Hi again, I just came across a huge clue. A friend had me look again at the java.net properties. This one makes the test program work: -Djava.net.preferIPv4Stack=true Now, as to why suddenly Java was preferring the IPv6 stack, I have no idea. On 2.11.10, Steve White wrote: > 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> > > -- > | - - - - - - - - - - - - - - - - - - - - - - - - - > | Steve White +49(331)7499-202 > | E-Science Zi. 27 Villa Turbulenz > | - - - - - - - - - - - - - - - - - - - - - - - - - > | Astrophysikalisches Institut Potsdam (AIP) > | An der Sternwarte 16, D-14482 Potsdam > | > | Vorstand: Prof. Dr. Matthias Steinmetz, Peter A. Stolz > | > | Stiftung privaten Rechts, Stiftungsverzeichnis Brandenburg: III/7-71-026 > | - - - - - - - - - - - - - - - - - - - - - - - - - > 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" ); > } > } > } -- | - - - - - - - - - - - - - - - - - - - - - - - - - | Steve White +49(331)7499-202 | E-Science Zi. 27 Villa Turbulenz | - - - - - - - - - - - - - - - - - - - - - - - - - | Astrophysikalisches Institut Potsdam (AIP) | An der Sternwarte 16, D-14482 Potsdam | | Vorstand: Prof. Dr. Matthias Steinmetz, Peter A. Stolz | | Stiftung privaten Rechts, Stiftungsverzeichnis Brandenburg: III/7-71-026 | - - - - - - - - - - - - - - - - - - - - - - - - -