Why is My Java Code Failing to Ping Hosts Other Than Localhost?

Troubleshooting Host Reachability Using Ping in Java

In Java, pinging an IP address can be done using the InetAddress.isReachable() method. However, it may sometimes encounter issues where hosts other than localhost appear unreachable.

Original Problem

Consider the following Java code snippet:

<code class="java">public static void main(String[] args) throws UnknownHostException, IOException {
    String ipAddress = "127.0.0.1";
    InetAddress inet = InetAddress.getByName(ipAddress);

    System.out.println("Sending Ping Request to " + ipAddress);
    System.out.println(inet.isReachable(5000) ? "Host is reachable" : "Host is NOT reachable");

    ipAddress = "173.194.32.38";
    inet = InetAddress.getByName(ipAddress);

    System.out.println("Sending Ping Request to " + ipAddress);
    System.out.println(inet.isReachable(5000) ? "Host is reachable" : "Host is NOT reachable");
}</code>

The output of this code is:

Sending Ping Request to 127.0.0.1
Host is reachable
Sending Ping Request to 173.194.32.38
Host is NOT reachable

This result suggests that pinging localhost is successful, but other hosts are unreachable.

Solution Explanation

According to the InetAddress.isReachable() method in the Java documentation:

"... A typical implementation will use ICMP ECHO REQUESTs if the
privilege can be obtained, otherwise it will try to establish a TCP
connection on port 7 (Echo) of the destination host..."

Option #1 (using ICMP ECHO REQUESTs) typically requires administrative (root) privileges. If these privileges are not granted, the method will attempt to use TCP to determine reachability.

Conclusion

When encountering issues with pinging hosts other than localhost, it's possible that ICMP privileges are not granted. Ensuring that the necessary privileges are granted can resolve the Host Not Reachable error.

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