.\" $NetBSD: tun.4,v 1.13 2002/08/20 15:47:46 wiz Exp $
.\" Based on PR#2411
.\"
.Dd March 10, 1996
.Dt TUN 4
.Os
.Sh NAME
.Nm tun
.Nd tunnel software network interface
.Sh SYNOPSIS
.Cd pseudo-device tun Op Ar count
.Sh DESCRIPTION
The
.Nm tun
interface is a software loopback mechanism that can be loosely
described as the network interface analog of the
.Xr pty 4 ,
that is,
.Nm tun
does for network interfaces what the
.Nm pty
driver does for terminals.
.Pp
The
.Nm tun
driver, like the
.Nm pty
driver, provides two interfaces: an interface like the usual facility
it is simulating
.Po
a network interface in the case of
.Nm tun ,
or a terminal for
.Nm pty
.Pc ,
and a character-special device
.Dq control
interface.
.Pp
To use a
.Nm tun
device, the administrator must first create the interface.  This can
be done by using the
.Xr ifconfig 8
.Sq create
command, or via the
.Dv SIOCIFCREATE
ioctl.  Note that an
.Fn open
call on
.Pa /dev/tun Ns Sy N ,
will indicate that the device is not configured
.Pq Er ENXIO
unless the corresponding network interface has been created.
.Pp
The network interfaces are named
.Sy tun Ns Ar 0 ,
.Sy tun Ns Ar 1 ,
etc, as many in all as the
.Ar count
figure given on the
.Sy pseudo-device
line.  Each one supports the usual network-interface
.Xr ioctl 2 Ns s ,
such as
.Dv SIOCSIFADDR
and
.Dv SIOCSIFNETMASK ,
and thus can be used with
.Xr ifconfig 8
like any other interface.  At boot time, they are
.Dv 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
.Po
it appears there as
.Dq output
.Pc ;
writing a packet to the control device generates an input
packet on the network interface, as if the
.Pq non-existent
hardware had just received it.
.Pp
The tunnel device, normally
.Pa /dev/tun Ns Sy N ,
is exclusive-open
.Po
it cannot be opened if it is already open
.Pc
and is restricted to the super-user
.Po
regardless of file system permissions
.Pc . A
.Fn read
call will return an error
.Pq Er EHOSTDOWN
if the interface is not
.Dq ready
(which means that the interface
address has not been set).
Once the interface is ready,
.Fn read
will return a packet if one is available; if not, it will either block
until one is or return
.Er 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
.Fn read ,
the extra data will be silently dropped.
.Pp
Packets can be optionally prepended with the destination address as presented
to the network interface output routine
.Pq Sq Li tunoutput .
The destination address is in
.Sq Li struct sockaddr
format. The actual length of the prepended address is in the member
.Sq Li sa_len .
The packet data follows immediately.
A
.Xr write 2
call passes a packet in to be
.Dq received
on the pseudo-interface.  Each
.Fn write
call supplies exactly one packet; the packet length is taken from the
amount of data provided to
.Fn write .
Writes will not block; if the packet cannot be accepted for a
transient reason
.Pq e.g., no buffer space available ,
it is silently dropped; if the reason is not transient
.Pq e.g., packet too large ,
an error is returned.
If
.Dq link-layer mode
is on
.Pq see Dv TUNSLMODE No below ,
the actual packet data must be preceded by a
.Sq Li struct sockaddr .
The driver currently only inspects the
.Sq Li sa_family
field.
The following
.Xr ioctl 2
calls are supported
.Pq defined in Aq Pa net/if_tun.h :
.Bl -tag -width TUNSIFMODE
.It Dv TUNSDEBUG
The argument should be a pointer to an
.Va int ;
this sets the internal debugging variable to that value.  What, if
anything, this variable controls is not documented here; see the source
code.
.It Dv TUNGDEBUG
The argument should be a pointer to an
.Va int ;
this stores the internal debugging variable's value into it.
.It Dv TUNSIFMODE
The argument should be a pointer to an
.Va int ;
its value must be either
.Dv IFF_POINTOPOINT
or
.Dv IFF_BROADCAST
(optionally
.Dv IFF_MULTICAST
may be or'ed into the value).  The type of the corresponding
.Em tun Ns Sy n
interface is set to the supplied type.  If the value is anything else,
an
.Er EINVAL
error occurs.  The interface must be down at the time; if it is up, an
.Er EBUSY
error occurs.
.It Dv TUNSLMODE
The argument should be a pointer to an
.Va int ;
a non-zero value turns on
.Dq link-layer
mode, causing packets read from the tunnel device to be prepended with
network destination address.
.It Dv FIONBIO
Turn non-blocking I/O for reads off or on, according as the argument
.Va int Ns 's
value is or isn't zero
.Pq Writes are always nonblocking .
.It Dv FIOASYNC
Turn asynchronous I/O for reads
.Po
i.e., generation of
.Dv SIGIO
when data is available to be read
.Pc off or on, according as the argument
.Va int Ns 's
value is or isn't zero.
.It Dv FIONREAD
If any packets are queued to be read, store the size of the first one
into the argument
.Va int ;
otherwise, store zero.
.It Dv TIOCSPGRP
Set the process group to receive
.Dv SIGIO
signals, when asynchronous I/O is enabled, to the argument
.Va int
value.
.It Dv TIOCGPGRP
Retrieve the process group value for
.Dv SIGIO
signals into the argument
.Va int
value.
.El
.Pp
The control device also supports
.Xr select 2
for read; selecting for write is pointless, and always succeeds, since
writes are always non-blocking.
.Pp
On the last close of the data device, by default, the interface is
brought down
.Po as if with
.Dq ifconfig tun Ns Sy n No down
.Pc .
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.
.Sh SEE ALSO
.Xr inet 4 ,
.Xr intro 4
.Sh BUGS
Currently is IPv4-only.