Being able to set conflicting options like noDns and useDnsmasq isn't great. One option could be to an enum as you suggest with a system option that applies the logic currently in place for systemd-resolved and unbound. The alternative is to get rid of the automatic detection and turn the dependency upside down so that setting dns to systemd-resolved automatically enabled it.

Maybe the automatic detection could enabled only if dns is null?

That doesn't seem intuitive. How about an auto option which is the default?

Even better! I'll get to work! Do I keep but deprecate the old useDnsmasq option?

You can deprecate by adding it here: https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/rename.nix and then removing it accordingly.

When deprecating an option please also add a notice to the release notes.

Aha, thanks!

And if you can fix the merge conflict, I'll go ahead and merge this.

@peterhoeg Awesome, thanks! Rebased :)

Thank you for doing up the PR and the quick turnaround time!