Small rmi program opens several ports; what and why?

Question

Hello,
I'm a bit confused about my small rmi program.
It seems that more ports are opened than I expected, which could give firewall-problems if we're not careful.
You can see below
- my testprogram.
- opend ports when my server app starts
fyi: my rmiregistry was opened on port 1091 and there is no client connected yet
  Proto  Local Address          Foreign Address        State
  TCP    0.0.0.0:220            0.0.0.0:0              LISTENING
  TCP    0.0.0.0:1091           0.0.0.0:0              LISTENING
  TCP    127.0.0.1:1091         127.0.0.1:2170         ESTABLISHED
  TCP    127.0.0.1:2170         127.0.0.1:1091         ESTABLISHED
  TCP    172.21.4.241:220       172.21.4.241:2171      ESTABLISHED
  TCP    172.21.4.241:2171      172.21.4.241:220       ESTABLISHEDdifferences:
- i see a listening connection on port 220 and 1091; i understand this
- i see a connection from 1091 to 2170 and vice versa; 1091 is my rmi registry. why is there suddenly an active connection with 2170 ? Can I avoid this or should'nt it disappear after the object is bound ?
- i see a connection from 220 to 2171 and vice versa; 220 is my exported object, but why is there suddenly an active connection with 2171 ? Can I avoid this or should'nt it disappear after the object is bound ?
public class rmisockettester {
     public static void main(String[] args) throws Exception {
         int port = 1091;
         String hostName = "localhost";
          System.out.println("start");
        System.setSecurityManager(new RMISecurityManager());
        String hostname="127.0.0.1";
        String rmiName = "//" + hostName + ":" + port + "/" + "serv2";
        System.out.print("Binding \"" + rmiName + "\"....");
        serverclock tsi = new serverclock();
        Naming.bind(rmiName, tsi);
        System.out.println("binding done");
        Thread.sleep(10000);
          System.out.println("end");
public class serverclock  extends UnicastRemoteObject implements serverclockinterface
    public serverclock()
    throws Exception
         super(220);
    public int setTime(Date d)
         return 9;
public interface serverclockinterface
    public int setTime(Date d)
    throws RemoteException;
}

Answer 1

I'm a bit confused about my small rmi program.
It seems that more ports are opened than I expected, which could give firewall-problems if we're not careful.
- i see a listening connection on port 220 and 1091; i understand thisGood.
- i see a connection from 1091 to 2170 and vice versa; 1091 is my rmi registry. why is there suddenly an active connection with 2170 ?That happens when the client connects to the Registry. I don't know why you think that's unexpected. This particular connection may be for DGC purposes, but there is nothing harmful or unexpected about it. Or it may be a client connection. Which is the entire purpose of the exercise.
Can I avoid thisWhy would you want to avoid it?
or should'nt it disappear after the object is bound ?Why?
why is there suddenly an active connection with 2171 ?Either also for DGC purposes or for a client connection. Why do you think this is a problem?
Can I avoid thisWhy would you want to avoid it?
or should'nt it disappear after the object is bound ?Why?
     super(220);Why are you using a reserved port number here? Port 220 is reserved for IMAP v3 servers. See [RFC 1700|http://www.ietf.org/rfc/rfc1700.txt]. Use a port number above 1024 please.
I think you're getting confused because you're running the server and the client on the same host. Run them on separate hosts, then running netstat on the server host will give you a much clearer picture.