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TUN(4) NetBSD Kernel Interfaces Manual TUN(4)
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tun -- tunnel software network interface
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 driver does for terminals.
The tun driver, like the pty driver, provides two interfaces: an inter-
face 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
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 net-
work interface with the same unit number of that device if it doesn't
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
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 pre-
sented to the network interface output routine (`tunoutput'). The desti-
nation 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 sup-
ported (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 any-
thing, this variable controls is not documented here; see the
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 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
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
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 fam-
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
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 asyn-
chronous 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.
When an application has opened the tun character device the link is con-
sidered up, otherwise down. As such, it is best to open the character
device once connectivity has been established so that Duplicate Address
Detection, if applicable, can be performed. If connectivity is lost, the
character device should be closed.
ioctl(2), read(2), select(2), write(2), inet(4), intro(4), ifconfig(8)
IPv6 support comes mostly from FreeBSD and was added in NetBSD 4.0 by
Rui Paulo <rpaulo@NetBSD.org>.
NetBSD 10.99 September 27, 2020 NetBSD 10.99