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TUN(4)                 FreeBSD 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
     interface 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
     device ``control'' interface.

     The network interfaces are named ``tun0'', ``tun1'', etc., one for each
     control 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
     unused 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
     SIOCAIFADDR and thus can be used with ifconfig(8) like any other inter-
     face.  At boot time, they are POINTOPOINT interfaces, but this can be
     changed; see the description of the control device, below.  When the sys-
     tem 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-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-
     interface.  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
     error 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
                 internal debugging variable to that value.  What, if any-
                 thing, 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.

     TUNSIFINFO  The argument should be a pointer to an struct tuninfo and
                 allows setting the MTU, the type, and the baudrate of the
                 tunnel 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
                 required.  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 is not 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 is not 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.

FreeBSD 6.2                     October 9, 2006                    FreeBSD 6.2


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