Unix & Linux: How can I know if IP Multicast is enabled? (4 Solutions!!)
Channel: Roel Van de Paar
Date: 08/08/2020

Unix & Linux: How can I know if IP Multicast is enabled?


The Question: I have scripts that run IP multicast tests; however, my scripts are failing on
a particular linux machine.
I know that I can look at CONFIG_IP_MULTICAST in the kernel configuration file
to determine whether the kernel was compiled with this. However, it would be
easier to flag missing requirements in my script if I could look at /proc or
sysctl and get the answer.
Is there a way to find if IP Multicast was compiled into the kernel without
looking at CONFIG_IP_MULTICAST?

Solutions: Please watch the whole video to see all solutions, in order of how many people found them helpful

== This solution helped 18 people ==
It seems the most kernels(post v1) do support multicast by default or have
CONFIG_IP_MULTICAST enabled while compiling. To check whether the compiled and
running kernel subscribes to any multicast group, I would use netstat -g.
It seems the most kernels(post v1) do support multicast by default.
[root@centos module]# grep CONFIG_IP_MULTICAST /usr/src/kernels/2.6.18-
274.7.1.el5-i686/.config
CONFIG_IP_MULTICAST=y
[root@centos module]# netstat -g
IPv6/IPv4 Group Memberships
Interface RefCnt Group
--------------- ------ ---------------------
lo 1 all-systems.mcast.net
eth0 1 224.0.0.251
eth0 1 all-systems.mcast.net
I see that eth0 interface on my host thinks its subscribed 224.0.0.251 basic
mcast group . Also alternatively, if I ping the group or the network and then
all the known multicast enabled host networks are returned to me, I would say
multicast is working on the host. Can you try netstat -g or cat /proc/net/igmp
and see if it returns any groups on your host?

== This solution helped 20 people ==
On your linux box: ip maddr show gives:
@4Gtest ~]$ ip maddr show
1: lo
inet 224.0.0.1
inet6 ff02::1
2: p3p1
link 01:00:5e:00:00:01
link 33:33:00:00:00:01
link 33:33:ff:bd:7e:0f
link 01:00:5e:00:00:fb
link 01:00:5e:7f:ff:05
inet 224.127.255.5
inet 224.0.0.251
inet 224.0.0.1
inet6 ff02::1:ffbd:7e0f
inet6 ff02::1

== This solution helped 8 people ==
# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 00:0C:29:F1:FF:EA
inet addr:78.46.74.27 Bcast:78.46.74.31 Mask:255.255.255.224
inet6 addr: fe80::20c:29ff:fef1:ffea/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:5339836 errors:0 dropped:0 overruns:0 frame:0
TX packets:5486444 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:820300389 (782.2 MiB) TX bytes:1929979381 (1.7 GiB)
Base address:0x1400 Memory:e8820000-e8840000
Disable Multicast:
ifconfig eth0 -multicast
result:
# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 00:0C:29:F1:FF:EA
inet addr:78.46.74.27 Bcast:78.46.74.31 Mask:255.255.255.224
inet6 addr: fe80::20c:29ff:fef1:ffea/64 Scope:Link
UP BROADCAST MTU:1500 Metric:1
RX packets:5339836 errors:0 dropped:0 overruns:0 frame:0
TX packets:5486444 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:820300389 (782.2 MiB) TX bytes:1929979381 (1.7 GiB)
Base address:0x1400 Memory:e8820000-e8840000
No Multicast present in the interface
Enable Multicast :
ifconfig eth0 multicast

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