semi-spread example: serve internal vhosts on the VPN addresses only, over IPv4 and IPv6

docs/examples/semi-spread/dns.nix

@@ -1,6 +1,6 @@
-# Names inside the VPN. Every swarm name that is not public resolves to the
-# VPN address of the machine serving it, through a static table written on
-# each host. Identical on every host.
+# Names inside the VPN. Every swarm name, the public ones included, resolves to
+# the VPN addresses (IPv4 and IPv6) of the machine serving it, through a static
+# table written on each host. Identical on every host.
 #
 # Two readers need the table. Processes on the host read /etc/hosts
 # (`networking.hosts`). Containers (agents, swarm services) ask the hive's
@@ -49,17 +49,20 @@ let
     };
 
   elsewhere = removeAttrs servedBy [ self ];
+
+  # Each machine's names, once per address family: A and AAAA answers.
+  addressesOf = machine: [
+    mesh.${machine}.address
+    mesh.${machine}.address6
+  ];
+  table = lib.concatMap (
+    machine: map (address: lib.nameValuePair address elsewhere.${machine}) (addressesOf machine)
+  ) (builtins.attrNames elsewhere);
 in
 {
-  networking.hosts = lib.mapAttrs' (
-    machine: names: lib.nameValuePair mesh.${machine}.address names
-  ) elsewhere;
+  networking.hosts = lib.listToAttrs table;
 
   services.dnsmasq.settings.addn-hosts = "${builtins.toFile "swarm-vpn-hosts" (
-    lib.concatStringsSep "\n" (
-      lib.mapAttrsToList (
-        machine: names: "${mesh.${machine}.address} ${lib.concatStringsSep " " names}"
-      ) elsewhere
-    )
+    lib.concatMapStrings (entry: "${entry.name} ${lib.concatStringsSep " " entry.value}\n") table
   )}";
 }