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 inter

faces: an
interface like the usual facility it is simulating (a net

work interface
in the case of tun, or a terminal for pty(4)), and a charac

ter-special
device ``control'' interface.

The network interfaces are named ``tun0'', ``tun1'', etc.,

one for each
control device that has been opened. These network inter

faces persist
until the if_tun.ko module is unloaded (if tun is built into

your kernel,
the network interfaces cannot be removed).

The tun interface permits opens on the special control de

vice /dev/tun.
When this device is opened, tun will return a handle for the

lowest
unused tun device (use devname(3) to determine which). Con

trol devices
(once successfully opened) persist until if_tun.ko is un

loaded in the
same way that network interfaces persist (see above).

Each interface supports the usual network-interface

ioctl(2)s, such as
SIOCSIFADDR and SIOCSIFNETMASK, and thus can be used with

ifconfig(8)
like any other interface. At boot time, they are POINTO

POINT 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-exis

tent) 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 re

turn
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

pseudointerface. If the TUNSIFHEAD ioctl has been set, the ad

dress 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 can

not 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

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 docu

mented 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 de

clared in

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_BROAD

CAST and
should have IFF_MULTICAST OR'd into the val

ue 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, causing packets read

from the tunnel
device to be prepended with the network des

tination
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 en

ables
``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 otherwise.

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., gen

eration 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 packets are queued to be read, store

the size of

the first one into the argument int; other

wise, store
zero.

TIOCSPGRP Set the process group to receive SIGIO sig

nals, 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; select

ing for write
is pointless, and always succeeds, since writes are always

non-blocking.

On the last close of the data device, by default, the inter

face 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), ifconfig(8)

AUTHORS

This manual page was originally obtained from NetBSD.

BSD June 5, 2001

BSD