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 goups. 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 maitain the membership of a group. We can capture the following packet when a program join 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 use the “ifconfig” command.
# ifconfig eth0 | grep -i multi
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
2. To disable multicast on a spcific 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 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.