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On an Ubuntu 20.04 server, netplan is used to configure the network. It can configure ordinary static IP addresses but I need additional IPv6 addresses with preferred_lft=0. Netplan can't do that (there's an open bug for that). So I found out that it actually goes like this:

  • Netplan reads its own limited config and transforms it into a config for the more powerful systemd-network.
  • systemd-network probably passes on its data to things like the ip command when a network device appears.
  • The network device then applies the actual configuration. Down here everything is possible.

If netplan can't help me, I tried to move on to systemd. Since I don't want to remove the apparently widely used and highly praised netplan entirely, I just need to add my additional addresses besides the basic netplan config.

I tried to create the file /etc/systemd/network/web-ipv6.conf with this content:

[Match]
Name=ens33 (this is the name from the existing netplan file)

[Address]
Address=fd9e:21a7:a92c:2323::2/64
PreferredLifetime=0

The address fd9e:21a7:a92c:2323::1 is configured statically via netplan. In this demo it's a local address, in reality it's my server's main IPv6 address. Additional addresses have a different interface value, like this ...::2.

I guess I have to apply my config with systemctl restart systemd-networkd but I'm not sure. The log says things I'd consider a success. But the new IPv6 address ...::2 doesn't show up in ip addr.

In the man page for systemd.network I've read that only a single file for a [Match] will be used. I clearly have two files now: one from netplan (/run/systemd/network/10-netplan-ens33.network) and my other one. So this doesn't seem to work. But I don't want to edit netplan's file as it will surely be overwritten sometime.

How should I continue with this? I'd like to have a separate and persistent file that only adds my additional addresses. I'm going to manage that file in a custom script.

Should I go back to ifupdown or what it was in the old days? In a previous server setup I use a file named /etc/network/if-up.d/dynamic6 that contains all the ip addr add ... dev ... preferred_lft 0 commands. From what I've seen this is today considered deprecated.

  • Is your NIC actually named ens33? If not, then it won't match. As for netplan, I prefer to dump it and configure networking directly. – Michael Hampton Jun 30 at 21:04
  • Yes, the name is correct. ip link shows that. (That dev system is a VMware guest machine if that's interesting.) – ygoe Jul 1 at 7:03
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Seems the old ifupdown functionality has been recreated for systemd-networkd. The package networkd-dispatcher, pre-installed on Ubuntu Server 20.04, lets me add a script to /etc/networkd-dispatcher/routable.d/ that is executed when a device comes up. In that script I can run ip addr add commands as I need. It is called on netplan apply, systemctl restart systemd-networkd and on system boot. In all cases, my additional IPv6 addresses were added as "deprecated" when they'd otherwise be missing.

Here's an article that explains it, with a short sample script: https://andreas.scherbaum.la/blog/archives/963-if-up-and-if-down-scripts-with-systemd.html

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