TUN(4) | Device Drivers Manual | TUN(4) |
tun
—
pseudo-device tun
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
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
TUNGDEBUG
TUNSIFMODE
IFF_POINTOPOINT
or
IFF_BROADCAST
(optionally
IFF_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, an
EINVAL
error occurs. The interface must be down at
the time; if it is up, an EBUSY
error occurs.TUNSLMODE
TUNGIFHEAD
TUNSIFHEAD
FIONBIO
FIOASYNC
SIGIO
when data is available to be read) off or
on, according as the argument int's value is or
isn't zero.FIONREAD
TIOCSPGRP
SIGIO
signals,
when asynchronous I/O is enabled, to the argument
int value.TIOCGPGRP
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.
April 8, 2006 | NetBSD 9.4 |