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POWhat is the quickest way to detect an unreachable host in Java?
 

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  1. POWhat is the quickest way to detect an unreachable host in Java?
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    <p>I would like the fastest and most accurate function <code>boolean isReachable(String host, int port)</code> that passes the following JUnit tests under the conditions below. Timeout values are specified by the JUnit test itself, and may be considered "unreachable."</p> <p><strong>Please note:</strong> All answers must be platform-independent. This means that <code>InetAddress.isReachable(int timeout)</code> is not going to work, since it relies on port <code>7</code> to do a ping on Windows (ICMP ping being an undocumented function on Windows), and this port is blocked in this setup.</p> <p>LAN Setup:</p> <ul> <li><code>thisMachine</code> (<code>192.168.0.100</code>)</li> <li><code>otherMachine</code> (<code>192.168.0.200</code>)</li> <li><strong>no</strong> machine is called <code>noMachine</code> or has the IP <code>192.168.0.222</code> (always unreachable)</li> <li>both machines are running Apache Tomcat on port <code>8080</code>; all other ports are unreachable (including port <code>7</code>)</li> <li><code>example.com</code> (<code>208.77.188.166</code>) is running a webserver on port <code>80</code> and is only reachable when the LAN is connected to the Internet</li> </ul> <p>Occasionally, the LAN is disconnected from the Internet in which case only local machines called by IP address are reachable (all others are unreachable; there's no DNS).</p> <p><strong>All tests are run on <code>thisMachine</code>.</strong></p> <pre><code>@Test(timeout=1600) // ~320ms per call (should be possible to do better) public void testLocalhost() { // We can always reach ourselves. assertTrue(isReachable("localhost", 8080)); assertTrue(isReachable("127.0.0.1", 8080)); assertTrue(isReachable("thisMachine", 8080)); // Even if there's no DNS! assertTrue(isReachable("192.168.0.100", 8080)); assertFalse(isReachable("localhost", 80)); // Nothing on that port. } @Test(timeout=5500) // ~1867ms per call (should be able to do better) public void testLAN() { assertTrue(isReachable("192.168.0.200", 8080)); // Always connected to the LAN. assertFalse(isReachable("192.168.0.222", 8080)); // No such a machine. assertFalse(isReachable("noMachine", 8080)); // No such machine. } </code></pre> <p>The following test is only run when the LAN is <strong>disconnected</strong> from the Internet.</p> <pre><code>@Test(timeout=5600) // ~1867ms per call (reasonable?) public void testNoDNS() { assertFalse(isReachable("otherMachine", 8080)); // No DNS. assertFalse(isReachable("example.com", 80)); // No DNS &amp; no Internet. assertFalse(isReachable("208.77.188.166", 80)); // No Internet. } </code></pre> <p>The following test is only run when the LAN is <strong>connected</strong> to the Internet.</p> <pre><code>@Test(timeout=5600) // ~1867ms per call (reasonable?) public void testHaveDNS() { assertTrue(isReachable("otherMachine", 8080)); // DNS resolves local names. assertTrue(isReachable("example.com", 80)); // DNS available. assertTrue(isReachable("208.77.188.166", 80)); // Internet available. } </code></pre>
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