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.