systemd-resolved - This is going to be good

[epoch1970]

I'm late to the party it seems but these days I am fiddling with ubuntu 18-02 machines and today suddenly my resolver stopped working on some. I use the old interfaces config file on these machines, the uplink is a simplistic dhcp client.

I had the delight to read, line 3 and 4...

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$ cat /etc/resolv.conf 
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
# 127.0.0.53 is the systemd-resolved stub resolver.
# run "systemd-resolve --status" to see details about the actual nameservers.
...

... that ubuntu had switched to using a systemd service to configure the resolver and it runs a local DNS forwarder by default.

I don't mind the DNS forwarder, although when I want one I configure dnsmasq myself. The worry was that somehow dhclient and resolved are not completely in tune so the DHCP-defined DNS servers were missing from /etc/resolv.conf on some machines (not all)...

EDIT: Caveat emptor. If you do what follows, you're butchering your system. Do not blindly remove system files on your OS. The VMs I was doing this on were disposable.

To revert to a working /etc/resolv.conf I had to run:

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sudo systemctl disable systemd-resolved.service
sudo systemctl stop systemd-resolved
sudo rm /etc/dhcp/dhclient-enter-hooks.d/resolved
sudo reboot

Look ma, no "chattr -i". Ok, an rm instead...

The man page for systemd-resolved doesn't have the "I don't like this, how do I disable this?" section you can find in their PNIN page. I suppose it will come in good time.

[pjf02536]

When using Network Manager in Raspbain Buster the cleanest way I've found to permanently leave resolv.conf alone is

To stop NetworkManager from touching /etc/resolv.conf, set main.dns=none with a configuration file in /etc/NetworkManager/conf.d/:

/etc/NetworkManager/conf.d/dns.conf
[main]
dns=none

(ref https://wiki.archlinux.org/index.php/Ne ... esolv.conf )