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TUN(4)                 FreeBSD Kernel Interfaces Manual                 TUN(4)

NAME
     tun - tunnel software network interface

SYNOPSIS
     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(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, or until removed with the
     ifconfig(8) command.

     tun devices are created using interface cloning.  This is done using the
     ``ifconfig tunN create'' command.  This is the preferred method of
     creating tun devices.  The same method allows removal of interfaces.  For
     this, use the ``ifconfig tunN destroy'' command.

     If the sysctl(8) variable net.link.tun.devfs_cloning is non-zero, 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).

     Disabling the legacy devfs cloning functionality may break existing
     applications which use tun, such as ppp(8) and ssh(1).  It therefore
     defaults to being enabled until further notice.

     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
     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-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
     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.  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
                     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.

     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
                     <net/if_tun.h>.

                     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 family.

     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 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 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
                     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.

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

AUTHORS
     This manual page was originally obtained from NetBSD.

FreeBSD 11.0-PRERELEASE        February 4, 2007        FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | AUTHORS

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