nixos/avahi-daemon: resolve mdns only over enabled protocols, disable…

… ipv6 by default

see https://github.com/lathiat/nss-mdns#:~:text=in%20such%20a%20situation%20causes%20long%20timeouts%20when%20resolving%20hosts
especially:
> libnss_mdns.so.2 resolves both IPv6 and IPv4 addresses, libnss_mdns4.so.2 only IPv4 addresses and
> libnss_mdns6.so.2 only IPv6 addresses. Due to the fact that most mDNS responders only register local IPv4
> addresses via mDNS, most people will want to use libnss_mdns4.so.2 exclusively. Using libnss_mdns.so.2
> or libnss_mdns6.so.2 in such a situation causes long timeouts when resolving hosts since most modern
> Unix/Linux applications check for IPv6 addresses first, followed by a lookup for IPv4.

nixos/doc/manual/release-notes/rl-2405.section.md

@@ -30,6 +30,9 @@ The pre-existing [services.ankisyncd](#opt-services.ankisyncd.enable) has been m
 - `mkosi` was updated to v19. Parts of the user interface have changed. Consult the
   [release notes](https://github.com/systemd/mkosi/releases/tag/v19) for a list of changes.
 
+- `services.avahi.nssmdns` got split into `services.avahi.nssmdns4` and `services.avahi.nssmdns6` which enable the mDNS NSS switch for IPv4 and IPv6 respectively.
+  Since most mDNS responders only register IPv4 addresses, most users want to keep the IPv6 support disabled to avoid long timeouts.
+
 ## Other Notable Changes {#sec-release-24.05-notable-changes}
 
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->

nixos/modules/services/networking/avahi-daemon.nix

@@ -42,6 +42,7 @@ in
 {
   imports = [
     (lib.mkRenamedOptionModule [ "services" "avahi" "interfaces" ] [ "services" "avahi" "allowInterfaces" ])
+    (lib.mkRenamedOptionModule [ "services" "avahi" "nssmdns" ] [ "services" "avahi" "nssmdns4" ])
   ];
 
   options.services.avahi = {
@@ -93,7 +94,7 @@ in
 
     ipv6 = mkOption {
       type = types.bool;
-      default = config.networking.enableIPv6;
+      default = false;
       defaultText = literalExpression "config.networking.enableIPv6";
       description = lib.mdDoc "Whether to use IPv6.";
     };
@@ -218,13 +219,28 @@ in
       };
     };
 
-    nssmdns = mkOption {
+    nssmdns4 = mkOption {
+      type = types.bool;
+      default = false;
+      description = lib.mdDoc ''
+        Whether to enable the mDNS NSS (Name Service Switch) plug-in for IPv4.
+        Enabling it allows applications to resolve names in the `.local`
+        domain by transparently querying the Avahi daemon.
+      '';
+    };
+
+    nssmdns6 = mkOption {
       type = types.bool;
       default = false;
       description = lib.mdDoc ''
-        Whether to enable the mDNS NSS (Name Service Switch) plug-in.
+        Whether to enable the mDNS NSS (Name Service Switch) plug-in for IPv6.
         Enabling it allows applications to resolve names in the `.local`
         domain by transparently querying the Avahi daemon.
+
+        ::: {.note}
+        Due to the fact that most mDNS responders only register local IPv4 addresses,
+        most user want to leave this option disabled to avoid long timeouts when applications first resolve the none existing IPv6 address.
+        :::
       '';
     };
 
@@ -257,8 +273,17 @@ in
     users.groups.avahi = { };
 
     system.nssModules = optional cfg.nssmdns pkgs.nssmdns;
-    system.nssDatabases.hosts = optionals cfg.nssmdns (mkMerge [
-      (mkBefore [ "mdns_minimal [NOTFOUND=return]" ]) # before resolve
+    system.nssDatabases.hosts = let
+      mdnsMinimal = if (cfg.nssmdns4 && cfg.nssmdns6) then
+        "mdns_minimal"
+      else if (!cfg.nssmdns4 && cfg.nssmdns6) then
+        "mdns6_minimal"
+      else if (cfg.nssmdns4 && !cfg.nssmdns6) then
+        "mdns4_minimal"
+      else
+        "";
+    in optionals (cfg.nssmdns4 || cfg.nssmdns6) (mkMerge [
+      (mkBefore [ "${mdnsMinimal} [NOTFOUND=return]" ]) # before resolve
       (mkAfter [ "mdns" ]) # after dns
     ]);