IP Address not obtained in java

Question

This code used to return my local ip address as 192.xxx.x.xxx but now it is returning 127.0.0.1 . Please help me why the same code is returning different value. Is there something that I need to watch at linux OS.

import java.util.*;
import java.lang.*;
import java.net.*;

public class GetOwnIP
{
  public static void main(String args[]) {
    try{
      InetAddress ownIP=InetAddress.getLocalHost();
      System.out.println("IP of my system is := "+ownIP.getHostAddress());
    }catch (Exception e){
      System.out.println("Exception caught ="+e.getMessage());
    }
  }
}

Answer 1

127.0.0.1 is the loopback adapter - it's a perfectly correct response to the (somewhat malfomed) question "what is my IP address?"

The problem is that there are multiple correct answers to that question.

EDIT: The docs for getLocalHost say:

If there is a security manager, its checkConnect method is called with the local host name and -1 as its arguments to see if the operation is allowed. If the operation is not allowed, an InetAddress representing the loopback address is returned.

Is it possible that the change in behaviour is due to a change in permissions?

EDIT: I believe that NetworkInterface.getNetworkInterfaces is what you need to enumerate all the possibilities. Here's an example which doesn't show virtual addresses, but works for "main" interfaces:

import java.net.*;
import java.util.*;

public class Test
{
    public static void main(String[] args)
        throws Exception // Just for simplicity
    {
        for (Enumeration<NetworkInterface> ifaces = 
               NetworkInterface.getNetworkInterfaces();
             ifaces.hasMoreElements(); )
        {
            NetworkInterface iface = ifaces.nextElement();
            System.out.println(iface.getName() + ":");
            for (Enumeration<InetAddress> addresses =
                   iface.getInetAddresses();
                 addresses.hasMoreElements(); )
            {
                InetAddress address = addresses.nextElement();
                System.out.println("  " + address);
            }
        }
    }
}

(I'd forgotten just how awful the Enumeration<T> type is to work with directly!)

Here are the results on my laptop right now:

lo:
  /127.0.0.1
eth0:
  /169.254.148.66
eth1:
eth2:
ppp0:
  /10.54.251.111

(I don't think that's giving away any hugely sensitive information :)

If you know which network interface you want to use, call NetworkInterface.getByName(...) and then look at the addresses for that interface (as shown in the code above).

Answer 2

When you use InetAddress.getLocalHost() you are not guaranteed to get a particular interface, ie. you could receive the loopback (127) interface or a connected one.

In order to ensure you always get an external interface, you should use the java.net.NetworkInterface class. The static getByName(String) class will give you the interface with the defined name (such as "eth0"). Then you can use the getInetAddresses() function to obtain the IP addresses (could be just one) bound to that interface.

NetworkInterface ni = NetworkInterface.getByName("eth1");
ni.getInetAddresses();