`hostnamectl hostname --static` should be allowed

[hacker1024]

Describe the bug

hostnamectl hostname --static is currently blocked.

There are valid use-cases for this tool on NixOS, though. I, for example, want to set the static hostname dynamically based on system properties. The transient hostname is not used by NetworkManager for DHCP requests, which is why the static hostname must be used.

Steps To Reproduce

A service like the following does not currently work.

{
  networking.hostName = "";
  systemd.services.set-hostname = {
    wants = [ "network-pre.target" ];
    before = [ "network-pre.target" ];
    wantedBy = [ "multi-user.target" ];
    path = with pkgs; [ coreutils ];
    script = ''
      hostnamectl hostname --static "$(sha256sum /sys/devices/virtual/dmi/id/board_serial | cut --characters=-8)"
    '';
    serviceConfig.Type = "oneshot";
  };
}

Expected behavior

hostnamectl hostname --static should not fail.

Notify maintainers

@NixOS/systemd

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

[user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
 - system: `"aarch64-linux"`
 - host os: `Linux 5.10.104, NixOS, 24.05 (Uakari), 24.05pre-git`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.18.1`
 - channels(root): `"nixos"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

---

Add a :+1: reaction to issues you find important.

[arianvp]

This makes sense to me.

[hacker1024]

Also see: https://github.com/NixOS/nixpkgs/issues/224080#issuecomment-1509170912

It might make sense to conditionally allow setting the static hostname interactively when networking.hostName is not set.

[flokli]

I'd be happy to review such patch.

[TheDelus]

Since I came here while searching for a way to dynamically change the hostname to the machine-id on boot I want to leave my (hacky) solution here:

  # /etc/hostname symlink is not created on empty hostName in config
  networking.hostName = lib.mkForce "";
  systemd.services.set-machine-id-as-hostname = {
    description = "Set hostname from /etc/machine-id";

    # Run after the system has booted and the filesystem is mounted
    after = [ "network.target" ];

    # Set to run before the hostname is set
    before = [ "systemd-hostnamed.service" ];

    serviceConfig = {
      User = "root";
      Group = "root";
      Type = "oneshot";
      RemainAfterExit = true;
    };

    script = ''
      ${pkgs.coreutils}/bin/echo "system-''$(cat /etc/machine-id)" > /etc/hostname
    '';

    # Ensure the service runs on boot
    wantedBy = [ "multi-user.target" ];
  };

It seems to be working fine. But I am not sure if this will break anything.