diff --git a/nixos/modules/services/networking/smokeping.nix b/nixos/modules/services/networking/smokeping.nix
index 0747ff6dd5a9..4470c18fd533 100644
--- a/nixos/modules/services/networking/smokeping.nix
+++ b/nixos/modules/services/networking/smokeping.nix
@@ -124,7 +124,8 @@ in
       };
       hostName = mkOption {
         type = types.str;
-        default = config.networking.hostName;
+        default = config.networking.fqdn;
+        defaultText = "\${config.networking.fqdn}";
         example = "somewhere.example.com";
         description = "DNS name for the urls generated in the cgi.";
       };
@@ -156,6 +157,7 @@ in
       ownerEmail = mkOption {
         type = types.str;
         default = "no-reply@${cfg.hostName}";
+        defaultText = "no-reply@\${hostName}";
         example = "no-reply@yourdomain.com";
         description = "Email contact for owner";
       };
@@ -239,18 +241,18 @@ in
       targetConfig = mkOption {
         type = types.lines;
         default = ''
-					probe = FPing
-					menu = Top
-					title = Network Latency Grapher
-					remark = Welcome to the SmokePing website of xxx Company. \
-									 Here you will learn all about the latency of our network.
-					+ Local
-					menu = Local
-					title = Local Network
-					++ LocalMachine
-					menu = Local Machine
-					title = This host
-					host = localhost
+          probe = FPing
+          menu = Top
+          title = Network Latency Grapher
+          remark = Welcome to the SmokePing website of xxx Company. \
+                   Here you will learn all about the latency of our network.
+          + Local
+          menu = Local
+          title = Local Network
+          ++ LocalMachine
+          menu = Local Machine
+          title = This host
+          host = localhost
         '';
         description = "Target configuration";
       };
diff --git a/nixos/modules/tasks/network-interfaces.nix b/nixos/modules/tasks/network-interfaces.nix
index afb9c5404169..f730ec82bdf5 100644
--- a/nixos/modules/tasks/network-interfaces.nix
+++ b/nixos/modules/tasks/network-interfaces.nix
@@ -398,6 +398,24 @@ 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 e598549ef1d2..2e92b4259a6a 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";
 }
diff --git a/nixos/tests/smokeping.nix b/nixos/tests/smokeping.nix
index 4ac672b814bd..ccacf60cfe4b 100644
--- a/nixos/tests/smokeping.nix
+++ b/nixos/tests/smokeping.nix
@@ -8,6 +8,7 @@ import ./make-test-python.nix ({ pkgs, ...} : {
     sm =
       { ... }:
       {
+        networking.domain = "example.com"; # FQDN: sm.example.com
         services.smokeping = {
           enable = true;
           port = 8081;