Motivation for this change

The current implementation of DEFAULT_HOSTNAME in version.nix does not account for cases where networking.hostName is an empty string. This can lead to incorrect or unexpected behavior in scenarios where networking.hostName is unset or explicitly set to an empty value, even when networking.domain is defined.

In such corner cases, fqdnOrHostName can produce inconsistent results or cause runtime errors due to assumptions about hostName always being set. By explicitly checking for an empty hostName, we can ensure that DEFAULT_HOSTNAME only propagates valid values.

Changes Made

• Updated the DEFAULT_HOSTNAME definition in nixos/modules/misc/version.nix to:

  DEFAULT_HOSTNAME = optionalString (config.networking.hostName != "") config.networking.fqdnOrHostName;

Impact

This change ensures that configurations with networking.hostName = ""; do not inadvertently propagate invalid or unexpected values for DEFAULT_HOSTNAME. It also prevents potential errors when hostName is empty but domain is defined, as fqdnOrHostName may behave unpredictably in such cases.

Things Done:

• Updated nixos/modules/misc/version.nix to handle empty networking.hostName.
• Built and tested NixOS system configurations with different networking.hostName and networking.domain settings to verify behavior.

Testing:

• Built on platform(s):
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested nixos-rebuild with varying configurations:
  • networking.hostName = ""; networking.domain = null;
  • networking.hostName = ""; networking.domain = "example.com";
  • networking.hostName = "example"; networking.domain = "example.com";
  • networking.hostName = null; networking.domain = null;

Submission Checklist:
Make sure you’ve followed the Nixpkgs contribution guidelines:

• Followed the [commit message guidelines](https://nixos.org/manual/nixpkgs/stable/#sec-writing-good-commit-messages).
• Squashed any unnecessary commits.
• Included a clear and concise explanation of the change.