TAP(4) BSD Kernel Interfaces Manual TAP(4)NAMEtap — virtual Ethernet device
SYNOPSIS
pseudo-device tapDESCRIPTION
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 applica‐
tion 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 ALSObridge(4), etherip(4), tun(4), ifconfig(8)HISTORY
The tap driver first appeared in NetBSD 3.0.
BSD March 10, 2009 BSD
