Import NetBSD's tun(4) man page.
Obtained from: NetBSD
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Joerg Wunsch
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share/man/man4/tun.4
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share/man/man4/tun.4
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.\" $NetBSD: tun.4,v 1.1 1996/06/25 22:17:37 pk Exp $
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.\" Based on PR#2411
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.\"
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.Dd March 10, 1996
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.Dt TUN 4
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.Os NetBSD 1.1
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.Sh NAME
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.Nm tun
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.Nd tunnel software network interface
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.Sh SYNOPSIS
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.Cd pseudo-device tun Op Ar count
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.Sh DESCRIPTION
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The
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.Nm tun
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interface is a software loopback mechanism that can be loosely
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described as the network interface analog of the
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.Xr pty 4 ,
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that is,
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.Nm tun
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does for network interfaces what the
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.Nm pty
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driver does for terminals.
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.Pp
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The
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.Nm tun
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driver, like the
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.Nm pty
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driver, provides two interfaces: an interface like the usual facility
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it is simulating
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.Po
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a network interface in the case of
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.Nm tun ,
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or a terminal for
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.Nm pty Pc ,
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and a character-special device
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.Dq control
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interface.
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.Pp
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The network interfaces are named
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.Sy tun Ns Ar 0 ,
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.Sy tun Ns Ar 1 ,
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etc, as many in all as the
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.Ar count
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figure given on the
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.Sy pseudo-device
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line. Each one supports the usual network-interface
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.Xr ioctl 2 Ns s ,
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such as
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.Dv SIOCSIFADDR
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and
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.Dv SIOCSIFNETMASK ,
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and thus can be used with
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.Xr ifconfig 8
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like any other interface. At boot time, they are
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.Dv POINTOPOINT
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interfaces, but this can be changed; see the description of the control
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device, below. When the system chooses to transmit a packet on the
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network interface, the packet can be read from the control device
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.Po
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it appears as
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.Dq input
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there
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.Pc ;
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writing a packet to the control device generates an input
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packet on the network interface, as if the
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.Pq non-existent
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hardware had just received it.
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.Pp
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The tunnel device, normally
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.Pa /dev/tun Ns Sy N ,
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is exclusive-open
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.Po
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it cannot be opened if it is already open
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.Pc
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and is restricted to the super-user. A
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.Fn read
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call will return an error
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.Pq Er EHOSTDOWN
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if the interface is not
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.Dq ready
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.Po
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which means that the control device is open and the interface's
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address has been set
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.Pc .
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Once the interface is ready,
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.Fn read
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will return a packet if one is available; if not, it will either block
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until one is or return
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.Er EWOULDBLOCK ,
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depending on whether non-blocking I/O has been enabled. If the packet
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is longer than is allowed for in the buffer passed to
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.Fn read ,
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the extra data will be silently dropped.
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.Pp
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Packets can be optionally prepended with the destination address as presented
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to the network interface output routine
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.Pq Sq Li tunoutput .
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The destination address is in
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.Sq Li struct sockaddr
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format. The actual length of the prepended address is in the member
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.Sq Li sa_len .
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The packet data follows immediately.
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A
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.Xr write 2
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call passes a packet in to be
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.Dq received
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on the pseudo-interface. Each
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.Fn write
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call supplies exactly one packet; the packet length is taken from the
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amount of data provided to
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.Fn write .
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Writes will not block; if the packet cannot be accepted for a
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transient reason
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.Pq e.g., no buffer space available ,
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it is silently dropped; if the reason is not transient
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.Pq e.g., packet too large ,
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an error is returned.
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If
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.Dq link-layer mode
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is on
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.Pq see Dv TUNSLMODE No below ,
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the actual packet data must be preceded by a
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.Sq Li struct sockaddr .
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The driver currently only inspects the
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.Sq Li sa_family
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field.
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The following
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.Xr ioctl 2
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calls are supported
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.Pq defined in Aq Pa net/if_tun.h Ns :
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.Bl -tag -width TUNSIFMODE
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.It Dv TUNSDEBUG
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The argument should be a pointer to an
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.Va int ;
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this sets the internal debugging variable to that value. What, if
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anything, this variable controls is not documented here; see the source
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code.
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.It Dv TUNGDEBUG
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The argument should be a pointer to an
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.Va int ;
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this stores the internal debugging variable's value into it.
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.It Dv TUNSIFMODE
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The argument should be a pointer to an
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.Va int ;
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its value must be
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.Dv IFF_POINTOPOINT
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or
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.Dv IFF_BROADCAST .
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The type of the corresponding
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.Em tun Ns Sy n
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interface is set to the supplied type. If the value is anything else,
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an
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.Er EINVAL
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error occurs. The interface must be down at the time; if it is up, an
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.Er EBUSY
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error occurs.
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.It Dv TUNSLMODE
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The argument should be a pointer to an
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.Va int ;
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a non-zero value turns on
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.Dq link-layer
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mode, causing packets read from the tunnel device to be prepended with
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network destination address.
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.It Dv FIONBIO
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Turn non-blocking I/O for reads off or on, according as the argument
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.Va int Ns 's
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value is or isn't zero
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.Pq Writes are always nonblocking .
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.It Dv FIOASYNC
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Turn asynchronous I/O for reads
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.Po
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i.e., generation of
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.Dv SIGIO
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when data is available to be read
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.Pc off or on, according as the argument
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.Va int Ns 's
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value is or isn't zero.
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.It Dv FIONREAD
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If any packets are queued to be read, store the size of the first one
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into the argument
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.Va int ;
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otherwise, store zero.
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.It Dv TIOCSPGRP
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Set the process group to receive
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.Dv SIGIO
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signals, when asynchronous I/O is enabled, to the argument
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.Va int
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value.
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.It Dv TIOCGPGRP
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Retrieve the process group value for
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.Dv SIGIO
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signals into the argument
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.Va int
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value.
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.El
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.Pp
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The control device also supports
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.Xr select 2
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for read; selecting for write is pointless, and always succeeds, since
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writes are always non-blocking.
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.Pp
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On the last close of the data device, by default, the interface is
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brought down
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.Po as if with
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.Dq ifconfig tun Ns Sy n No down
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.Pc .
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All queued packets are thrown away.
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.Po If the interface is up when the data device is not open
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output packets are always
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thrown away rather than letting them pile up
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.Pc .
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.Sh SEE ALSO
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.Xr intro 4 ,
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.Xr inet 4
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.Sh BUGS
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Currently is IP-only.
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