332 lines
7.3 KiB
Groff
332 lines
7.3 KiB
Groff
.\" $FreeBSD$
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.\" Based on PR#2411
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.\"
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.Dd April 10, 2015
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.Dt TAP 4
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.Os
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.Sh NAME
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.Nm tap
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.Nd Ethernet tunnel software network interface
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.Sh SYNOPSIS
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.Cd device tap
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.Sh DESCRIPTION
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The
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.Nm
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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
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does for network interfaces what the
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.Xr pty 4
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driver does for terminals.
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.Pp
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The
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.Nm
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driver, like the
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.Xr pty 4
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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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(an Ethernet network interface in the case of
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.Nm ,
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or a terminal for
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.Xr pty 4 ) ,
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and a character-special device
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.Dq control
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interface.
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A client program transfers Ethernet frames to or from the
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.Nm
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.Dq control
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interface.
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The
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.Xr tun 4
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interface provides similar functionality at the network layer:
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a client will transfer IP (by default) packets to or from a
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.Xr tun 4
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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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.Dq Li tap0 ,
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.Dq Li tap1 ,
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etc., one for each control device that has been opened.
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These Ethernet network interfaces persist until
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.Pa if_tap.ko
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module is unloaded, or until removed with "ifconfig destroy" (see below).
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.Pp
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.Nm
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devices are created using interface cloning.
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This is done using the
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.Dq ifconfig tap Ns Sy N No create
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command.
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This is the preferred method of creating
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.Nm
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devices.
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The same method allows removal of interfaces.
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For this, use the
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.Dq ifconfig tap Ns Sy N No destroy
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command.
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.Pp
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If the
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.Xr sysctl 8
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variable
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.Va net.link.tap.devfs_cloning
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is non-zero, the
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.Nm
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interface
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permits opens on the special control device
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.Pa /dev/tap .
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When this device is opened,
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.Nm
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will return a handle for the lowest unused
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.Nm
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device (use
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.Xr devname 3
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to determine which).
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.Pp
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.Bf Em
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Disabling the legacy devfs cloning functionality may break existing
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applications which use
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.Nm ,
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such as
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.Tn VMware
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and
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.Xr ssh 1 .
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It therefore defaults to being enabled until further notice.
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.Ef
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.Pp
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Control devices (once successfully opened) persist until
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.Pa if_tap.ko
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is unloaded or the interface is destroyed.
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.Pp
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Each interface supports the usual Ethernet network interface
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.Xr ioctl 2 Ns s
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and thus can be used with
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.Xr ifconfig 8
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like any other Ethernet interface.
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When the system chooses to transmit
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an Ethernet frame on the network interface, the frame can be read from
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the control device
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(it appears as
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.Dq input
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there);
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writing an Ethernet frame to the control device generates an input frame on
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the network interface, as if the
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(non-existent)
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hardware had just received it.
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.Pp
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The Ethernet tunnel device, normally
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.Pa /dev/tap Ns Sy N ,
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is exclusive-open
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(it cannot be opened if it is already open)
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and is restricted to the super-user, unless the
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.Xr sysctl 8
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variable
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.Va net.link.tap.user_open
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is non-zero.
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If the
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.Xr sysctl 8
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variable
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.Va net.link.tap.up_on_open
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is non-zero, the tunnel device will be marked
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.Dq up
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when the control device is opened.
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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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Once the interface is ready,
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.Fn read
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will return an Ethernet frame if one is available; if not, it will
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either block 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.
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If the frame
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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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A
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.Xr write 2
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call passes an Ethernet frame in to be
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.Dq received
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on the pseudo-interface.
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Each
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.Fn write
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call supplies exactly one frame; the frame 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 frame cannot be accepted
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for a transient reason
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(e.g., no buffer space available),
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it is silently dropped; if the reason is not transient
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(e.g., frame too large),
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an error is returned.
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The following
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.Xr ioctl 2
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calls are supported
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(defined in
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.In net/if_tap.h ) :
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.Bl -tag -width VMIO_SIOCSETMACADDR
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.It Dv TAPSIFINFO
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Set network interface information (line speed, MTU and type).
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The argument should be a pointer to a
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.Va struct tapinfo .
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.It Dv TAPGIFINFO
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Retrieve network interface information (line speed, MTU and type).
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The argument should be a pointer to a
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.Va struct tapinfo .
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.It Dv TAPSDEBUG
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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.
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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 TAPGDEBUG
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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 TAPGIFNAME
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Retrieve network interface name.
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The argument should be a pointer to a
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.Va struct ifreq .
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The interface name will be returned in the
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.Va ifr_name
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field.
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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 is not zero
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(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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(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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off or on, according as the argument
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.Va int Ns 's
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value is or is not zero.
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.It Dv FIONREAD
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If any frames are queued to be read, store the size of the first one 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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.It Dv SIOCGIFADDR
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Retrieve the Media Access Control
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.Pq Dv MAC
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address of the
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.Dq remote
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side.
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This command is used by the VMware port and expected to be executed on
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descriptor, associated with control device
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(usually
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.Pa /dev/vmnet Ns Sy N
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or
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.Pa /dev/tap Ns Sy N ) .
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The
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.Va buffer ,
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which is passed as the argument, is expected to have enough space to store
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the
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.Dv MAC
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address.
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At the open time both
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.Dq local
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and
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.Dq remote
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.Dv MAC
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addresses are the same, so this command could be used to retrieve the
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.Dq local
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.Dv MAC
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address.
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.It Dv SIOCSIFADDR
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Set the Media Access Control
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.Pq Dv MAC
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address of the
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.Dq remote
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side.
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This command is used by VMware port and expected to be executed on
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a descriptor, associated with control device
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(usually
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.Pa /dev/vmnet Ns Sy N ) .
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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, the interface is
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brought down
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(as if with
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.Dq ifconfig tap Ns Sy N No down )
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unless the device is a
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.Em VMnet
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device, or has
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.Dv IFF_LINK0
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flag set.
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All queued frames are thrown away.
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If the interface is up when the data
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device is not open, output frames are thrown away rather than
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letting them pile up.
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.Pp
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The
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.Nm
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device can also be used with the VMware port as a replacement
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for the old
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.Em VMnet
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device driver.
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The driver uses the minor number
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to select between
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.Nm
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and
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.Nm vmnet
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devices.
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.Em VMnet
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minor numbers begin at
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.Va 0x800000
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+
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.Va N ;
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where
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.Va N
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is a
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.Em VMnet
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unit number.
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In this case the control device is expected to be
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.Pa /dev/vmnet Ns Sy N ,
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and the network interface will be
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.Sy vmnet Ns Ar N .
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Additionally,
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.Em VMnet
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devices do not
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.Xr ifconfig 8
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themselves down when the
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control device is closed.
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Everything else is the same.
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.Pp
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In addition to the above mentioned
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.Xr ioctl 2
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calls, there is an additional one for the VMware port.
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.Bl -tag -width VMIO_SIOCSETMACADDR
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.It Dv VMIO_SIOCSIFFLAGS
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VMware
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.Dv SIOCSIFFLAGS .
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.El
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.Sh SEE ALSO
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.Xr inet 4 ,
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.Xr intro 4 ,
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.Xr tun 4
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