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
TUN(4) BSD Kernel Interfaces Manual TUN(4)
NAME
tun -- tunnel software network interface
SYNOPSIS
device tun
DESCRIPTION
The tun interface is a software loopback mechanism that can be loosely described as the network interface analog of the pty(4), that is, tun
does for network interfaces what the pty(4) driver does for terminals.
The tun driver, like the pty(4) driver, provides two interfaces: an interface like the usual facility it is simulating (a network interface
in the case of tun, or a terminal for pty(4)), and a character-special device ``control'' interface. A client program transfers IP (by
default) packets to or from the tun ``control'' interface. The tap(4) interface provides similar functionality at the Ethernet layer: a
client will transfer Ethernet frames to or from a tap(4) ``control'' interface.
The network interfaces are named ``tun0'', ``tun1'', etc., one for each control device that has been opened. These network interfaces per-
sist until the if_tun.ko module is unloaded, or until removed with the ifconfig(8) command.
tun devices are created using interface cloning. This is done using the ``ifconfig tunN create'' command. This is the preferred method of
creating tun devices. The same method allows removal of interfaces. For this, use the ``ifconfig tunN destroy'' command.
If the sysctl(8) variable net.link.tun.devfs_cloning is non-zero, the tun interface permits opens on the special control device /dev/tun.
When this device is opened, tun will return a handle for the lowest unused tun device (use devname(3) to determine which).
Disabling the legacy devfs cloning functionality may break existing applications which use tun, such as ppp(8) and ssh(1). It therefore
defaults to being enabled until further notice.
Control devices (once successfully opened) persist until if_tun.ko is unloaded in the same way that network interfaces persist (see above).
Each interface supports the usual network-interface ioctl(2)s, such as SIOCAIFADDR and thus can be used with ifconfig(8) like any other
interface. At boot time, they are POINTOPOINT interfaces, but this can be changed; see the description of the control device, below. When
the system chooses to transmit a packet on the network interface, the packet can be read from the control device (it appears as ``input''
there); writing a packet to the control device generates an input packet on the network interface, as if the (non-existent) hardware had just
received it.
The tunnel device (/dev/tunN) is exclusive-open (it cannot be opened if it is already open). A read(2) call will return an error (EHOSTDOWN)
if the interface is not ``ready'' (which means that the control device is open and the interface's address has been set).
Once the interface is ready, read(2) will return a packet 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 packet is longer than is allowed for in the buffer passed to
read(2), the extra data will be silently dropped.
If the TUNSLMODE ioctl has been set, packets read from the control device will be prepended with the destination address as presented to the
network interface output routine, tunoutput(). The destination address is in struct sockaddr format. The actual length of the prepended
address is in the member sa_len. If the TUNSIFHEAD ioctl has been set, packets will be prepended with a four byte address family in network
byte order. TUNSLMODE and TUNSIFHEAD are mutually exclusive. In any case, the packet data follows immediately.
A write(2) call passes a packet in to be ``received'' on the pseudo-interface. If the TUNSIFHEAD ioctl has been set, the address family must
be prepended, otherwise the packet is assumed to be of type AF_INET. Each write(2) call supplies exactly one packet; the packet length is
taken from the amount of data provided to write(2) (minus any supplied address family). Writes will not block; if the packet 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., packet too
large), an error is returned.
The following ioctl(2) calls are supported (defined in <net/if_tun.h>):
TUNSDEBUG 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.
TUNGDEBUG The argument should be a pointer to an int; this stores the internal debugging variable's value into it.
TUNSIFINFO The argument should be a pointer to an struct tuninfo and allows setting the MTU, the type, and the baudrate of the tunnel
device. The struct tuninfo is declared in <net/if_tun.h>.
The use of this ioctl is restricted to the super-user.
TUNGIFINFO The argument should be a pointer to an struct tuninfo, where the current MTU, type, and baudrate will be stored.
TUNSIFMODE The argument should be a pointer to an int; its value must be either IFF_POINTOPOINT or IFF_BROADCAST and should have
IFF_MULTICAST OR'd into the value if multicast support is required. The type of the corresponding ``tunN'' interface is set to
the supplied type. If the value is outside the above range, an EINVAL error is returned. The interface must be down at the
time; if it is up, an EBUSY error is returned.
TUNSLMODE The argument should be a pointer to an int; a non-zero value turns off ``multi-af'' mode and turns on ``link-layer'' mode, caus-
ing packets read from the tunnel device to be prepended with the network destination address (see above).
TUNSIFPID Will set the pid owning the tunnel device to the current process's pid.
TUNSIFHEAD The argument should be a pointer to an int; a non-zero value turns off ``link-layer'' mode, and enables ``multi-af'' mode, where
every packet is preceded with a four byte address family.
TUNGIFHEAD The argument should be a pointer to an int; the ioctl sets the value to one if the device is in ``multi-af'' mode, and zero oth-
erwise.
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 non-
blocking.)
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 argu-
ment int's value is or is not zero.
FIONREAD If any packets 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.
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, by default, the interface is brought down (as if with ifconfig tunN down). All queued packets are
thrown away. If the interface is up when the data device is not open output packets are always thrown away rather than letting them pile up.
SEE ALSO
ioctl(2), read(2), select(2), write(2), devname(3), inet(4), intro(4), pty(4), tap(4), ifconfig(8)
AUTHORS
This manual page was originally obtained from NetBSD.
BSD
November 30, 2014 BSD