No internet after minor update - problem with resolv.conf

Question

Today I updated my 18.04 system, and after reboot I lost all internet connection both through wifi and ethernet cable. Wifi and cable work normally for other devices. No router or network settings were changed recently.

After trying many things (I will detail below), there is only one thing that partially works: add a nameserver to the /etc/resolv.conf file. However, this file gets rewritten upon reboot and whenever I try to initialize my VPN connection. So currently I cannot go on the internet with a VPN.

Things I tried and did nothing (most - but not all - based on the discussion here: No internet after upgrade from 16.04 to 18.04)

• downgrade all the linux-* packages I found that were updated in the dpkg and apt logs using dpkg -i and the cached versions (all other packages updates at this time do not seem to have any relation to networking: thunderbird (which I don't use), vim, xxd, element-desktop)
• boot an old kernel version using GRUB
• add the nameserver to /etc/resolvconf/resolv.conf.d/base and /head (the resolv.conf.d dir didn't exist on my system)
• add DNS=8.8.8.8 (and others) to /etc/systemd/resolved.conf
• add my ethernet and wifi interfaces to /etc/network/interface with iface <device> inet dhcp
• set static ip and manually add DNS through the GUI

All the above attempts did not work individually nor in various combinations, even after rebooting the system in each attempt.

Some diagnostics and general info:

• I'm running ubuntu native on this machine (not through dual boot, nor through a VM)
• I can ping 8.8.8.8, and other public IPs fine
• my device connects to the local network and router fine, receives ip configurations as normal (both wifi and cable)
• settings for both the wired and wireless connections have always been set to automatic for ip configs, DNS and DHCP which used to work (and continues to work on other devices)
• the update was from kernel 5.4.0.94 to 5.4.0.96 (if I'm using the correct terminology here)

Hence, I believe it has something to do with the DNS not being found correctly and/or with the systemd-resolved service, but I don't know what.

Answer 1

Well well, of course I figure out the solution 12 minutes after posting this question... It is simply

sudo apt-get install resolvconf

In a couple of the answers, restarting resolvconf was the final step, but it wasn't present on my system. Internet started working immediately after installing it. I have no idea what caused the problem in the first place, if the update perhaps removed resolvconf or something...