java.net.SocketException: Network is unreachable

I hit this about a week ago . First time I saw it was on my office desktop running Debian unstable. Since I was not doing too much Java on it I decided it was a problem with JConsole. I nearly lost a bet out of this:

I was pretty sure JConsole was able to attach to local processes even when they were started without any JMX options enabled. Borislav Tonchev was pretty sure it wasn't. I quickly wrote a Java class with its main method sleeping for 100 seconds and tried to attach to its process. Unfortunately I wasn't able to do so. At that point Borislav walked away with 10 bucks coming out of my pocket.

I was curious enough to check this stuff and at first appeared that Java didn't like the bsdgroups option my ext3 /tmp file system was mounted with. Trying the same thing on my home PC, with bsdgroups disabled showed this java.net.SocketException: Network is unreachable. At this point I was starting to loose ground. I decided to check the docs ( http://java.sun.com/javase/6/docs/technotes/guides/management/jconsole.html ) and they confirmed my point. I checked the documented behavior in a JVM running inside an Windows XP installation I have ( VirtualBox image for the corporate stuff in the office ) and Borislav unhappily brought my money back.

At this point I decided the exception under Debian was caused by a bug in JConsole - probably it was not maintained too much in recent releases as a similar tool appeared - VisualVM.

Several days after this long background, on Saturday, I've also hit the same exception on a production server running Tomcat. Pretty damn strange. I was not able to figure it out immediately. The actual problem was introduced in Debian in the beginning of December last year, with the netbase package setting:

# cat /etc/sysctl.d/bindv6only.conf
net.ipv6.bindv6only=1

This did not showed up on the server immediately, since the netbase upgrade did not apply the new setting. The exception appeared after a restart almost two months after the upgrade.

The workaround is to set the above to "0" as it was before, or to add the option -Djava.net.preferIPv4Stack=true to each Java process you start. I prefer the former as I did not want to configure every Java program (e.g. I use azureus/vuze) manually.

More information could be found in Debian bug #560044