Checking broadcast and non-IP traffic
The tcpdump and wireshark tools are useful for troubleshooting an "almost-working" OpenVPN setup. Wireshark is available for Linux, Mac OS X, and Windows. It can be used as a command-line tool but most often the GUI-based version is used. On most Unix/Linux-based platforms, the command-line tool tcpdump is also available.
We will now use tcpdump and wireshark to view the flow of packets over a tap-based VPN setup.
Address Resolution Protocol traffic
One of the most basic types of Ethernet traffic present on all networks is Address Resolution Protocol (ARP) traffic. ARP is a prime example of an Ethernet protocol that does not travel across point-to-point links (such as tun-based OpenVPN setups). The physical layer ...