Support DuplicateAddressDetection by default

[Lukas Märdian]

netplan & networkd only use systemd-networkd's default of DuplicateAddressDetection=ipv6, thus not checking for IPv4 conflicts.

From what I know RedHat based systems on the other hand use the arping tool during ifup in their ifcfg system (ARPCHECK=yes by default) to check for IP conflicts. In order to reach a similar result in Ubuntu, we would need to place an override configuration snippet in /etc/systemd/network/10-netplan-eth0.network.d/override.conf, specifying an [Address] section and corresponding DuplicateAddressDetection=both setting for each IP address [0].

This is probably even something that netplan should be doing by default, but it is not currently.

[0] https://www.freedesktop.org/software/systemd/man/systemd.network.html#DuplicateAddressDetection=

[Aleksandar Ivanisevic (aleksandar-e)]

conf override still requires you to know the interface name, I'm using a solution where an interface is killed from the network dispatcher if a duplicate is detected. IP will be up for a short time so some packets might be lost, but it works universally, no metter how many interfaces are there or how they are called.

cat<<'EOF'>/etc/networkd-dispatcher/routable.d/dad.sh
#!/bin/bash
# check for duplicate IP and either kill the interface or proceed
if arping -0 -c 2 -w 2 $ADDR; then
   ip link set $IFACE down
   echo conflict detected on $IFACE for $ADDR, it has been shut down
else
   echo no duplicate IPs detected on $IFACE addr $ADDR
fi
EOF
chmod +x /etc/networkd-dispatcher/routable.d/dad.sh