From 971f0b45ef7129073a6e996f1c63a1f2b470a7e0 Mon Sep 17 00:00:00 2001
From: Michael Weiss <michael@primeos.dev>
Date: Sat, 10 Oct 2020 11:30:47 +0000
Subject: [PATCH] nixos/networking: Add a read-only option for the FQDN

This is a convenience option that can be used to quickly obtain the
configured FQDN.
---
 nixos/modules/tasks/network-interfaces.nix | 18 ++++++++++++++++++
 nixos/tests/hostname.nix                   | 14 ++++++++++----
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/nixos/modules/tasks/network-interfaces.nix b/nixos/modules/tasks/network-interfaces.nix
index d369aab5457..63a704e31f4 100644
--- a/nixos/modules/tasks/network-interfaces.nix
+++ b/nixos/modules/tasks/network-interfaces.nix
@@ -394,6 +394,24 @@ in
       '';
     };
 
+    networking.fqdn = mkOption {
+      readOnly = true;
+      type = types.str;
+      default = if (cfg.hostName != "" && cfg.domain != null)
+        then "${cfg.hostName}.${cfg.domain}"
+        else throw ''
+          The FQDN is required but cannot be determined. Please make sure that
+          both networking.hostName and networking.domain are set properly.
+        '';
+      defaultText = literalExample ''''${networking.hostName}.''${networking.domain}'';
+      description = ''
+        The fully qualified domain name (FQDN) of this host. It is the result
+        of combining networking.hostName and networking.domain. Using this
+        option will result in an evaluation error if the hostname is empty or
+        no domain is specified.
+      '';
+    };
+
     networking.hostId = mkOption {
       default = null;
       example = "4e98920d";
diff --git a/nixos/tests/hostname.nix b/nixos/tests/hostname.nix
index 3b87303d73e..8a77afc2173 100644
--- a/nixos/tests/hostname.nix
+++ b/nixos/tests/hostname.nix
@@ -7,9 +7,12 @@ with import ../lib/testing-python.nix { inherit system pkgs; };
 with pkgs.lib;
 
 let
-  makeHostNameTest = hostName: domain:
+  makeHostNameTest = hostName: domain: fqdnOrNull:
     let
       fqdn = hostName + (optionalString (domain != null) ".${domain}");
+      getStr = str: # maybeString2String
+        let res = builtins.tryEval str;
+        in if (res.success && res.value != null) then res.value else "null";
     in
       makeTest {
         name = "hostname-${fqdn}";
@@ -26,13 +29,16 @@ let
           ];
         };
 
-        testScript = ''
+        testScript = { nodes, ... }: ''
           start_all()
 
           machine = ${hostName}
 
           machine.wait_for_unit("network-online.target")
 
+          # Test if NixOS computes the correct FQDN (either a FQDN or an error/null):
+          assert "${getStr nodes.machine.config.networking.fqdn}" == "${getStr fqdnOrNull}"
+
           # The FQDN, domain name, and hostname detection should work as expected:
           assert "${fqdn}" == machine.succeed("hostname --fqdn").strip()
           assert "${optionalString (domain != null) domain}" == machine.succeed("dnsdomainname").strip()
@@ -60,7 +66,7 @@ let
 
 in
 {
-  noExplicitDomain = makeHostNameTest "ahost" null;
+  noExplicitDomain = makeHostNameTest "ahost" null null;
 
-  explicitDomain = makeHostNameTest "ahost" "adomain";
+  explicitDomain = makeHostNameTest "ahost" "adomain" "ahost.adomain";
 }