How can I know if IP Multicast is enabled

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

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?

# 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