Updated: 2022/Sep/29

Please read Privacy Policy. It's for your privacy.

TUN(4)                       Device Drivers 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 driver does for terminals.

     The tun driver, like the pty 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), and a character-special
     device "control" interface.

     To use a tun device, the administrator must first create the interface.
     This can be done by using the ifconfig(8) create command, or via the
     SIOCIFCREATE ioctl.  An open() call on /dev/tunN will also create a
     network interface with the same unit number of that device if it doesn't
     exist yet.

     The network interfaces should be named tun0, tun1, etc.  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 there as "output");
     writing a packet to the control device generates an input packet on the
     network interface, as if the (non-existent) hardware had just received

     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
     (regardless of file system permissions).  A read() call will return an
     error (EHOSTDOWN) if the interface is not "ready" (which means that the
     interface address has not been set).  Once the interface is ready, read()
     will return a packet if one is available; if not, it will either block
     until one is or return EAGAIN, 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(), 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
     immediately.  A write(2) call passes a packet in to be "received" on the
     pseudo-interface.  Each write() call supplies exactly one packet; the
     packet length is taken from the amount of data provided to write().
     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.

     TUNSIFMODE  The argument should be a pointer to an int; its value must be
                 either IFF_POINTOPOINT or IFF_BROADCAST (optionally
                 IFF_MULTICAST may be or'ed into the value).  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 off "multi-af" mode and turns on "link-layer" mode,
                 causing packets read from the tunnel device to be prepended
                 with network destination address.

     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.

     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

     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

     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 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.

     When an application has opened the tun character device the link is
     considered up, otherwise down.  As such, it is best to open the character
     device once connectivity has been established so that Duplicate Address
     Detection, if applicable, can be performed.  If connectivity is lost, the
     character device should be closed.

     ioctl(2), read(2), select(2), write(2), inet(4), intro(4), ifconfig(8)

     IPv6 support comes mostly from FreeBSD and was added in NetBSD 4.0 by
     Rui Paulo <rpaulo@NetBSD.org>.

NetBSD 10.99                  September 27, 2020                  NetBSD 10.99