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 (layer 1) is generally an electrical or optical connection between systems. In the case of a VPN, the tunnel takes the place of that physical connection. The next step in the OSI model is the Ethernet layer (layer 2). The ARP protocol is often...