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TUN(4)			 BSD Kernel Interfaces Manual			TUN(4)

     tun -- tunnel software network interface

     device tun

     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(4) driver	does for terminals.

     The tun driver, like the pty(4) driver, provides two interfaces: an in-
     terface 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 de-
     vice "control" interface.

     The network interfaces are	named "tun0", "tun1", etc., one	for each con-
     trol device that has been opened.	These network interfaces persist until
     the if_tun.ko module is unloaded (if tun is built into your kernel, the
     network interfaces	cannot be removed).

     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 un-
     used tun device (use devname(3) to	determine which).  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
     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, be-
     low.  When	the system chooses to transmit a packet	on the network inter-
     face, 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	net-
     work 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-inter-
     face.  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 er-
     ror 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 in-
		 ternal	debugging variable to that value.  What, if anything,
		 this variable controls	is not documented here;	see the	source

     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 al-
		 lows setting the MTU, the type, and the baudrate of the tun-
		 nel device.  The struct tuninfo is declared in

		 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	or IFF_BROADCAST and should have
		 IFF_MULTICAST OR'd into the value if multicast	support	is re-
		 quired.  The type of the corresponding	"tunN" interface is
		 set to	the supplied type.  If the value is outside the	above
		 range,	an EINVAL error	is returned.  The interface must be
		 down at the time; if it is up,	an EBUSY 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 fam-

     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 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.

     ioctl(2), read(2),	select(2), write(2), devname(3), inet(4), intro(4),
     pty(4), ifconfig(8)

     This manual page was originally obtained from NetBSD.

BSD				 June 5, 2001				   BSD


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