Linux 7 Multicast Configuration
Internal
Concepts
Procedure
Multicast at Kernel Level
Make sure multicast is enabled at kernel level.
To compile multicast support into the kernel, CONFIG_IP_MULTICAST must be present in the kernel configuration file. The kernels are usually compiled with multicast support. If netstat -ng shows multicast groups being registered to, it means multicast support is available in kernel. For more details, see Troubleshooting below.
Multicast Support at Interface Level
Verify that the network interface you plan to use for multicast traffic has multicast enabled, and if not, turn it on. This can be done with the ip utility, as described here Change MULTICAST Flag on Device.
ip addr
Look for MULTICAST flag:
[eap@app01 ~]$ ip addr
...
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
link/ether 08:00:27:cd:76:d7 brd ff:ff:ff:ff:ff:ff
...
To turn on:
ip link set dev <interface-name> multicast on
Another way of determining whether multicast is configured on a network interface is to use lshw as described below:
lshw -class network
Multicast configuration will be listed under the "configuration" section:
configuration: ... multicast=yes ...
Multicast Routing
Configure multicast routing, ensuring the multicast traffic is routed to the appropriate interface, and the route configuration survives reboot. The route we need is 224.0.0.0/4.
ip route add 224.0.0.0/4 dev <device-name>
For more details, see Configure Multicast Routing.
iptables Configuration
If iptables runs on the system, it must be configured to allow multicast traffic for the interface that will handle multicast traffic. Details on how to do that are available here:
IP Forwarding
Consider enabling IP Forwarding.
Test
Ping and generate multicast traffic. For more details see the Troubleshooting section.
Troubleshooting
This section describes the sequence of steps to troubleshoot O/S (Linux)-level multicast problems.
Display Multicast Group Membership Information
netstat -g
displays multicast group membership information for IPv4 and IPv6. Optionally use -n to prevent DNS lookups (faster).
The same information can be obtained from /proc:
cat /proc/net/dev_mcast cat /proc/net/igmp cat /proc/net/igmp6
For more details on these files see /proc above.
ip has multicast querying abilities. See:
Ping
A host with multicast enabled will listen by default to 224.0.0.1, so all the hosts on the network can be pinged.
ping -c 10 224.0.0.1
The networking subsystem might have to be reconfigured to not ignore broadcast ping:
echo "0" > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
Send Multicast Traffic
./bin/ntg receive --protocol=multicast --address=225.5.5.5:5555 ./bin/ntg send --protocol=multicast --address=225.5.5.5:5555
For more details, see
Other Ideas
strace -f socat - UDP4-DATAGRAM:$GRP:$PORT,ip-add-membership=$GRP:$IFACE,bind=0.0.0.0:$PORT,multicast-loop=0
/proc
/proc/net/dev_mcast
Contains Layer2 multicast groups which a device is listening to (interface index, label, number of references, number of bound addresses).
/proc/net/igmp
Contains IPv4 multicast groups joined by this host.
/proc/net/igmp6
Contains IPv6 multicast groups joined by this host.