From 857c6490832fb5dccbac6ca7a679496e85a85e21 Mon Sep 17 00:00:00 2001
From: Graham Christensen <graham@grahamc.com>
Date: Tue, 23 Mar 2021 11:40:54 -0400
Subject: [PATCH] network-interfaces: set hostname from sysctl if defined

The `networking.hostname` option was changed to not permit periods
in names, due to a strict reading of RFC 1123. For users who need
the hostname to be fully qualified, the networking.hostName option
suggests using boot.kernel.sysctl."kernel.hostname" as a workaround.

This option works correctly at boot time, but every "nixos-rebuild
switch" will change the hostname back to the unqualified version.

This commit brings the activation script in line with the
documentation's recommendation.
---
 nixos/modules/tasks/network-interfaces.nix | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/nixos/modules/tasks/network-interfaces.nix b/nixos/modules/tasks/network-interfaces.nix
index f730ec82bdf5..a3929428f694 100644
--- a/nixos/modules/tasks/network-interfaces.nix
+++ b/nixos/modules/tasks/network-interfaces.nix
@@ -1115,9 +1115,10 @@ in
     # Set the host and domain names in the activation script.  Don't
     # clear it if it's not configured in the NixOS configuration,
     # since it may have been set by dhcpcd in the meantime.
-    system.activationScripts.hostname =
-      optionalString (cfg.hostName != "") ''
-        hostname "${cfg.hostName}"
+    system.activationScripts.hostname = let
+        effectiveHostname = config.boot.kernel.sysctl."kernel.hostname" or cfg.hostName;
+      in optionalString (effectiveHostname != "") ''
+        hostname "${effectiveHostname}"
       '';
     system.activationScripts.domain =
       optionalString (cfg.domain != null) ''