tap(4) is a virtual Ethernet device driver.  Each tap(4) device appears as a
regular Ethernet NIC to the kernel, with a MAC address and a set of media
interfaces.  It is a clonable network interface, which means any number of
such devices can be created by the administrator.

Just like tun(4), tap(4) offers a TTY interface to each of its devices which
allows an application to read and inject Ethernet frames into the network
stack.  tap(4) devices can be created and used individually for system-wide
configurations, but an application can also use a special clonable device
node to create interfaces on demand.

As a LKM must be compiled with the same environment as the kernel with respect
to a handful of definition (DEBUG, DIAGNOSTIC, MULTIPROCESSOR, LOCKDEBUG and
MALLOCLOG), the package provides a variable, TAP_KERNEL_ENV, to hold the
correct addition to the compilation command line.  For example, you might want
to define it the following way in mk.conf:

  TAP_KERNEL_ENV=    -DDIAGNOSTIC -DMULTIPROCESSOR