add ability to specify listening interfaces

system/config.example.json

@@ -37,7 +37,9 @@
   "ExternalAddress": "10.0.1.5",
 
   // Port to listen for DNS queries on. To disable the DNS server, comment this
-  // out or set it to zero.
+  // out or set it to zero. By default, the DNS server listens on all
+  // interfaces, but you can specify an interface by replacing this with a list
+  // of [interface_addr_or_name, port].
   "DNSServerPort": 53,
 
   // Ports to listen for game connections on.
@@ -45,6 +47,12 @@
     // Format of entries in this dictionary:
     // name: [port, version, behavior]
 
+    // port is normally just an integer (which will cause the server to listen
+    // on that port on all interfaces), but you can also replace the integer
+    // with a 2-list of [address, port] to listen in a specific port. For
+    // example, that might look like:
+    // "xb-login": [["en0", 9500], "xb", "login_server"],
+
     // Various versions of PSO hardcode these ports in the clients. Don't change
     // these unless you don't want to support certain versions of PSO.
     // Note: The pc_console_detect behavior is used for separating PSO PC and