Support Split DNS on Linux using systemd-resolved

[tolfsh]

Hi!
This PR aims to add split DNS support --domains-vpn-dns on Linux using systemd-resolved/resolvectl. I mimicked the way it is made on Mac by extending the SplitDnsProvider class. The program check if systemd-resolved is used and then add the nameservers and domain using resolvectl.

I needed the tun name, so I also edited the configure_domain_vpn_dns definition. This fixes #157 and follows the recommendations in #158.

Tested on Debian 13

[dc138]

I've been using the branch Linux_split_DNS_PR118_rework through this AUR package on Arch Linux for a while and it works great. However, it seems (as far as I can tell) that it has been abandoned without hope of being merged.

I'm not too familiar with the codebase, so I can't provide a very thorough review of this PR, but it seems to implement the same functionality a bit more robustly, and on an up-to-date master.

Looking forward for this to be merged.

[Knud3]

Thanks for the PR!

Just sharing some thoughts:

• Is it wanted that domains also work as search domains? If not, then ~ prefix for domains needs to be added. By default resolvectl configure both routing domain and search domain.
• Checking systemd-resolved comment is not most safe thing to do. How about either:
  • Check if /run/systemd/resolve/stub-resolv.conf exist.
  • Or check if systemd-resolved.service is running.
    • systemctl is-active systemd-resolved.
    • Or resolvectl status.
• Check If /etc/resolv.conf points to 127.0.0.1 as then there is e.g. dnscrypt-proxy, dnsmasq, stubby, or unbound installed.
• Would cleanup need some fail safe? If VPN software crashes, or if interface gets stuck, DNS settings might remain attached.
  • Maybe running resolvectl revert <interface_name> before destroying interface?
• Maybe something for future:
  • Also NetworkManager handles resolv.conf if dns=default in /etc/NetworkManager/NetworkManager.conf.
  • For fail safe/older systems use resolvconf/openresolv/nmcli to set routing domains.

[tolfsh]

Hello!

Thank you for this review! I'll modify this as soon as I can.

(I hope I'll be able to add a commit to this PR, this is like my second PR ever on any project XD)

[tolfsh]

Ok I added the following :

• Prefixed domains with ~
• Checking activeness of systemd-resolved service
• Checking presence of 127.0.0.53 in /etc/resolv.conf, but printing a warning if it is not
• Failsafe by reverting interface DNS configuration

For the rest, I think people with particular configuration know what they are doing, and I don't want the script to just do nothing if something is missing. I feel checking that systemd-resolved is running and that /etc/resolv.conf contains 127.0.0.53 is enough.

[dc138]

Works as expected for me running systemd 260.2 with systemd-resolved and systemd-networkd on kernel 7.0.10.