Hi, I tried to create Java RMI program and everything goes very well when I used one machine or I used host and VM. However, the programs is not working when I run it on two different VM(s).

on VM1, I run 'rmiregistry' and 'java server <serverIP>' - Works

on VM2, I run 'java client <serverIP>' - ERROR

Error :

Exception in thread "main" java.rmi.ConnectIOException: Exception creating connection to: 192.168.122.30; nested exception is: java.net.NoRouteToHostException: No route to host (Host unreachable)

I have tried :

Put the specific port under the implementation.java 'super()'

Use the hostname@<serverIP>

Inject the ipaddress in code

Run server using -Djava.rmi.server.hostname=<serverIP>

Still isnt working.

SSH, telnet, ping, and traceroute are working
Here my Code

Client.java

import java.rmi.*;  import java.rmi.Naming; // for Naming.lookup()  import java.io.*;  import java.net.MalformedURLException;           public class ConverterClient           {               public static void main( String args[] ) throws Exception               {                   // Check for hostname argument                   if( args.length != 1 )                   {                       System.out.println                           ( "Syntax: ConverterClient <hostname>" );                       System.exit( 1 );                   }                int port = 1091;                   ConverterInterface service =                       (ConverterInterface) Naming.lookup                           ( "//" + args[0] +":"+port+ "/ConverterInterface");                   DataInputStream din = new DataInputStream( System.in );                   for( ;; ){                       System.out.println();                       System.out.println                         ( "1 - Convert Fahrenheit degree to celsius degree" );                       System.out.println                         ( "2 - Convert celsius degree to Fahrenheit degree" );                       System.out.println                         ( "3 - Exit" ); System.out.println();                       System.out.print( "Choice: " );                       String line = din.readLine();                       Integer choice = new Integer( line );                       int value = choice.intValue();                       float tempValue;                       switch( value )                       {                           case 1:                               System.out.print( "Farenheit degree: " );                               line = din.readLine(); System.out.println();                               tempValue = (Float.valueOf(line)).floatValue();                               System.out.println                                   ( "The equivalent Celsius degree: "                                   + service.f2c( tempValue ) );                               break;                           case 2:                               System.out.print( "Celsius degree: " );                               line = din.readLine();                               tempValue = (Float.valueOf(line)).floatValue();                               // Call remote method                               System.out.println                                   ( "The equivalent Fahrenheit degree: "                                   + service.c2f( tempValue ) );                           break;                           case 3:                               System.exit(0);                           default :                               System.out.println ("Invalid option");                           break;                       }                   }               }           }
Server.java

import java.net.MalformedURLException; import java.rmi.Naming; import java.rmi.RemoteException; import java.rmi.registry.LocateRegistry; import java.rmi.*; import java.rmi.server.*; public class ConverterServer {    public static void main( String args[] ) throws RemoteException    {        System.out.println("Launching server..");        ConverterImpl svr = new ConverterImpl();        int port = 1091;        try {            LocateRegistry.createRegistry(port);            System.out.println("java RMI registry created.");        } catch (RemoteException e) {            System.out.println("java RMI registry already exists.");        }        try{            // then bind it with the rmi registry            Naming.bind ( "//"+ args[0]+":"+port+"/ConverterInterface", svr );            System.out.println("Addition Server is ready at:" + "//"+ args[0]+":"+port+"/ConverterInterface");            System.out.println ("Service listening....");        }        catch( Exception e )        { System.out.println( "Server Failed: " + e ); }    } }

Thank you!

Jane Cour wrote:Exception in thread "main" java.rmi.ConnectIOException: Exception creating connection to: 192.168.122.30; nested exception is: java.net.NoRouteToHostException: No route to host (Host unreachable)

This error doesn't actually have anything to do with RMI -- it simply means that the client JVM can't connect to the server JVM because there's no network connection between the two machines they are running on.

But you said that ping was working? Does that mean that from the machine where VM2 is running you got a command line and tried to ping the machine where VM1 is running, and ping connected? If that's the case then I think you probably have to find somebody to help you with the network configuration, perhaps there's a firewall in the way or something like that.

Seconded. It's almost certainly firewall issues. An RMI server publishes its registry on tcp port 1099. However, when a client connects, the port that it then uses is unpredictable. The system assigns a separate port number for each client/server session.

RMI, like CORBA, was designed in an era when there was more trust on networks. CORBA is mostly history, but RMI still has occasional uses. For more information on how to deal with RMI in an age of firewalls, see this: https://docs.oracle.com/javase/7/docs/platform/rmi/spec/rmi-arch6.html

The firewall issue is one reason that web services are so popular now, since virtually no machines block the http and https ports. Message Brokers also fill a niche there. However EJB3 remote beans do use RMI-IIOP.

Thanks for all the responses!

I already figured it out how to solve it and I hope this will be helpful for others.

To get the vm talking each other, the programs do not have any problems but the firewall on vm(s) is. So before running the programs, please make sure, the firewalld is disabled and stop running. Then, It will work.

Run this command in the vm(s)

systemctl status firewalld   systemctl disable firewalld   systemctl stop firewalld   systemctl status firewalld

Dont forget to enable and run your firewalld after this!

Thanks!

I really don't recommend that. Athough you may not have any direct-to-Internet connections on that VM, I prefer to lower my guards as little as possible. So if I'm running services like RMI or backend databases on a DMZ LAN, I set up the firewall to permit requests originating from within the DMZ, while still blocking external requests. So, for example, if my DMZ is subnet 10.14.0.0/16, I'd add this rule:
-I INPUT -p tcp -s 10.14.0.0/16 -j ACCEPT

There actually are firewall services that understand mechanisms like RMI and can dynamically allow the RMI connection - something like that is used with the FTP rules. But I'm not conversant with anything specific to RMI itself.