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TUN(4)                 FreeBSD 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
     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, 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-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.  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
     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

     A write(2) call passes a packet in to be ``received'' on the pseudo-
     interface.  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 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.

     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

     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 error 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 address.

     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; this stores
                     one if the device is in ``multi-af'' mode, and zero
                     otherwise in it.

     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

     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.

     inet(4), intro(4)

     This manual page was originally obtained from NetBSD.

FreeBSD 11.0-PRERELEASE         March 10, 1996         FreeBSD 11.0-PRERELEASE


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