How to configure multicast on an IP address (interface)?

Multicast can be used to send IP packets to a group of interested receivers. Compared with broadcast, the packets are only sent to the members of a multicast group with the help of a multicast router.

 

Multicast Addresses

 

Class D addresses (224.0.0.0 – 239.255.255.255) are assigned to multicast. So the first 4 bits of a multicast address should be 1110 and the remaining 28 bits represent different groups. Some addresses are reserved and have been assigned to specified groups. For example,

 

224.0.0.1 – all hosts group, all hosts configured IP multicast will join this group when an interface starts up.

224.0.0.2 – all multicast routers group

224.0.0.22 – IGMP Multicast router. An IGMP member report will be sent this addresses when a host joins a multicast group, then the multicast router can receive that report and maintain the membership of a group. We can capture the following packet when a program joins the multicast group 225.0.0.37.

 

Enabling multicast on an IP address

 

1. To check if multicast is enabled already on an IP address using the “ifconfig” command.

# ifconfig eth0 | grep -i multi
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

 

Note: Please replace ethX with appropriate Ethernet. If multicast is enabled, a flag will be displaying.

 

2. To disable multicast on a specific ip/interface:

# ifconfig ethX -multicast

 

3. To enable multicast on a specific ip/interface :

# ifconfig ethX multicast

 

Listing IPs subscribed to a multicast group

 

In order to look at the IP address which has subscribed the multicast group, one can take a look at in the content of the following file /proc/net/igmp. For example:

# cat /proc/net/igmp
Idx	Device    : Count Querier	Group    Users Timer	Reporter
1	lo        :     1      V3
				010000E0     1 0:00000000		0
2	eth0      :     1      V3
				010000E0     1 0:00000000		0

 

Based on multicast application, IP will be receiving or transferring packets.

 

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