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

Question

InetAddress byName = InetAddress.getByName("173.39.161.140");
System.out.println(byName);
System.out.println(byNam         


        

Answer 1

 private boolean isReachable(int nping, int wping, String ipping) throws Exception {

    int nReceived = 0;
    int nLost = 0;

    Runtime runtime = Runtime.getRuntime();
    Process process = runtime.exec("ping -n " + nping + " -w " + wping + " " + ipping);
    Scanner scanner = new Scanner(process.getInputStream());
    process.waitFor();
    ArrayList<String> strings = new ArrayList<>();
    String data = "";
    //
    while (scanner.hasNextLine()) {
        String string = scanner.nextLine();
        data = data + string + "\n";
        strings.add(string);
    }

    if (data.contains("IP address must be specified.")
            || (data.contains("Ping request could not find host " + ipping + ".")
            || data.contains("Please check the name and try again."))) {
        throw new Exception(data);
    } else if (nping > strings.size()) {
        throw new Exception(data);
    }

    int index = 2;

    for (int i = index; i < nping + index; i++) {
        String string = strings.get(i);
        if (string.contains("Destination host unreachable.")) {
            nLost++;
        } else if (string.contains("Request timed out.")) {
            nLost++;
        } else if (string.contains("bytes") && string.contains("time") && string.contains("TTL")) {
            nReceived++;
        } else {
        }
    }

    return nReceived > 0;
}

nping is number of try to ping ip(packets), if you have busy network or systems choose biger nping numbers.
wping is time waiting for pong from ip, you can set it 2000ms
for using this method u can write this:

isReachable(5, 2000, "192.168.7.93");

Answer 2

I would suggest that the ONLY reliable way to test an internet connection is to actually connect AND download a file, OR to parse the output of an OS ping call via exec(). You cannot rely on the exit code for ping and isReachable() is crap.

You cannot rely on a ping exit code as it returns 0 if the ping command executes correctly. Unfortunately, ping executes correctly if it can't reach the target host but gets a "Destination host unreachable" from your home ADSL router. This is kind of a reply that gets treated as a successfull hit, thus exit code = 0. Have to add though that this is on a Windows system. Not checked *nixes.

Answer 3

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.

Answer 4

Just mentioning it explicitly since the other answers don't. The ping part of isReachable() requires root access on Unix. And as pointed out by bestsss in 4779367:

And if you ask why ping from bash doesn't, actually it does need as well. Do that ls -l /bin/ping.

Since using root was not an option in my case the solution was to allow access to port 7 in the firewall to the specific server I was interested in.