I've recently had to deal with the application connectivity issue (details on stackoverflow ) that appeared after the migration of the application to a new server. It resulted in "Connection timed out" Java exceptions in certain cases. The answer was on the surface but I didn't know where to look at exactly. So I had to investigate and apply network sniffing tools such as tcpdump and Wireshark . Here I'd like to share my experience with the network analysis. The issue and the cause. Wireshark. Tcpdump. The issue and the cause The following exception was thrown by Saxon XSLT processor when the document function was invoked: Caused by: org.apache.commons.lang.exception.NestableRuntimeException: net.sf.saxon.trans.DynamicError: net.sf.saxon.trans.DynamicError: java.net.ConnectException: Connection timed out at java.net.PlainSocketImpl.socketConnect(Native Method) at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:333) at j...