cups: Add a listenAddresses option

nixos/modules/services/printing/cupsd.nix

@@ -56,6 +56,15 @@ in
         '';
       };
 
+      listenAddresses = mkOption {
+        type = types.listOf types.str;
+        default = [ "127.0.0.1:631" ];
+        example = [ "*:631" ];
+        description = ''
+          A list of addresses and ports on which to listen.
+        '';
+      };
+
       bindirCmds = mkOption {
         type = types.lines;
         internal = true;
@@ -154,7 +163,9 @@ in
 
         SystemGroup root wheel
 
-        Listen localhost:631
+        ${concatMapStrings (addr: ''
+          Listen ${addr}
+        '') cfg.listenAddresses}
         Listen /var/run/cups/cups.sock
 
         # Note: we can't use ${cups}/etc/cups as the ServerRoot, since

nixos/tests/printing.nix

@@ -7,9 +7,9 @@ import ./make-test.nix ({pkgs, ... }: {
     server =
       { config, pkgs, ... }:
       { services.printing.enable = true;
+        services.printing.listenAddresses = [ "*:631" ];
         services.printing.cupsdConf =
           ''
-            Listen server:631
             <Location />
               Order allow,deny
               Allow from all