RHEL : What is Multicast and how to configure Network interface with Multicast Address?
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Multicast is method of sending IP packetes 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 multicast router.What IP Addresses can be used for Multicast purpose?
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. In Simple:
Bit –> 0 31 Address Range:
+-+—————————-+
|0| Class A Address | 0.0.0.0 – 127.255.255.255
+-+—————————-+
+-+-+————————–+
|1 0| Class B Address | 128.0.0.0 – 191.255.255.255
+-+-+————————–+
+-+-+-+————————+
|1 1 0| Class C Address | 192.0.0.0 – 223.255.255.255
+-+-+-+————————+
+-+-+-+-+———————-+
|1 1 1 0| MULTICAST Address | 224.0.0.0 – 239.255.255.255
+-+-+-+-+———————-+
+-+-+-+-+-+——————–+
|1 1 1 1 0| Reserved | 240.0.0.0 – 247.255.255.255
+-+-+-+-+-+——————–+
Additionally,
- -> 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.
How to enable or disable multicst for a NIC
Enable Multicast for eth0:
# ifconfig eth0 multicast
# 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 for eth0:
# ifconfig eth0 -multicast
# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 00:0C:34:FE:AB:6A
inet addr:10.0.0.100 Bcast:10.0.0.255 Mask:255.255.255.0
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
Other useful command to test Multicast Configuration:
Command Description
- cat /proc/net/igmp List multicast group to which the host is subscribed. Use “Internet Group Management Protocol”.
- cat /proc/net/dev_mcast List multicast interfaces.
- ping -L 239.192.104.2 To check connectivity with another multicast address
- ping -L 224.0.0.1 All hosts configured for multicast will respond with their IP addresses
- ping 224.0.0.2 All routers configured for multicast will respond
- ping 224.0.0.3 All PIM routers configured for multicast will respond
- ping 224.0.0.4 All DVMRP routers configured for multicast will respond
- ping 224.0.0.5 All OSPF routers configured for multicast will respond
Check If Multicast enabled in the kernel or not:
#grep -i multi /boot/config-<Kernel version>
* CONFIG_IP_MULTICAST=y <–
* CONFIG_IP_ROUTER=y
* CONFIG_IP_MROUTE=y
* CONFIG_NET_IPIP=y
Configuring Map Network Interface to Send Multicast Traffic
To Configure a NIC to send multicast network, we need to add a default route for muticast traffic , to the specific NIC. For example, Below command will allow eth0 to send multicast traffic:
# route add -net 224.0.0.0 netmask 240.0.0.0 dev eth0
And check the route settings with
# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.0.1 0.0.0.0 255.255.255.0 U 0 0 0 eth0
224.0.0.0 0.0.0.0 240.0.0.0 U 0 0 0 eth0
0.0.0.0 192.168.0.254 0.0.0.0 UG 0 0 0 eth0
How to Verify the Mutlicast groups in our current system
# netstat -g
IPv6/IPv4 Group Memberships
Interface RefCnt Group
————— —— ———————
lo 1 all-systems.mcast.net
bond0 1 239.192.153.137
bond0 2 all-systems.mcast.net
$ cat proc/net/igmp
Idx Device : Count Querier Group Users Timer Reporter
1 lo : 0 V2
010000E0 1 0:00000000 0
2 eth0 : 2 V2
9B9B9BEA 1 0:00000000 1
010000E0 1 0:00000000 0
4 eth2 : 1 V2
010000E0 1 0:00000000 0
5 eth3 : 1 V2
010000E0 1 0:00000000 0
Above output shows that eth0 interface configures with Multicast address 9B9B9BEA = 234.155.155.155 ( that means – EA = 234; 9B = 155; 9B = 155; 9B = 155 ). Same information will be reflected from the file /proc/net/igmp
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