Why Can\'t My Java Program Ping Hosts Other Than Localhost?

Troubleshooting Pinging Issue in Java

When attempting to ping an IP address using Java, some users may encounter the problem where the localhost is reachable but all other hosts return an "Host is NOT reachable" message. This article explores the underlying cause and provides a possible solution.

Code Illustration

The provided code snippet attempts to ping two IP addresses: localhost (127.0.0.1) and an external address (173.194.32.38). The isReachable() method returns true if the host is reachable within the specified timeout (5000 milliseconds), otherwise false.

Root Cause

According to the Java documentation, the isReachable() method typically employs ICMP ECHO REQUESTs to determine host reachability. However, using ICMP often requires elevated privileges (root permissions).

Solution

Option #1: Obtain Administrative Privileges

If ICMP is not blocked by the system firewall, the issue may be that the code is not running with sufficient permissions to perform ICMP operations. Try running the program with administrative privileges.

// Windows
> runas /profile /user:administrator java -jar myapp.jar

// Unix/Linux
> sudo java -jar myapp.jar

If ICMP is blocked, the second option is to use TCP instead.

Option #2: Enable TCP Ping

<code class="java">// This example uses the Google DNS server
String ipAddress = "173.194.32.38";
try {
    Socket socket = new Socket();
    socket.connect(new InetSocketAddress(ipAddress, 80), 5000);
    System.out.println("Host " + ipAddress + " is reachable");
} catch (IOException e) {
    System.out.println("Host " + ipAddress + " is NOT reachable");
}</code>

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