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

     tun -- tunnel software network interface

     pseudo-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, as many as were
     made by MAKEDEV(8).  Each one 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 as "input" there); writing a packet to	the control device
     generates an input	packet on the network interface, as if the (non-exis-
     tent) 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.	 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.

     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 imme-

     A write(2)	call passes a packet in	to be "received" on the	pseudo-inter-
     face.  Each write(2) call supplies	exactly	one packet; the	packet length
     is	taken from the amount of data provided to write(2).  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 pre-
     ceded 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 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

     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.  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 er-
		 ror occurs.

     TUNSLMODE	 The argument should be	a pointer to an	int; a non-zero	value
		 turns on "link-layer" mode, causing packets read from the
		 tunnel	device to be prepended with network destination	ad-

     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; this stores one
		 if the	device is in "multi-af"	mode, and zero otherwise in

     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.

     inet(4), intro(4)

     This manual page was originally obtained from NetBSD.

BSD				March 10, 1996				   BSD


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