mikas blog

I’m working for a customer who’s using IBM blades. Remote access isn’t limited to e.g. SoL but also possible through a Remote Console feature using a Java applet.

After migrating one of my 32bit systems to a fresh 64bit system I suddenly couldn’t use this Remote Console feature any longer. The error message was (leaving it for search engines and help other affected users):

load: class vnc.VncViewer.class not found.
java.lang.ClassNotFoundException: vnc.VncViewer.class
	at sun.plugin2.applet.Applet2ClassLoader.findClass(Applet2ClassLoader.java:152)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:303)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
	at sun.plugin2.applet.Plugin2ClassLoader.loadCode(Plugin2ClassLoader.java:447)
	at sun.plugin2.applet.Plugin2Manager.createApplet(Plugin2Manager.java:2880)
	at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Plugin2Manager.java:1397)
	at java.lang.Thread.run(Thread.java:619)
Caused by: java.net.ConnectException: Network is unreachable
	at java.net.PlainSocketImpl.socketConnect(Native Method)
	at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:333)
	at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:195)
	at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:182)
	at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366)
	at java.net.Socket.connect(Socket.java:525)
	at sun.net.NetworkClient.doConnect(NetworkClient.java:161)
	at sun.net.www.http.HttpClient.openServer(HttpClient.java:394)
	at sun.net.www.http.HttpClient.openServer(HttpClient.java:529)
	at sun.net.www.http.HttpClient.<init>(HttpClient.java:233)
	at sun.net.www.http.HttpClient.New(HttpClient.java:306)
	at sun.net.www.http.HttpClient.New(HttpClient.java:323)
	at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:860)
	at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:801)
	at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:726)
	at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1049)
	at java.net.HttpURLConnection.getResponseCode(HttpURLConnection.java:373)
	at sun.plugin2.applet.Applet2ClassLoader.getBytes(Applet2ClassLoader.java:458)
	at sun.plugin2.applet.Applet2ClassLoader.access$000(Applet2ClassLoader.java:46)
	at sun.plugin2.applet.Applet2ClassLoader$1.run(Applet2ClassLoader.java:126)
	at java.security.AccessController.doPrivileged(Native Method)
	at sun.plugin2.applet.Applet2ClassLoader.findClass(Applet2ClassLoader.java:123)
	... 6 more
Exception: java.lang.ClassNotFoundException: vnc.VncViewer.class

The error message might not be obvious at a glance and that’s why I’m writing about it actually. It’s NOT the:

load: class vnc.VncViewer.class not found.

why it’s failing but instead the real reason for the failure is the:

java.net.ConnectException: Network is unreachable

As you can read in Debian’s Bug Tracking System in bug #560044:

Netbase has recently introduced the sysctl-setting
net.ipv6.bindv6only=1 in /etc/sysctl.d/bindv6only.conf and this setting will probably be the default in squeeze.

This setting breaks networking in java, and any traffic will always
result in a “java.net.SocketException: Network is unreachable”.

To quote /etc/sysctl.d/bindv6only.conf:

When disabled, IPv6 sockets will also be able to send and receive IPv4 traffic with addresses in the form ::ffff:192.0.2.1 and daemons listening on IPv6 sockets will also accept IPv4 connections.

When IPV6_V6ONLY is enabled, daemons interested in both IPv4 and IPv6 connections must open two listening sockets.

To work around this issue you can either execute the Java process through "java -Djava.net.preferIPv4Stack=true" or to change the IPv6 behaviour system wide execute "sysctl -w net.ipv6.bindv6only=0". To make this setting permanent across reboots adjust the setting inside /etc/sysctl.d/bindv6only.conf. After applying this workaround the Remote Console should work again.

This entry was posted by mika on Tuesday, April 6th, 2010 at 20:28 and is filed under Computer, Debian, English, Hardware. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.