A dedicated post collecting solutions to minor NixOS headaches.

nixos-rebuild shows no network activity

On rare occasions, a system rebuild will get stuck while downloading a package from a source. No network activity, no timeout, no writes to the nix store.

root@rahu /h/u/dotfiles (main)# nixos-rebuild boot --flake .
building the system configuration...
[0/9 built, 1/0/1 copied (0.0/681.0 MiB), 0.0/670.5 MiB DL] fetching linux-firmware-20250808-zstd from https://cache.nixos.org

This issue persists if the build command is simply rerun. According to @manveru from the NixOS discourse, nix-daemon has a bug where it might not close a connection to the source, with no timeout context. Force kill it with

sudo pkill -9 nixos-daemon

Now reissue the build.

command-not-found unable to connect to database

The programs.sqlite is only generated for the nixos- prefixed channels. This likely means that you are using NixOS unstable. If this is the case, ensure you use the unstable channel using these commands as root:

nix-channel --add https://nixos.org/channels/nixos-unstable nixos
nix-channel --update

Further reading: NixOS discourse.

Check if a package builds (flakes enabled)

(import <nixpkgs> {}).callPackage ./package.nix { }

nix build -f ./builder.nix

Get current NixOS generation number

readlink /nix/var/nix/profiles/system | cut -d- -f2