TUN(4)                    OpenBSD Programmer's Manual                   TUN(4)

NAME
     tun - network tunnel pseudo-device

SYNOPSIS
     pseudo-device tun

     #include <sys/types.h>
     #include <net/if_tun.h>

DESCRIPTION
     The tun driver provides a network interface pseudo-device.  Packets sent
     to this interface can be read by a userland process and processed as de-
     sired.  Packets written by the userland process are injected back into
     the kernel networking subsystem.

     A tun interface can be created at runtime using the ifconfig tunN create
     command or by opening the character special device /dev/tunN.

     Both layer 3 and layer 2 tunneling is supported.  Layer 3 tunneling is
     the default mode; to enable layer 2 tunneling mode the link0 flag needs
     to be set with ifconfig(8), or by setting up a hostname.if(5) configura-
     tion file for netstart(8).  In layer 2 mode the tun interface is simulat-
     ing an Ethernet network interface.

     Each device has the exclusive open property; it cannot be opened if it is
     already open and in use by another process.  Each read returns at most
     one packet; if insufficient buffer space is provided, the packet is trun-
     cated.  Each write supplies exactly one packet.  Each packet read or
     written is prefixed with a tunnel header consisting of a 4-byte network
     byte order integer containing the address family in the case of layer 3
     tunneling.  In layer 2 mode the 4-byte tunnel header is replaced with an
     Ethernet header.  On the last close of the device, all queued packets are
     discarded.  If the device was created by opening /dev/tunN, it will be
     automatically destroyed.  Devices created via ifconfig(8) are only marked
     as not running and traffic will be dropped returning EHOSTDOWN.

     Writes never block.  If the protocol queue is full, the packet is
     dropped, a ``collision'' is counted, and ENOBUFS is returned.

     In addition to the usual network interface ioctl commands described in
     netintro(4), the following special commands defined in <net/if_tun.h> are
     supported:

     TUNGIFINFO struct tuninfo *
     TUNSIFINFO struct tuninfo *
             Get or set the interface characteristics.

             /* iface info */
             struct tuninfo {
                     u_int   mtu;
                     u_short type;
                     u_short flags;
                     u_int   baudrate;
             };

             flags sets the interface flags, and can include one or more of
             IFF_UP, IFF_POINTOPOINT, IFF_MULTICAST, IFF_BROADCAST.  Flags
             given will be set; flags omitted will be cleared; flags not in
             this list will not be changed even when given.  Flags default to
             IFF_POINTOPOINT for layer 3 and to IFF_BROADCAST | IFF_MULTICAST
             for layer 2 mode.  It is an error to set both IFF_POINTOPOINT and
             IFF_BROADCAST.  type defaults to IFT_TUNNEL for layer 3 and
             IFT_ETHER for layer 2 tunneling mode.  This sets the interface
             media address header type.

     TUNSIFMODE int *flags
             Set just the interface flags.  The same restrictions as for
             TUNSIFINFO apply.

     FIONBIO int *flag
             Set non-blocking I/O.

     FIOASYNC int *flag
             Cause signal SIGIO to be sent when a packet can be read.

     TIOCSPGRP int *pgrp
     TIOCGPGRP int *pgrp
             Get or set the process group to which signals might be sent via
             FIOASYNC.

     FIONREAD int *count
             Get the byte count of the next packet available to be read.

     SIOCGIFADDR struct ether_addr *addr
     SIOCSIFADDR struct ether_addr *addr
             Get or set the Ethernet address of the device in layer 2 mode.

FILES
     /dev/tun*

ERRORS
     If open fails, errno(2) may be set to one of:

     [EPERM]       Only the superuser may open the device.

     [ENXIO]       Not that many devices configured.

     [EBUSY]       Device was already open.

     If a write(2) call fails, errno(2) is set to one of:

     [EMSGSIZE]    The packet supplied was too small or too large.  The maxi-
                   mum sized packet allowed is currently 16384 bytes.

     [ENOBUFS]     There were no mbufs, or the queue for the outgoing protocol
                   is full.

     [EAFNOSUPPORT]
                   The address family specified in the tunnel header was not
                   recognized.

     Ioctl commands may fail with:

     [EINVAL]      Attempt to set both IFF_POINTOPOINT and IFF_BROADCAST with
                   TUNSIFMODE or using SIOCGIFADDR or SIOCSIFADDR in layer 3
                   mode.

     [ENOTTY]      Unrecognized ioctl command.

     A read(2) call may fail because of:

     [EHOSTDOWN]   The device is not ready.  The device must have an inet(4)
                   interface address assigned to it, such as via SIOCSIFADDR.

     [EWOULDBLOCK]
                   Non-blocking I/O was selected and no packets were avail-
                   able.

     An attempt to send a packet out via the interface may fail with:

     [EHOSTDOWN]   The device is not ready.  The device must have an inet(4)
                   interface address assigned to it, such as via SIOCSIFADDR.

SEE ALSO
     inet(4), intro(4), netintro(4), hostname.if(5), ifconfig(8), netstart(8)

OpenBSD 4.7                      May 31, 2007                                3
