The simplest way to do this is to act like a hacker. Snoop around the network traffic, either on the device itself or a port on the network. In the case of IPsec, for example, you would see encapsulating security payload (ESP) frames (Protocol 50). Yet, when you look inside the packet payload, you will only see garbled characters -- no clear text at all. Network snooping tools are easily available on the Internet and are simple to use. Of these, Wireshark is probably the most popular tool. You may find this resource on how to do penetration testing on your VPN useful.
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This was first published in April 2012