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.

Hi Philip,
thanks for sharing your snippet gist,
I've testing in my ubuntu 21.04

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:15:5d:01:b6:27 brd ff:ff:ff:ff:ff:ff
    inet 10.55.254.126/24 brd 10.55.254.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet 224.10.10.10/24 scope global autojoin eth0
       valid_lft forever preferred_lft forever

But when i've test ping to 224.10.10.10 its getting error

root@zu:/home/d# ping 224.10.10.10
ping: connect: Invalid argument

what's wrong with my config?

Hi, same problem for me !

Be sure to start a multicast ping from another host and the one from which you've bound to the multicast address. Also be sure to issue ping -t 16 224.10.10.10 otherwise all packets will leave with a TTL=1.

still seeing "connect: Invalid argument" after adding "-t 16" argument to ping

is multicast correctly enabled on your system ?

do you mean set "sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=0"?

yes, the command given above works well on my ubuntu.

I see. Maybe there is some issue on my end. Thanks Surya

sudo sysctl -a | grep "net.ipv4.icmp_echo_ignore_broadcasts"
net.ipv4.icmp_echo_ignore_broadcasts = 0

ip -f inet maddr show dev ztmosn2bor
6:      ztmosn2bor
        inet  224.10.10.10
        inet  224.0.0.252
        inet  224.0.0.251 users 2
        inet  224.0.0.1

ip -c addr show dev ztmosn2bor
6: ztmosn2bor: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 2800 qdisc fq_codel state UNKNOWN group default qlen 1000
    link/ether 9a:05:17:7a:90:31 brd ff:ff:ff:ff:ff:ff
    inet 10.147.19.11/24 brd 10.147.19.255 scope global ztmosn2bor
       valid_lft forever preferred_lft forever
    inet 224.10.10.10/24 scope global autojoin ztmosn2bor
       valid_lft forever preferred_lft forever
    inet6 fe80::9805:17ff:fe7a:9031/64 scope link proto kernel_ll
       valid_lft forever preferred_lft forever

/usr/bin/ping -v -t 15 224.10.10.10
/usr/bin/ping: sock4.fd: 3 (socktype: SOCK_DGRAM), sock6.fd: 4 (socktype: SOCK_DGRAM), hints.ai_family: AF_UNSPEC

ai->ai_family: AF_INET, ai->ai_canonname: '224.10.10.10'
/usr/bin/ping: connect: Invalid argument

/usr/bin/ping -v 224.10.10.10
/usr/bin/ping: sock4.fd: 3 (socktype: SOCK_DGRAM), sock6.fd: 4 (socktype: SOCK_DGRAM), hints.ai_family: AF_UNSPEC

ai->ai_family: AF_INET, ai->ai_canonname: '224.10.10.10'
/usr/bin/ping: connect: Invalid argument

/usr/bin/ping -V
ping from iputils 20240905
libcap: yes, IDN: yes, NLS: yes, error.h: yes, getrandom(): yes, __fpending(): yes

connect: Invalid argument does not seem to be related to -t flag.

nc -u 224.10.10.10 42069

meanwhile

nc -u 224.0.0.1 42069

works but only on wlan0 interface in my case

"hints.ai_family: AF_UNSPEC"

Address Family unspecified. The multicast address put in argument is not recognized.

No, not really. By passing in -4 hints.ai_family changes from AF_UNSPEC to AF_INET. However no matter if the flag is present or not it's getting detected correctly as ai->ai_family: AF_INET.

/usr/bin/ping -v -4 -t 15 224.10.10.10
/usr/bin/ping: sock4.fd: 3 (socktype: SOCK_DGRAM), sock6.fd: -1 (socktype: 0), hints.ai_family: AF_INET

ai->ai_family: AF_INET, ai->ai_canonname: '224.10.10.10'
/usr/bin/ping: connect: Invalid argument

that's strange. the very same command works well on my ubuntu 22.04