Hi,

A Java networking problem has arisen very recently on one of our servers,
an Opeteron with Scientific Linux 5.5, running the distro Java without
problems--until a week or so ago.
 
The effect is that all Java apps have become unable to do DNS host name
resolution, and throw an UnknownHostException.  However nothing seems
otherwise amiss with networking on this machine.  All non-Java programs
resolve outside host names fine.

I wrote a small test program, attached.  It works fine on other machines
here.  Fails on the server, with an IP address.  But if I give it a valid
IP number, or the name of the server itself, it succeeds.

I have done security updates fairly regularly; this may have something to
do with the problem, but I really don't know for sure.

Also tried:

* Cut the hosts file down to the bare minimum, and also the resolv.conf
  file and re-started networking.  No improvement.

* Logged in as various users.  All show the same problem.

* Cut down and exported all related environment variables to bare minimum
	(JAVA_HOME, PATH, *LIBRARY_PATH, etc),
  No effect.

* Reversed the updates from last week, and re-booted with the
  previous kernel.  No improvement.

* Fiddled with various Java system properties, as in
  http://download.oracle.com/javase/6/docs/technotes/guides/net/properties.html
  Nothing seemed to help.

* Installed a Java 1.5, built the test program with it, ran the test program
  under that.  It failed just like the distro version.

Any ideas?

=========================================================
To run the attached test
	javac nettest.java
	java nettest <hostname> <optional-port-number>

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import java.net.Socket;

public class
nettest
{
	public static void
	main( String [] a )
	{
		if( a.length > 0 )
		{
			String host = a[0];
			int    port = 80;
			if( a.length > 1 )
			{
				port = Integer.parseInt( a[1] );
			}
			try
			{
				Socket si = new Socket( host, port );
				System.out.println( "Success connecting to "
					+ host );
			}
			catch( Exception e )
			{
				System.err.println( e.toString() );
				e.printStackTrace();
			}
		}
		else
		{
			System.err.println( "please provide host string" );
		}
	}
}