Why Does `InetAddress.isReachable()` Sometimes Return False Even When a Host is Pingable?

Investigating False Reachability from InetAddress.isReachable()

InetAddress, a Java class for working with IP addresses, provides methods for various network-related tasks. One such method is isReachable(), used to determine if a host is accessible via a network. However, in some cases, this method might return false even when pinging the IP address succeeds.

Consider the following code snippet that attempts to check the reachability of a host:

InetAddress byName = InetAddress.getByName("173.39.161.140");
System.out.println(byName);
System.out.println(byName.isReachable(1000));

If you run this code, you might encounter a situation where isReachable() returns false, despite the fact that you can ping the specified IP address.

To address this issue, an alternative approach is presented, which remains platform-independent and leverages information about open ports on the remote machine.

private static boolean isReachable(String addr, int openPort, int timeOutMillis) {
    // Any Open port on other machine
    // openPort =  22 - ssh, 80 or 443 - webserver, 25 - mailserver etc.
    try (Socket soc = new Socket()) {
        soc.connect(new InetSocketAddress(addr, openPort), timeOutMillis);
        return true;
    } catch (IOException ex) {
        return false;
    }
}

By utilizing this code, you can determine the reachability of a host by attempting to establish a connection to a known open port on that host.

Update:

For a more concise version of the code snippet, refer to the following:

private static boolean isReachable(String addr, int openPort, int timeOutMillis) {
    try (Socket soc = new Socket()) {
        soc.connect(new InetSocketAddress(addr, openPort), timeOutMillis);
        return true;
    } catch (IOException ex) {
        return false;
    }
}

By employing this approach, you can reliably determine the reachability of a host, even in scenarios where InetAddress.isReachable() does not provide an accurate indication.

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