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tap(4)

NAME

     tap -- Ethernet tunnel software network interface

SYNOPSIS

     device tap

DESCRIPTION

     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
     control device that has been opened.  These Ethernet network interfaces
     persist until if_tap.ko module is unloaded (if tap is built into your
     kernel, 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
     unused 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.  A read() call will return an error (EHOSTDOWN) if the interface is
     not ``ready''.  Once the interface is ready, read() will return an Ether-
     net 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
     enabled.  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.

     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 isn't
			  zero.

     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
			  value.

     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,
			  associated 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
     device 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.

     VMIO_SIOCSIFFLAGS	  VMware SIOCSIFFLAGS.

SEE ALSO

     inet(4), intro(4)

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