How to set a custom DNS server within Nixos

The 127.0.1.1 entry is most likely placed there by dnsmasq which is a local daemon for serving (and crucially caching) dns and dhcp. It's possible to configure NetworkManager to not run dnsmasq as follows: edit the file /etc/NetworkManager/NetworkManager.conf and comment out the line dns=dnsmasq by placing a # at the beginning of the line.

sudo nano /etc/NetworkManager/NetworkManager.conf

Now restart NetworkManager:

sudo service network-manager restart

Next check that the 127.0.1.1 entry has vanished from /etc/resolv.conf and replaced with those obtained from your router.

To upgrade NixOS:

1. Ensure you have a backup of your NixOS installation and that you know how to restore from the backup, if the need arises.
2. Review the NixOS release notes to ensure you account for any changes that need to be done manually. In particular, sometimes options change in backward-incompatible ways.
3. As the root user, replace the NixOS channel so it points to the one you want to upgrade to, while ensuring it is named nixos:

   nix-channel --add https://nixos.org/channels/nixos-18.09 nixos
   

   and update the channel (nix-channel --update).
4. As the root user, build your system:

   nixos-rebuild --upgrade boot
   

5. Reboot to enter your newly-built NixOS.

If things go wrong you can reboot, select the previous generation, use nix-channel to add the old channel, and then nixos-rebuild boot to make the working generation the default; I think it's more reliable to rebuild than to use nixos-rebuild --rollback.

Alternative process

If you want to try the upgrade without messing around with channels, you can use a GIT clone of the nixpkgs repo:

cd nixpkgs
git checkout release-18.03
nixos-rebuild -I nixpkgs="$PWD" build

If all is well...

sudo nixos-rebuild -I nixpkgs="$PWD" boot

The downside to this approach is that subsequent calls to Nix tools, such as nixos-rebuild, require the -I flag to specify the correct nixpkgs.  That is, until you update the channel.