Why Does Inetaddress.Isreachable Return False, When I Can Ping the Ip Address

Why does InetAddress.isReachable return false, when I can ping the IP address?

The "isReachable" method has not been worthy of using for me in many cases. You can scroll to the bottom to see my alternative for simply testing if you're online and capable of resolving external hosts (i.e. google.com) ... Which generally seems to work on *NIX machines.

The issue

There is alot of chatter about this :

Here are other, similar questions :
Detect internet Connection using Java
How do I test the availability of the internet in Java?
And even a reported bug on this same matter :
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4921816

Part 1 : A reproducible example of the problem

Note that in this case, it fails.

       //also, this fails for an invalid address, like "www.sjdosgoogle.com1234sd" 
       InetAddress[] addresses = InetAddress.getAllByName("www.google.com");
      for (InetAddress address : addresses) {
        if (address.isReachable(10000))
        {   
           System.out.println("Connected "+ address);
        }
        else
        {
           System.out.println("Failed "+address);
        }
      }
          //output:*Failed www.google.com/74.125.227.114*

Part 2 : A Hackish Workaround

As an alternative, you can do this :

// in case of Linux change the 'n' to 'c'
    Process p1 = java.lang.Runtime.getRuntime().exec("ping -n 1 www.google.com");
    int returnVal = p1.waitFor();
    boolean reachable = (returnVal==0);

The -c option of ping will allow ping to simply try to reach the server once(as opposed to the infinite ping which we're used to using at the terminal).

This will return 0 if the host is reachable. Otherwise, you will get "2" as a return value.

Much simpler - but of course it is platform specific.
And there may be certain privilege caveats to using this command - but I find it works on my machines.

PLEASE Note that :
1) This solution is not production quality. Its a bit of a hack. If google is down, or your internet is temporarily slow, or maybe even if there is some funniness in your privileges/system settings, if could return false negatives (i.e. it could fail even though the input address is reachable).
2) The isReachable failure is an outstanding issue. Again - there are several online resources indicating that there is no "perfect" way of doing this at the time of this writing, due to the way the JVM tries to reach hosts - I guess it is an intrinsically platform specific task which, although simple, hasn't yet been abstracted sufficiently by the JVM.

isReachable always returning true regardless of what the IP address is

A way to check if some address is reachable, via java.net.InetAddress.isReachable() methods.
The implementation of these methods goes native and tries to do its best to "ping" the address represented by the InetAddress.

Surprisingly, there are many differences between the Windows and the Linux/Unix implementation of java.net.InetAddress.isReachable().

Windows, as strange as it seems, does not officially support an ICMP "ping" system call. The Java SE 5 implementation hence tries to open a TCP socket on port 7 (the echo service) and hopes to get some sort of reply.

Linux/Unix, instead, supports an ICMP "ping" system call. So the implementation of java.net.InetAddress.isReachable() first tries to perform the "ping" system call; if this fails, it falls back trying to open a TCP socket on port 7, as in Windows.

It turns out that in Linux/Unix the ping system call requires root privileges, so most of the times java.net.InetAddress.isReachable() will fail, because many Java programs are not run as root.

The correct approach is the ICMP protocol. This is what ping uses internally. It is recommended see THIS to gather knowledge and proceed.

FROM: Simone Bordet's Blog