Address Space
Multicast ip addresses run from 224.0.0.0 - 239.255.255.255. Some of these however are reserved (e.g. 224.0.0.251 for DNS-SD that uses multicast DNS, a.k.a zeroconf). If you’re building your own application, make use of the 239.0.0.0-239.255.255.255 range.
Kernel Support
Confguration is updated by various programs in /etc/resolv.conf. This is managed by systemd-resolved and will get cached & overwritten when pursuing activities like vpn.
$ cat /boot/config-<kernel-version> | grep CONFIG_IP_MULTICAST CONFIG_IP_MULTICAST=y
Interfaces
Introspection:
# Check if an interface is enabled or not
$ ifconfig enp0s31f6 | grep MULTICAST
enp0s31f6: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
# Check what multicast groups it is subscribed to
$ ip maddr show enp0s31f6
2: enp0s31f6
....
inet 239.0.0.1
inet 224.0.0.251 users 5
inet 224.0.0.1
# Check all interfaces/group associations
$ netstat -g
IPv6/IPv4 Group Memberships
Interface RefCnt Group
--------------- ------ ---------------------
lo 1 224.0.0.251
lo 1 all-systems.mcast.net
enp0s31f6 5 224.0.0.251
enp0s31f6 1 all-systems.mcast.net
# Get IP's instead of names
$ netstat -ng
IPv6/IPv4 Group Memberships
Interface RefCnt Group
--------------- ------ ---------------------
lo 1 224.0.0.251
lo 1 224.0.0.1
enp0s31f6 5 224.0.0.251
enp0s31f6 1 224.0.0.1
Enable/disable:
$ ifconfig eth0 multicast $ ifconfig eth0 -multicast
Testing
IPerf:
# On the sender side $ iperf -c 224.0.0.1 -u -T 32 -t 3 -i 1 ------------------------------------------------------------ Client connecting to 224.0.0.1, UDP port 5001 Sending 1470 byte datagrams, IPG target: 11215.21 us (kalman adjust) Setting multicast TTL to 32 UDP buffer size: 208 KByte (default) ------------------------------------------------------------ [ 3] local 192.168.1.4 port 44915 connected with 224.0.0.1 port 5001 [ ID] Interval Transfer Bandwidth [ 3] 0.0- 1.0 sec 131 KBytes 1.07 Mbits/sec [ 3] 1.0- 2.0 sec 128 KBytes 1.05 Mbits/sec [ 3] 2.0- 3.0 sec 128 KBytes 1.05 Mbits/sec [ 3] 0.0- 3.0 sec 386 KBytes 1.05 Mbits/ # On the listener side $ iperf -s -u -B 224.0.0.1 -i 1 ------------------------------------------------------------ Server listening on UDP port 5001 Binding to local address 224.0.0.1 Joining multicast group 224.0.0.1 Receiving 1470 byte datagrams UDP buffer size: 208 KByte (default) ------------------------------------------------------------ [ 3] local 224.0.0.1 port 5001 connected with 192.168.1.4 port 49386 [ ID] Interval Transfer Bandwidth Jitter Lost/Total Datagrams [ 3] 0.0- 1.0 sec 129 KBytes 1.06 Mbits/sec 0.189 ms 0/ 90 (0%) [ 3] 1.0- 2.0 sec 128 KBytes 1.05 Mbits/sec 0.230 ms 0/ 89 (0%) [ 3] 2.0- 3.0 sec 128 KBytes 1.05 Mbits/sec 0.175 ms 0/ 89 (0%) [ 3] 0.0- 3.0 sec 386 KBytes 1.05 Mbits/sec 0.182 ms 0/ 269 (0%)
Ping 224.0.0.1:
# From kernel 2.6 the ping response from subscribers to multicast groups has been disabled by default. # https://0x657573.wordpress.com/2010/11/22/localhost-does-not-response-to-ping-to-multicast-address-224-0-0-1/ # To re-enable $ echo "0" | sudo tee -a /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts # Then from the same computer, or another computer on the multicast group $ ping 224.0.0.1 PING 224.0.0.1 (224.0.0.1) 56(84) bytes of data. 64 bytes from 192.168.1.4: icmp_seq=1 ttl=64 time=0.030 ms
Create MCast Groups
# Create a new mcast group.
# Remember to keep it in the 239.0.0.0-239.255.255.255 range.
$ sudo smcroute -j enp0s31f6 239.0.0.1
# Verify
$ ip maddr show enp0s31f6
inet 239.0.0.1
inet 224.0.0.251 users 5
inet 224.0.0.1
inet6 ff02::fb
$ ping 239.0.0.1
PING 239.0.0.1 (239.0.0.1) 56(84) bytes of data.
64 bytes from 192.168.1.4: icmp_seq=1 ttl=64 time=0.031 ms
# Leave
$ sudo smcroute -l enp0s31f6 239.0.0.1
Zeroconf
Uses multicast, typically on IPv4 address 224.0.0.251 or IPv6 address ff02::fb and UDP port 5353.