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(4) driver does for terminals.

     The tap driver, like the pty(4) driver, provides two interfaces: an interface like the usual
     facility it is simulating (an Ethernet network interface in the case of tap, or a terminal
     for pty(4)), and a character-special device “control” interface.  A client program transfers
     Ethernet frames to or from the tap “control” interface.  The tun(4) interface provides
     similar functionality at the network layer: a client will transfer IP (by default) packets
     to or from a tun(4) “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,
     or until removed with "ifconfig destroy" (see below).

     tap devices are created using interface cloning.  This is done using the “ifconfig tapN
     create” command.  This is the preferred method of creating tap devices.  The same method
     allows removal of interfaces.  For this, use the “ifconfig tapN destroy” command.

     If the sysctl(8) variable net.link.tap.devfs_cloning is non-zero, 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).

     Disabling the legacy devfs cloning functionality may break existing applications which use
     tap, such as VMware and ssh(1).  It therefore defaults to being enabled until further
     notice.

     Control devices (once successfully opened) persist until if_tap.ko is unloaded or the
     interface is destroyed.

     Each interface supports the usual Ethernet network interface ioctl(2)s 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 network 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
     net.link.tap.user_open is non-zero.  If the sysctl(8) variable net.link.tap.up_on_open is
     non-zero, the tunnel device will be marked “up” when the control device is opened.  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
     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>):

     TAPSIFINFO           Set network interface information (line speed and MTU).  The type must
                          be the same as returned by TAPGIFINFO or set to IFT_ETHER else the
                          ioctl(2) call will fail.  The argument should be a pointer to a struct
                          tapinfo.

     TAPGIFINFO           Retrieve network interface information (line speed, MTU and type).  The
                          argument should be a pointer to a struct tapinfo.

     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 it.

     TAPGIFNAME           Retrieve network interface name.  The argument should be a pointer to a
                          struct ifreq.  The interface name will be returned in the ifr_name
                          field.

     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 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
                          descriptor, 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, or has IFF_LINK0 flag set.  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), tun(4)