ifupdown on Ubuntu 22.04 fails to set DNS correctly -> DNS resolution does not work

[Thomas Dreibholz]

On a Ubuntu 22.04 system with /etc/network/interfaces configuration (ifupdown) instead of Netplan, DNS is not set correctly. Therefore, DNS resolution does not work.

This explains the issue: https://unix.stackexchange.com/questions/714901/dns-broken-when-using-ifupdown-and-systemd-resolved-after-upgrade-to-ubuntu-22-0

Solution:

Fix in /etc/network/if-up.d/resolved:
Old: "$DNS"="$NEW_DNS"
New: $DNS="$NEW_DNS"

Old: "$DOMAINS"="$NEW_DOMAINS"
New: $DOMAINS="$NEW_DOMAINS"

/etc/network/if-up.d/resolved writes files /run/network/ifupdown-<PROTOCOL>-<INTERFACE> containing the DNS configuration. The fix ensures that the files /run/network/ifupdown-<PROTOCOL>-<INTERFACE> contain the correct syntax without quotes for the variable name.

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: ifupdown 0.8.36+nmu1ubuntu3
ProcVersionSignature: Ubuntu 5.19.0-41.42~22.04.1-generic 5.19.17
Uname: Linux 5.19.0-41-generic x86_64
ApportVersion: 2.20.11-0ubuntu82.4
Architecture: amd64
CasperMD5CheckResult: pass
Date: Wed Apr 26 15:04:14 2023
InstallationDate: Installed on 2022-12-08 (139 days ago)
InstallationMedia: Ubuntu-Server 22.04.1 LTS "Jammy Jellyfish" - Release amd64 (20220809)
ProcEnviron:
 LANGUAGE=nb:de:en_US
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ifupdown
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.network.if-up.d.resolved: 2023-04-26T14:36:08.277304