NAME
tun
—
tunnel software network
interface
SYNOPSIS
pseudo-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
driver does for terminals.
The tun
driver, like the
pty
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
),
and a character-special device “control” interface.
To use a tun
device, the
administrator must first create the interface. This can be done by using the
ifconfig(8) create
command, or via the
SIOCIFCREATE
ioctl. An
open
() call on
/dev/tunN will also create a
network interface with the same unit number of that device if it doesn't
exist yet.
The network interfaces should be named
tun0,
tun1, etc. 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
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 there as “output”); 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, normally
/dev/tunN, is exclusive-open (it
cannot be opened if it is already open) and is restricted to the super-user
(regardless of file system permissions). A
read
() call
will return an error (EHOSTDOWN
) if the interface is
not “ready” (which means that the interface address has not
been set). Once the interface is ready, read
() will
return a packet if one is available; if not, it will either block until one
is or return EAGAIN
, 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
(), the extra data
will be silently dropped.
Packets can be optionally 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
’. The packet data follows
immediately. A write(2) call passes a packet in to be
“received” on the pseudo-interface. Each
write
() call
supplies exactly one packet; the packet length is taken from the amount of
data provided to write
(). 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. If “link-layer mode”
is on (see TUNSLMODE
below),
the actual packet data must be preceded by a ‘struct
sockaddr
’. The driver currently only inspects the
‘sa_family
’ field. 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.
TUNSIFMODE
- The argument should be a pointer to an int; its
value must be either
IFF_POINTOPOINT
orIFF_BROADCAST
(optionallyIFF_MULTICAST
may be or'ed into the value). The type of the corresponding tunn interface is set to the supplied type. If the value is anything else, anEINVAL
error occurs. The interface must be down at the time; if it is up, anEBUSY
error occurs. 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 network destination address.
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.
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.
FIONBIO
- Turn non-blocking I/O for reads off or on, according as the argument int's value is or isn't 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 isn't 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
HISTORY
IPv6 support comes mostly from FreeBSD and
was added in NetBSD 4.0 by
Rui Paulo ⟨rpaulo@NetBSD.org⟩.