I believe Adrian found the problem -- IPv6: see http://www.jboss.com/index.html?module=bb&op=viewtopic&t=123463 . I've updated testsuite/imports/server-configs.xml to set java.net.preferIPv4Stack=true on all configs. If I run the JGroups McastSenderTest and McastReceiverTest I can send messages from a process bound to localhost to a process bound to $MYTESTIP_1 on a number of QA lab machines. But a process bound to localhost cannot receive the messages sent from localhost. Specifying -Djava.net.preferIPv4Stack=true solves the problem. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4103910#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...