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tun - tunnel software network interface



      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 con‐
      trol device that has been opened.  These network interfaces persist until
      the if_tun.ko module is unloaded, or until removed with the ifconfig(8)
      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 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 inter‐
      face.  At boot time, they are POINTOPOINT interfaces, but this can be
      changed; see the description of the control device, below.  When the sys‐
      tem 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 net‐
      work 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-inter‐
      face.  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
      TUNSDEBUG   The argument should be a pointer to an int; this sets the
                  internal debugging variable to that value.  What, if any‐
                  thing, 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
                  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 pro‐
                  cess’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 fam‐
      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
      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 asyn‐
                  chronous 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.


Based on BSD UNIX
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