table of contents
TUN(4) | Device Drivers Manual | TUN(4) |
NAME¶
tun
—
tunnel software network interface
SYNOPSIS¶
device tun
DESCRIPTION¶
Thetun
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.
The network interfaces are named “tun0
”,
“tun1
”, etc., one for each control
device that has been opened. These network interfaces persist 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
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
tun
, such
as ppp(8) and
ssh(1). It therefore defaults to being enabled
until further notice.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
orIFF_BROADCAST
and should haveIFF_MULTICAST
OR'd into the value if multicast support is required. The type of the corresponding “tun
N” interface is set to the supplied type. If the value is outside the above range, anEINVAL
error is returned. The interface must be down at the time; if it is up, anEBUSY
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 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 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., 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 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.
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.February 4, 2007 | Debian |