RMI, sockets and NAT

Question

Apparently a RMI server in a LAN with NAT cannot be seen outside of LAN. Thus, it is possible to manage polling client in LAN and server outside of LAN, but I cannot make bidirectional RMI connection (where server could invoke client's methods, that is), right? Now, is there same kind of a problem with sockets?
Thanks,
Yuzaa

Answer 1

Yuzza,
There is no problem with sockets, NAT is causing the problem. Happily, there is an easy work-around.
When you create a remote object, you can specify two additional arguments, an RMIServerSocketFactory, and an RMIClientSocketFactory.
The RMIClientSocketFactory is an interface which defines one method:
Socket createSocket(String host, int port);
Your RMI client gets an object implementing this interface from your server, to connect back to it. Currently your server gives your client a default RMIClientSocketFactory object. This object will let the client connect to the address inside your NAT subnet, this will not work with NAT. That is the problem.
How can you fix this? Simply implement your own RMIClientSocketFactory, something like this:   class YuzzaCSF implements RMIClientSocketFactory, Serializable {
      final String host;
      YuzzaCSF(String host) { this.host = host; }
      public Socket createSocket(String host, int port) throws IOException {
         return new Socket(this.host, port);
   }Instantiate one using your server's address from outside NAT, and it will force the client to connect correctly.
This is the core of the solution, there are a few more small steps, but I think you can figure out the rest from here. It would not be as satisfying if I did it all for you :-)
John