[ 1 Address Space ] [ 2 Kernel Support ] [ 3 Interfaces ] [ 4 Testing ] [ 5 Create MCast Groups ] [ 6 Zeroconf ] [ 7 Programming ]

Address Space

Multicast Adress 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:

Ping 224.0.0.1:

Create MCast Groups

Zeroconf

When zeroconf, or more officially named DNS-SD (DNS Service Discovery) uses multicast (it doesn’t have to), it will use IPv4 address 224.0.0.251 or IPv6 address ff02::fb and UDP port 5353.

Programming

GitHub - mjansson/mdns: Public domain mDNS/DNS-SD library in C

Technical Notes

Multicast DNS

Analytics