MCJOIN(1) General Commands Manual MCJOIN(1)


mcjointiny multicast testing tool


mcjoin [-dhjsv] [-c COUNT] [-i IFNAME] [-l LEVEL] [-p PORT] [-r SEC] [-t TTL] [-w SEC] [[SOURCE,]GROUP0 .. [SOURCE,]GROUPN | [SOURCE,]GROUP+NUM]


mcjoin can be used to join IPv4 and IPv6 multicast groups, display progress as multicast packets are received, and also send multicast packets on select groups.
mcjoin can help verify intended IGMP snooping functionality in layer-2 bridges (switches), as well as verify forwarding of multicast in static or dynamic multicast routing setups.
mcjoin supports source-specific multicast, SSM (S,G), as well as any-source multicast, ASM (*,G). The source IP of an (S,G) pair is an optional argument that must precede the group and be separated with a comma. No spaces are allowed between source and group in this form. Multiple (S,G) pairs are separated with space.


With no options given mcjoin acts as a multicast receiver (or sink), joining the default group, listening on the default interface, eth0, binding to the default port, 1234.
Use the following options to adjust this behavior:
Stop sending/receiving after COUNT number of packets
Run as a daemon in the background, detached from the current terminal. All output, except progress is sent to syslog(3).
Print a summary of the options and exit
Interface to use for sending/receiving multicast, default: eth0
Join groups, default unless acting as sender
Control mcjoin log level; none, notice, debug. Default: notice.
UDP port number to send/listen to, default: 1234
Act as sender, sends packets to select groups, 1/100 msec, default: no
-r SEC
Do leave/join repeatedly every SEC seconds. Should not be needed anymore, kept for backwards compatibility.
-t TTL
TTL to use when sending multicast packets, default: 1
Show version information
-w SEC
Initial wait, sleep SEC seconds before starting anything. Useful for scripting test systems that launch mcjoin at boot without syncing with creation of networking.


To verify multicast connectivity, the simplest way is to run mcjoin on one system, without arguments, and on the other with the command option -s. In this setup one system joins the group waiting for packets to arrive, and the other end starts sending packets to the same group. To verify routing of multicast, make sure to add the -t TTL option to the sender since the default TTL is 1 and every router (simplified) decrements the TTL.
For a more advanced example, say you want to verify that your topology can forward 20 consecutive groups in the MCAST_TEST_NET, as defined in RFC5771. Simply add the following as a standalone argument to both the receiver and the sender:
For non-consecutive groups, simply add them in any order you want, up to 250 groups are supported:
To run mcjoin as both a sender and a receiver on the same host you will likely need to employ something like network namespaces (Linux netns) for at least one of them. Otherwise the network stack will likely let the sender's data stream take a short cut to the receiver, without passing through an actual wire.
Also, like most network applications, to run properly mcjoin needs both the loopback (lo) interface and a default route set up. At least in the default (receiver) case. In a network namespace neither of these are set up by default.


omping(1), ping(1), mgen(1)


Use the project's GitHub page to file bug reports, feature requests or patches (preferably as GitHub pull requests), or questions at ⟨⟩


Originally written by David Stevens, currently maintained by Joachim Nilsson at GitHub.
March 8, 2020 Debian