Add support for resolvconf interface

[basos9]

For systems with more complex DNS configurations, that also do not want to use systemd-resolved, it would be convenient to add support for resolvconf interface, in the same way as NetworkManager also supports this.

[dsommers]

Do you have any pointers to how NetworkManager integrates with resolvconf?

The challenge with /etc/resolv.conf based DNS resolver configuration is that it does not support split-DNS. So I don't quite fully understand how that fits into "more complex DNS configurations". Using only /etc/resolv.conf will always send DNS queries to the first responding nameserver line.

I've been pondering on adding support for dnsmasq support, though, which can also be used by NetworkManager as well. There might still need to be a few "running methods", depending on if NetworkManager is to be involved or not.

Otherwise, to use a similar approach to resolvconf interacting directly with /etc/resolv.conf, OpenVPN 3 Linux can use that as well instead of systemd-resolved:

  # openvpn3-admin netcfg-service --config-unset systemd-resolved
  # openvpn3-admin netcfg-service --config-set resolv-conf /etc/resolv.conf

Then you need to ensure the openvpn3-service-netcfg service is stopped and restarted. This will require all VPN sessions to be stopped first; this process is responsible for providing the tun/dco interfaces to the VPN client processes (openvpn3-service-client).