TUN(4)                                                     BSD 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 per-
     sist 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, caus-
                 ing 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 oth-
                 erwise.

     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 argu-
                 ment 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.

BSD                                                              February 4, 2007                                                              BSD