I was recently called out to a customer to fix a networking issue on WebLogic.

The problem was that they had two different instances. Instance one makes a transactional request to an EJB on the second instance. On the second instance it was clear that the transaction was commited, but the first node never got the response. We tracked the issue down to the network level. Server one made the request using one IP version and the second instance responded on another. Apparently this mix of IP versions was the culprit.

The solution, we chose, was to force the JVM for the instances to use the IPv4 protocol. This is done by setting the following startup parameters.

-Djava.net.preferIPv6Addresses=false -Djava.net.preferIPv4Stack=true

Starting WebLogic with those parameters fixed the issue.