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tap -- Ethernet tunnel software network interface
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
interface like the usual facility it is simulating (an Ethernet network
interface 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, 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,
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
network interface, as if the (non-existent) hardware had just received
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, MTU
and type). The argument should be a pointer to a
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
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
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,
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
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
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.