Testing Multicast Traffic on Linux

multicast-on-linux.md

Testing Multicast Traffic on Linux

Prerequisites

By default Linux ignores Broadcast and Multicast ICMP messages. That's why you need to enable it first:

sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=0

Join and Test Multicast Traffic

To join any mutlicast address (e.g. 224.10.10.10/24) just add it to your active interface (e.g. eth0) and append the keyword autojoin at the end:

ip addr add 224.10.10.10/24 dev eth0 autojoin

Now you can verify the join:

ip -f inet maddr show dev eth0

Finally to test the Multicast communication, ping to the chosen IP address from another host:

ping 224.10.10.10

You should now get an answer from your Multicast enabled host. That's it.

Change IGMP Version

The Linux Kernel normally uses IGMP version 3 to join Multicast groups. If you would rather use IGMP version 2, just change it via the command down here:

echo "2" > /proc/sys/net/ipv4/conf/eth0/force_igmp_version

Multicast Group Join Inside Docker Container

In order to get the commands above here working inside a Docker container, it must be started with the Linux capability NET_ADMIN:

docker run --rm -it --cap-add NET_ADMIN alpine /bin/sh

Fun fact: GNS3 starts all containers with all Linux capabilities added and in priviledged mode.

socat -u UDP4-RECV:3671,reuseaddr,ip-add-membership=224.0.23.12:eth0 STDOUT

https://unix.stackexchange.com/questions/657353/how-to-check-if-someone-is-streaming-rtp-over-a-specific-multicast-address

In an environment where IP multicast isn't simply flooded to all ports of a dumb switch, using tcpdump alone would likely capture no traffic: the switch snooping IGMP reports and queries (or doing itself queries) seeing there is no need for this multicast traffic on machine B's port would just not send it. This also requires a multicast client that used setsockopt(fd, SOL_IP, IP_ADD_MEMBERSHIP, ...) on its socket.

Install socat and use:

socat -d -d -u udp4-recv:5005,reuseaddr,ip-add-membership=239.0.0.1:192.168.0.21 /dev/null
this will:

join the multicast group 239.0.0.1 on the interface with 192.168.0.21 by sending an initial IGMP membership report, which when processed by an IGMP snooping bridge in presence of a querier will then make the smart switch send traffic to machine B's switch port

Optional reuseaddr here allows to run multiple times the same command (multicast at the system level then just makes each command receive a copy of the single multicast flow).

-d -d makes socat verbose: it will also display the source of the received packets.

Once joined, anything related to 239.0.0.1 can now be captured with tcpdump.

If 192.168.0.20 is sending to 239.0.0.1, the received packets will have 192.168.0.20 as source showing what system sent it.

receive the data and forget it to /dev/null. If the data is a video stream (really on port 5005, not say 5004), replacing /dev/null with - | mpv - would display the video using the mpv command.

When socat is stopped, an IGMP leave message will be sent by the kernel. Depending on switch's settings, it will immediately stop sending this multicast destination to machine B or this will happen within the next minute(s). tcpdump might thus continue to capture traffic, but this won't last.

note:

In an environment where the network is handled by a dumb switch (or a virtual linux bridge where the mcast_querier toggle is not set) then tcpdump would capture the multicast traffic in all cases because the switch is flooding it on all of its ports.