Multicast DNS

Owned by Daniel Stonier
Last updated: May 01, 2021
3 min read
[ 1 Address Space ] [ 2 Kernel Support ] [ 3 Interfaces ] [ 4 Testing ] [ 5 Create MCast Groups ] [ 6 Zeroconf ] [ 7 Programming ]

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:

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