Generally, ifupdown is no longer recommended (or supported). It's recommended to move to netplan, or at least to directly configuring networkd or network-manager.
For your specific issue, it's been a while since I looked at ifupdown, but from an old bug comment: https://bugs.launchpad.net/ubuntu/+source/ifupdown/+bug/1628552/comments/9 I think ifupdown static configuration of dns does not work with systemd- resolved; you must have resolvconf installed for that to work (since resolvconf installs the ifupdown hook, while resolved ignores ifupdown completely). Maybe your system upgrade removed the resolvconf package? ** Changed in: systemd (Ubuntu) Status: New => Incomplete -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1883412 Title: 20.04: DNS servers from /etc/network/interfaces not honored by systemd-resolve To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1883412/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
