TAP(4) BSD Kernel Interfaces Manual TAP(4)
NAME
tap -- virtual Ethernet device
SYNOPSIS
pseudo-device tap
DESCRIPTION
The tap driver allows the creation and use of virtual Ethernet devices. Those interfaces appear just as any real Ethernet NIC to the kernel,
but can also be accessed by userland through a character device node in order to read frames being sent by the system or to inject frames.
In that respect it is very similar to what tun(4) provides, but the added Ethernet layer allows easy integration with machine emulators or
virtual Ethernet networks through the use of bridge(4) with tunneling.
INTERFACE CREATION
Interfaces may be created in two different ways: using the ifconfig(8) create command with a specified device number, or its ioctl(2) equiva-
lent, SIOCIFCREATE, or using the special cloning device /dev/tap.
The former works the same as any other cloning network interface: the administrator can create and destroy interfaces at any time, notably at
boot time. This is the easiest way of combining tap and bridge(4). Later, userland will actually access the interfaces through the specific
device nodes /dev/tapN.
The latter is aimed at applications that need a virtual Ethernet device for the duration of their execution. A new interface is created at
the opening of /dev/tap, and is later destroyed when the last process using the file descriptor closes it.
CHARACTER DEVICES
Whether the tap devices are accessed through the special cloning device /dev/tap or through the specific devices /dev/tapN, the possible
actions to control the matching interface are the same.
When using /dev/tap though, as the interface is created on-the-fly, its name is not known immediately by the application. Therefore the
TAPGIFNAME ioctl is provided. It should be the first action an application using the special cloning device will do. It takes a pointer to
a struct ifreq as an argument.
Ethernet frames sent out by the kernel on a tap interface can be obtained by the controlling application with read(2). It can also inject
frames in the kernel with write(2). There is absolutely no validation of the content of the injected frame, it can be any data, of any
length.
One call of write(2) will inject a single frame in the kernel, as one call of read(2) will retrieve a single frame from the queue, to the
extent of the provided buffer. If the buffer is not large enough, the frame will be truncated.
tap character devices support the FIONREAD ioctl which returns the size of the next available frame, or 0 if there is no available frame in
the queue.
They also support non-blocking I/O through the FIONBIO ioctl. In that mode, EWOULDBLOCK is returned by read(2) when no data is available.
Asynchronous I/O is supported through the FIOASYNC, FIOSETOWN, and FIOGETOWN ioctls. The first will enable SIGIO generation, while the two
other configure the process group that will receive the signal when data is ready.
Synchronisation may also be achieved through the use of select(2), poll(2), or kevent(2).
ETHERNET ADDRESS
When a tap device is created, it is assigned an Ethernet address of the form f2:0b:a4:xx:xx:xx. This address can later be changed using
ifconfig(8) to add an active link layer address, or directly via the SIOCALIFADDR ioctl on a PF_LINK socket, as it is not available on the
ioctl handler of the character device interface.
FILES
/dev/tap cloning device
/dev/tap[0-9]* individual character device nodes
SEE ALSO
bridge(4), etherip(4), tun(4), ifconfig(8)
HISTORY
The tap driver first appeared in NetBSD 3.0.
BSD
March 10, 2009 BSD
Check Out this Related Man Page
TAP(4) BSD Kernel Interfaces Manual 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 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 inter-
faces 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 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, MTU and type). 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.
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 let-
ting 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)
BSD September 8, 2008 BSD