xen service: Forward DNS queries from Xen guests

Provide the option forwardDns in virtualisation.xen.bridge, which
enables forwarding of DNS queries to the default resolver, allowing
outside internet access for the xen guests.

nixos/modules/virtualisation/xen-dom0.nix

@@ -100,6 +100,16 @@ in
             subnet.
           '';
         };
+
+        forwardDns = mkOption {
+          default = false;
+          description = ''
+            If set to <literal>true</literal>, the DNS queries from the
+            hosts connected to the bridge will be forwarded to the DNS
+            servers specified in /etc/resolv.conf .
+            '';
+        };
+
       };
 
     virtualisation.xen.stored =
@@ -339,7 +349,6 @@ in
         interface=${cfg.bridge.name}
         except-interface=lo
         bind-interfaces
-        auth-server=dns.xen.local,${cfg.bridge.name}
         auth-zone=xen.local,$XEN_BRIDGE_NETWORK_ADDRESS/${toString cfg.bridge.prefixLength}
         domain=xen.local
         addn-hosts=/var/run/xen/dnsmasq.hostsfile
@@ -347,8 +356,11 @@ in
         strict-order
         no-hosts
         bogus-priv
-        no-resolv
-        no-poll
+        ${optionalString (!cfg.bridge.forwardDns) ''
+          no-resolv
+          no-poll
+          auth-server=dns.xen.local,${cfg.bridge.name}
+        ''}
         filterwin2k
         clear-on-reload
         domain-needed