TUN(4)

NAME
tun - tunnel software network interface

SYNOPSIS
pseudo-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 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 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
    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 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 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.

  LINK STATE
When an application has opened the tun character device the link is considered 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.

SEE ALSO
ioctl(2), read(2), select(2), write(2), inet(4), intro(4), ifconfig(8)

HISTORY
-split IPv6 support comes mostly from FreeBSD and was added in NetBSD 4.0 by Rui Paulo <rpaulo@NetBSD.org>.