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How to setup netplan/networkd to detect an IP conflict being putting networking online?

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We've hit an issue where our Ubuntu 20.04 systems seem to bring networking up without verifying there's no IP conflict that will occur. Our Red Hat based systems check for any collision and if detected don't bring the interface up.

We're using static IP assignments in netplan with networkd as the renderer. Is there a configuration option or setting that needs to be adjusted so Ubuntu will check to make sure there is no IP conflict before it actually brings the newly provisioned system online?

Thanks!

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cn flag

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, you could be placing an override configuration snippet in /etc/systemd/network/10-netplan-eth0.network.d/override.conf ("eth0" depending on your netplan ID/interface name), specifying an [Address] section and corresponding DuplicateAddressDetection=both setting for your IP address [0].

This is probably even something that netplan should be doing by default, but it is not currently. I've added this as a feature request to the netplan bug tracker [1].

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

[1] https://bugs.launchpad.net/netplan/+bug/1959190

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