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m c j o i n - tiny multicast testing tool

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mcjoin(1) is a simple and easy-to-use tool to test IPv4 and IPv6 multicast, featuring:

  • a multicast generator (server)
  • a multicast data sink (client)
  • support for join/send to one or more groups
  • support for both any source and source specific multicast:
    • ASM (*,G)
    • SSM (S,G)
  • support for both IPv4 & IPv6
  • support for the following operating systems:
    • Linux (GLIBC, musl libc)
    • FreeBSD
    • Apple macOS
    • OpenSolaris/Illumos
  • no support for the following due to lack of RFC3678 support:
    • NetBSD
    • OpenBSD

latest release available here:


this is a fairly odd example, joining multiple ipv6 asm groups and a single ipv4 ssm group. the purpose is only to show that it's possible.

mcjoin ff2e::42 ff2e::43 ff2e::44 ff2e::45

mcjoin receiver

the sender needs to have an ipv6 (and ipv4) address on the egressing interface. here the system only has an ipv6 address.

mcjoin -s ff2e::42

mcjoin sender

by default, mcjoin use the ipv4 group (which is very easy to spot also when translated to mac multicast, RFC1112). however, for testing purposes you may want to instead use the MCAST_TEST_NET from RFC5771,, or possibly 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!


without any arguments mcjoin defaults to act as a receiver, performing an IPv4 ASM join (*,G) of group, UDP port 1234. to act as a sender of the same group and port, add -s to the command line.

$ mcjoin -h

Usage: mcjoin [-dhjosv] [-c COUNT] [-f MSEC] [-i IFACE] [-l LEVEL]
              [-p PORT] [-t TTL] [-w SEC]

  -b BYTES    Payload in bytes over IP/UDP header (42 bytes), default: 100
  -c COUNT    Stop sending/receiving after COUNT number of packets
  -d          Run as daemon in background, output except progress to syslog
  -f MSEC     Frequency, poll/send every MSEC milliseconds, default: 100
  -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
  -o          Old (plain/ordinary) output, no fancy progress bars
  -p PORT     UDP port number to listen to, default: 1234
  -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
  -W SEC      Timeout, in seconds, before mcjoin exits

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 2048 groups can be joined.


the multicast producer, mcjoin -s, can send without a default route, but the sink (your receiver) need a net route back to the sender (or a default route), or reverse-path filtering (RPF) disabled to be able to receive the UDP stream. the sink will be able to start without an IP address or route, as long as the interface is UP and allows MULTICAST, the IGMP or MLD join frames will also be sent while you wait for an address+route, but the kernel will (likely) not forward any frames to mcjoin even though it may be arriving at the interface if you check with tcpdump.

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

depending on the route setup, and number of interfaces on a multihomed system, you may also need to verify that you don't have strict reverse path filtering (RPF) enabled. on Linux rp_filter can be set to either 0 (no filtering), 1 (strict), or 2 (loose filtering), the latter is the most common for distributions today. the difference between 1 and 2 is that 1 (strict) checks for the best route, while 2 checks for any route back to the sender. see RFC3704 for more on reverse path filtering.


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.

the same applies to ipv6(7), although the IPV6_MULTICAST_ALL socket option has only existed since Linux 4.20.

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, maybe 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.

testing on the same machine

in many cases while using mcjoin for testing networking equipment, you need to use at least two local network interfaces (nics): one acting as multicast sender and one as receiver. (often you need multiple sender interfaces, which can be physical, virtual or vlan interfaces.)

        |       |
    .---+  dut  +---.
    |   |       |   |
    |   '-------'   |
    |               |
.- eth0 ---------- eth1 -.
|                        |
|           pc           |
|                        |

to get this to work on linux you need to disable the rp_filter and enable accept_local sysctl settings for the involved interfaces. here is an example of how to adjust this for all interfaces. use with care, this can cause a lot of other problems if you use the same pc for other purposes as well:

$ cd /etc/sysctl.d/
$ cat 10-network-security.conf
# Allow receiving IP packets from local interfaces, useful for testing
# rigs where looping packets via networking infrastructure.

# Disable Source Address Verification in all interfaces to, usually set
# to 1 to prevent some spoofing attacks.  But for a testing rig this is
# usually the source of many woes, in particular for multicast testing.

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