.\"	$OpenBSD: tun.4,v 1.35 2007/02/21 15:01:28 claudio Exp $
.\"
.\" Copyright (c) 2003 Marcus D. Watts  All rights reserved.
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.Dd March 9, 2003
.Dt TUN 4
.Os
.Sh NAME
.Nm tun
.Nd network tunnel pseudo-device
.Sh SYNOPSIS
.Cd "pseudo-device tun"
.Pp
.Fd #include <sys/types.h>
.Fd #include <net/if_tun.h>
.Sh DESCRIPTION
The
.Nm
driver provides a network interface pseudo-device.
Packets sent to this interface can be read by a userland process
and processed as desired.
Packets written by the userland process are injected back into
the kernel networking subsystem.
.Pp
A
.Nm
interface can be created at runtime using the
.Ic ifconfig tun Ns Ar N Ic create
command or by opening the character special device
.Pa /dev/tunN .
.Pp
Both layer 3 and layer 2 tunneling is supported.
Layer 3 tunneling is the default mode; to enable layer 2 tunneling mode the
.Ar link0
flag needs to be set with
.Xr ifconfig 8 ,
or by setting up a
.Xr hostname.if 5
configuration file for
.Xr netstart 8 .
In layer 2 mode the
.Nm
interface is simulating an Ethernet network interface.
.Pp
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 truncated.
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
.Pa /dev/tunN ,
it will be automatically destroyed.
Devices created via
.Xr ifconfig 8
are only marked as not running and traffic will be dropped returning
.Er EHOSTDOWN .
.Pp
Writes never block.
If the protocol queue is full, the packet is dropped, a
.Dq collision
is counted, and
.Er ENOBUFS
is returned.
.Pp
In addition to the usual network interface
ioctl commands described in
.Xr netintro 4 ,
the following special commands defined in
.Aq Pa net/if_tun.h
are supported:
.Pp
.Bl -tag -width indent -compact
.It Dv TUNGIFINFO Fa "struct tuninfo *"
.It Dv TUNSIFINFO Fa "struct tuninfo *"
Get or set the interface characteristics.
.Bd -literal
/* iface info */
struct tuninfo {
        u_int   mtu;
        u_short type;
        u_short flags;
        u_int   baudrate;
};
.Ed
.Pp
.Va flags
sets the interface flags, and
can include one or more of
.Dv IFF_UP ,
.Dv IFF_POINTOPOINT ,
.Dv IFF_MULTICAST ,
.Dv 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
.Dv IFF_POINTOPOINT
for layer 3 and to
.Dv IFF_BROADCAST \&| IFF_MULTICAST
for layer 2 mode.
It is an error to set both
.Dv IFF_POINTOPOINT
and
.Dv IFF_BROADCAST .
.\" should say what type affects...
.Va type
defaults to
.Dv IFT_TUNNEL
for layer 3 and
.Dv IFT_ETHER
for layer 2 tunneling mode.
This sets the interface media address header type.
.Pp
.It Dv TUNSIFMODE Fa int *flags
Set just the interface flags.
The same restrictions as for
.Dv TUNSIFINFO
apply.
.Pp
.It Dv FIONBIO Fa int *flag
Set non-blocking I/O.
.Pp
.It Dv FIOASYNC Fa int *flag
Cause signal
.Dv SIGIO
to be sent when a packet can be read.
.Pp
.It Dv TIOCSPGRP Fa int *pgrp
.It Dv TIOCGPGRP Fa int *pgrp
Get or set the process group to which signals might be sent
via
.Dv FIOASYNC .
.Pp
.It Dv FIONREAD Fa int *count
Get the byte count of the next packet available to be read.
.Pp
.It Dv SIOCGIFADDR Fa struct ether_addr *addr
.It Dv SIOCSIFADDR Fa struct ether_addr *addr
Get or set the Ethernet address of the device in layer 2 mode.
.El
.Sh FILES
.Bl -tag -width /dev/tun* -compact
.It Pa /dev/tun*
.El
.Sh ERRORS
If open fails,
.Xr errno 2
may be set to one of:
.Bl -tag -width Er
.It Bq Er EPERM
Only the superuser may open the device.
.It Bq Er ENXIO
Not that many devices configured.
.It Bq Er EBUSY
Device was already open.
.El
.Pp
If a
.Xr write 2
call fails,
.Xr errno 2
is set to one of:
.Bl -tag -width Er
.It Bq Er EMSGSIZE
The packet supplied was too small or too large.
The maximum sized packet allowed is currently 16384 bytes.
.It Bq Er ENOBUFS
There were no mbufs, or
the queue for the outgoing protocol is full.
.It Bq Er EAFNOSUPPORT
The address family specified in the tunnel header was not
recognized.
.El
.Pp
Ioctl commands may fail with:
.Bl -tag -width Er
.It Bq Er EINVAL
Attempt to set both
.Dv IFF_POINTOPOINT
and
.Dv IFF_BROADCAST
with
.Dv TUNSIFMODE
or using
.Dv SIOCGIFADDR
or
.Dv SIOCSIFADDR
in layer 3 mode.
.It Bq Er ENOTTY
Unrecognized ioctl command.
.El
.Pp
A
.Xr read 2
call may fail because of:
.Bl -tag -width Er
.It Bq Er EHOSTDOWN
The device is not ready.
The device must have an
.Xr inet 4
interface address assigned to it, such as via
.Dv SIOCSIFADDR .
.It Bq Er EWOULDBLOCK
Non-blocking I/O was selected and no packets were available.
.El
.Pp
An attempt to send a packet out via the interface may fail with:
.Bl -tag -width Er
.It Bq Er EHOSTDOWN
The device is not ready.
The device must have an
.Xr inet 4
interface address assigned to it, such as via
.Dv SIOCSIFADDR .
.El
.Sh SEE ALSO
.Xr inet 4 ,
.Xr intro 4 ,
.Xr netintro 4 ,
.Xr hostname.if 5 ,
.Xr ifconfig 8 ,
.Xr netstart 8