
  • supports joining one or more groups:
    • ASM (*,G) support
    • SSM (S,G) support
  • IPv4
  • IPv6

the latest release is always available from GitHub at


sender$ mcjoin -s

without any arguments mcjoin defaults to an IPv4 ASM (*,G) join of, UDP port 1234. see the usage section below for more help.

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

for testing purposes you may want to use the MCAST_TEST_NET from RFC5771,, or possibly the ompoing(8) test group, UDP port 4321, as defined in this IETF draft.

for testing IPv6 you can use ff2e::42. for ipv6 groups the ipv6 address of the outbound interface will be used.

remember: to set ipv4 and/or ipv6 address on the outbound interface!


    $ mcjoin -h
    Usage: mcjoin [-dhjsv] [-c COUNT] [-i IFACE] [-p PORT] [-r SEC] [-t TTL] [-w SEC]
                  [[SOURCE,]GROUP0 .. [SOURCE,]GROUPN | [SOURCE,]GROUP+NUM]
      -c COUNT    Stop sending/receiving after COUNT number of packets
      -d          Run as daemon in background, output except progress to syslog
      -h          This help text
      -i IFACE    Interface to use for sending/receiving multicast, default: eth0
      -j          Join groups, default unless acting as sender
      -l LEVEL    Set log level; none, notice*, debug
      -p PORT     UDP port number to listen to, default: 1234
      -r SEC      Do a join/leave every SEC seconds (backwards compat. option)
      -s          Act as sender, sends packets to select groups
      -t TTL      TTL to use when sending multicast packets, default 1
      -v          Display program version
      -w SEC      Initial wait before opening sockets
    Bug report address :
    Project homepage   :

the SOURCE argument is optional, but when used it must be of the same address family as the group. to join multiple groups, either list them all on the command line, separated with space, or use the +NUM syntax. at the moment max 250 groups can be joined.


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


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.

build & install

the GNU Configure & Build system use /usr/local as the default install prefix. for most use-cases this is fine, but if you want to change this to /usr use the --prefix=/usr configure option:

$ ./configure --prefix=/usr
$ make -j5
$ sudo make install-strip

building from git

if you want to contribute, or simply just try out the latest but unreleased features, then you need to know a few things about the GNU Configure & Build system:

  • and a per-directory are key files
  • configure and are generated from, they are not stored in GIT but automatically generated for the release tarballs
  • Makefile is generated by configure script

to build from GIT; clone the repository and run the script. this requires automake and autoconf to be installed on your system. (if you build from a released tarball you don't need them.)

git clone
cd mcjoin/
./configure && make
sudo make install-strip
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