Configuring Linux For Network Multicast:

Regular network exchanges of data are peer to peer unicast transactions. An HTTP request to a web server (TCP/IP), email SNMP (TCP/IP), DNS (UDP), FTP (TCP/IP), ... are all peer to peer unicast transactions. If one wants to transmit a video, audio or data stream to multiple nodes with one transmission stream instead of multiple individual peer to peer connections, one for each node, one may use multicasting to reduce network load. Note that multicast and a network broadcast are different. Multicast messages are only "heard" by the nodes on the network that have "joined the multicast group" which are those that are interested in the information.

The Linux kernel is Level-2 Multicast-Compliant. It meets all requirements to send, receive and act as a router for multicast datagrams. For a process to receive multicast datagrams it has to request the kernel to join the multicast group and bind the port receiving the datagrams. When a process is no longer interested in the multicast group, a request is made to the kernel to leave the group. It is the kernel/host which joins the multicast group and not the process. Kernel configuration requires "CONFIG_IP_MULTICAST=y". In order for the Linux kernel to support multicast routing, set the following in the kernel config:

• CONFIG_IP_MULTICAST=y
• CONFIG_IP_ROUTER=y
• CONFIG_IP_MROUTE=y
• CONFIG_NET_IPIP=y

The default Red Hat / Fedora kernels are compiled to support multicast.

See the YoLinux tutorial on optimization and rebuilding the Linux kernal.

Note that on multihomed systems (more than one IP address/network card), only one device can be configured to handle multicast.

Class D networks with a range of IP addresses from 224.0.0.0 to 239.255.255.255 (See Network Classes above) have typically been reserved for multicast.

Usefull commands:

Command	Description
cat /proc/net/igmp	List multicast group to which the host is subscribed. Use "Internet Group Management Protocol". (See /usr/src/linux/net/core/igmp.c)
cat /proc/net/dev_mcast	List multicast interfaces. (See /usr/src/linux/net/core/dev_mcast.c)
ping 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

Multicast transmissions are achieved through proper routing, router configuration (if communicating through subnets) and programatically with the use of the following "C" function library calls:

Function Call	Description
setsockopt()	Pass information to the Kernel.
getsockopt()	Retrieve information broadcast using multicast.

For more on multicast programming see: Multicast Howto.

The multicast application will specify the multicast loopback interface, TTL (network time to live), network interface and the multicast group to add or drop.

Add route to support multicast:

• route add 224.0.0.0 netmask 240.0.0.0 dev eth0

Note that if adding a route to forward packets through a router, that the router MUST be configured to forward multicast packets. Many routers do not support forwarding of multicast packets or have a default configuration which does not. The internet by default does not forward multicast packets.