This is somewhat related to previous discussion at [1] and related JGRP-1152 [2]. Since we've upgraded JBoss AS trunk to JGroups 2.10.0.Alpha3, it will no longer start on a machine that supports both IPv4 and IPv6. This is because we don't specify java.net.preferIPv4Stack in our startup scripts, so Util.getIpStackType() returns StackType.IPv6. But we configure IPv4 multicast addresses by default so the JGRP-1152 check fails the startup. Since we're specifying IPv4 addresses by default, perhaps our startup scripts could set -Djava.net.preferIPv4Stack=true. Jason can comment if he likes; it may be necessary although I don't really like it (yet another config in an obscure location). I'm wondering if JGroups can handle this situation more flexibly. I've attached an *untested* patch that shows what I was thinking: 1) Add a new StackType.Either which Util.getIpStackType() returns if the OS supports both stack types and no java.net.preferXXX property is set. 2) Configurator.setupProtocolStack() recognizes StackType.Either and when it's analyzing the InetAddresses it's seeing, it uses the first one it sees to switch to either StackType.IPv4 or StackType.IPv6. So in this case the users choice of addresses controls the behavior. 3) Thereafter, the normal JGRP-1152 checks apply, so illegal inconsistencies are caught and rejected. I checked for other uses of Util.getIpStackType() and tweaked the one that needed it to deal with a return value of StackType.Either. I haven't checked *all* uses of StackType though; wanted to get some feedback first. -- Brian Stansberry Lead, AS Clustering JBoss by Red Hat [1] http://old.nabble.com/Protocol-stack-issue-on-dual-stack-%28IPv4-and-v6%2... [2] https://jira.jboss.org/jira/browse/JGRP-1152