TUN(4)                    BSD 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
     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 (/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

     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

     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 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

     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
                 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.

     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.

BSD                               June 5, 2001                               BSD