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TAP(4)			 BSD Kernel Interfaces Manual			TAP(4)

     tap -- Ethernet tunnel software network interface

     device tap

     The tap interface is a software loopback mechanism	that can be loosely
     described as the network interface	analog of the pty(4), that is, tap
     does for network interfaces what the pty driver does for terminals.

     The tap driver, like the pty driver, provides two interfaces: an inter-
     face like the usual facility it is	simulating (an Ethernet	network	inter-
     face in the case of tap, or a terminal for	pty), and a character-special
     device "control" interface.

     The network interfaces are	named "tap0", "tap1", etc., one	for each con-
     trol device that has been opened.	These Ethernet network interfaces per-
     sist until	if_tap.ko module is unloaded (if tap is	built into your	ker-
     nel, the network interfaces cannot	be removed).

     The tap interface permits opens on	the special control device /dev/tap.
     When this device is opened, tap will return a handle for the lowest un-
     used tap device (use devname(3) to	determine which).  Control devices
     (once successfully	opened)	persist	until if_tap.ko	is unloaded in the
     same way that network interfaces persist (see above).

     Each interface supports the usual Ethernet	network	interface ioctl(2)s,
     such as SIOCSIFADDR and SIOCSIFNETMASK, and thus can be used with
     ifconfig(8) like any other	Ethernet interface.  When the system chooses
     to	transmit an Ethernet frame on the network interface, the frame can be
     read from the control device (it appears as "input" there); writing an
     Ethernet frame to the control device generates an input frame on the net-
     work interface, as	if the (non-existent) hardware had just	received it.

     The Ethernet tunnel device, normally /dev/tapN, is	exclusive-open (it
     cannot be opened if it is already open) and is restricted to the super-
     user, unless the sysctl(8)	variable	is non-zero.
     A read() call will	return an error	(EHOSTDOWN) if the interface is	not
     "ready".  Once the	interface is ready, read() will	return an Ethernet
     frame if one is available;	if not,	it will	either block until one is or
     return EWOULDBLOCK, depending on whether non-blocking I/O has been	en-
     abled.  If	the frame is longer than is allowed for	in the buffer passed
     to	read(),	the extra data will be silently	dropped.

     A write(2)	call passes an Ethernet	frame in to be "received" on the
     pseudo-interface.	Each write() call supplies exactly one frame; the
     frame length is taken from	the amount of data provided to write().
     Writes will not block; if the frame cannot	be accepted for	a transient
     reason (e.g., no buffer space available), it is silently dropped; if the
     reason is not transient (e.g., frame too large), an error is returned.
     The following ioctl(2) calls are supported	(defined in <net/if_tap.h>):

     TAPSDEBUG		  The argument should be a pointer to an int; this
			  sets the internal debugging variable to that value.
			  What,	if anything, this variable controls is not
			  documented here; see the source code.

     TAPGDEBUG		  The argument should be a pointer to an int; this
			  stores the internal debugging	variable's value into

     FIONBIO		  Turn non-blocking I/O	for reads off or on, according
			  as the argument int's	value is or is not zero
			  (Writes are always nonblocking).

     FIOASYNC		  Turn asynchronous I/O	for reads (i.e., generation of
			  SIGIO	when data is available to be read) off or on,
			  according as the argument int's value	is or is not

     FIONREAD		  If any frames	are queued to be read, store the size
			  of the first one into	the argument int; otherwise,
			  store	zero.

     TIOCSPGRP		  Set the process group	to receive SIGIO signals, when
			  asynchronous I/O is enabled, to the argument int

     TIOCGPGRP		  Retrieve the process group value for SIGIO signals
			  into the argument int	value.

     SIOCGIFADDR	  Retrieve the Media Access Control (MAC) address of
			  the "remote" side.  This command is used by the
			  VMware port and expected to be executed on descrip-
			  tor, associated with control device (usually
			  /dev/vmnetN or /dev/tapN).  The buffer, which	is
			  passed as the	argument, is expected to have enough
			  space	to store the MAC address.  At the open time
			  both "local" and "remote" MAC	addresses are the
			  same,	so this	command	could be used to retrieve the
			  "local" MAC address.

     SIOCSIFADDR	  Set the Media	Access Control (MAC) address of	the
			  "remote" side.  This command is used by VMware port
			  and expected to be executed on a descriptor, associ-
			  ated with control device (usually /dev/vmnetN).

     The control device	also supports select(2)	for read; selecting for	write
     is	pointless, and always succeeds,	since writes are always	non-blocking.

     On	the last close of the data device, the interface is brought down (as
     if	with "ifconfig tapN down") unless the device is	a VMnet	device.	 All
     queued frames are thrown away.  If	the interface is up when the data de-
     vice is not open, output frames are thrown	away rather than letting them
     pile up.

     The tap device can	also be	used with the VMware port as a replacement for
     the old VMnet device driver.  The driver uses the minor number to select
     between tap and vmnet devices.  VMnet minor numbers begin at 0x800000 +
     N;	where N	is a VMnet unit	number.	 In this case the control device is
     expected to be /dev/vmnetN, and the network interface will	be vmnetN.
     Additionally, VMnet devices do not	ifconfig(8) themselves down when the
     control device is closed.	Everything else	is the same.

     In	addition to the	above mentioned	ioctl(2) calls,	there is an additional
     one for the VMware	port.


     inet(4), intro(4)

BSD				 July 9, 2000				   BSD


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