236 lines
8.4 KiB
Groff
236 lines
8.4 KiB
Groff
.\"
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.\" Copyright (c) 2004 Bruce M. Simpson <bms@spc.org>,
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.\" Darron Broad <darron@kewl.org>,
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.\" David Young <dyoung@pobox.com>.
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\" $Id: ieee80211_radiotap.9,v 1.3 2004/03/04 11:38:52 bruce Exp $
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.\"
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.Dd March 17, 2008
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.Dt IEEE80211_RADIOTAP 9
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.Os
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.Sh NAME
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.Nm ieee80211_radiotap
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.Nd software 802.11 stack packet capture definitions
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.Sh SYNOPSIS
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.In net80211/ieee80211_var.h
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.In net80211/ieee80211_ioctl.h
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.In net80211/ieee80211_radiotap.h
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.In net/bpf.h
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.\"
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.Sh DESCRIPTION
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The
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.Nm
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definitions provide a device-independent
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.Xr bpf 4
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attachment for the
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capture of information about 802.11 traffic which is not part of
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the 802.11 frame structure.
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.Pp
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Radiotap was designed to balance the desire for a capture format
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that conserved CPU and memory bandwidth on embedded systems,
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with the desire for a hardware-independent, extensible format
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that would support the diverse capabilities of virtually all
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802.11
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radios.
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.Pp
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These considerations led radiotap to settle on a format consisting of
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a standard preamble followed by an extensible bitmap indicating the
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presence of optional capture fields.
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.Pp
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The capture fields were packed into the header as compactly as possible,
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modulo the requirements that they had to be packed swiftly,
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with suitable alignment, in the same order as the bits indicating
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their presence.
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.Pp
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This typically includes information such as signal quality and
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timestamps.
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This information may be used by a variety of user agents, including
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.Xr tcpdump 1 .
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It is requested by using the
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.Xr bpf 4
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data-link type
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.Dv DLT_IEEE802_11_RADIO .
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.Pp
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.\"
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Each frame using this attachment has the following header prepended to it:
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.Bd -literal -offset indent
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struct ieee80211_radiotap_header {
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u_int8_t it_version; /* set to 0 */
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u_int8_t it_pad;
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u_int16_t it_len; /* entire length */
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u_int32_t it_present; /* fields present */
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} __attribute__((__packed__));
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.Ed
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.Pp
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.\"
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A device driver implementing
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.Vt radiotap
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typically defines a packed structure embedding an instance of
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.Vt "struct ieee80211_radiotap_header"
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at the beginning,
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with subsequent fields in the appropriate order,
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and a macro to set the bits of the
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.Va it_present
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bitmap to indicate which fields exist and are filled in by the driver.
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.\"
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.Pp
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Radiotap headers are copied to the userland via a separate bpf attachment.
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It is necessary for the driver to create this attachment after calling
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.Xr ieee80211_ifattach 9
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by calling
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.Fn bpfattach2
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with the data-link type set to
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.Dv DLT_IEEE802_11_RADIO .
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.Pp
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.\"
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When the the information is available,
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usually immediately before a link-layer transmission or after a receive,
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the driver copies it to the bpf layer using the
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.Fn bpf_mtap2
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function.
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.Pp
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.\"
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The following extension fields are defined for
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.Vt radiotap ,
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in the order in which they should appear in the buffer copied to userland:
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.Bl -tag -width indent
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.It Dv IEEE80211_RADIOTAP_TSFT
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This field contains the unsigned 64-bit value, in microseconds,
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of the MAC's 802.11 Time Synchronization Function timer,
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when the first bit of the MPDU arrived at the MAC.
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This field should be present for received frames only.
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.It Dv IEEE80211_RADIOTAP_FLAGS
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This field contains a single unsigned 8-bit value, containing a bitmap
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of flags specifying properties of the frame being transmitted or received.
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.It Dv IEEE80211_RADIOTAP_RATE
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This field contains a single unsigned 8-bit value, which is the data rate in
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use in units of 500Kbps.
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.It Dv IEEE80211_RADIOTAP_CHANNEL
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This field contains two unsigned 16-bit values.
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The first value is the frequency upon which this PDU was transmitted
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or received.
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The second value is a bitmap containing flags which specify properties of
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the channel in use.
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These are documented within the header file,
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.In net80211/ieee80211_radiotap.h .
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.It Dv IEEE80211_RADIOTAP_FHSS
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This field contains two 8-bit values.
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This field should be present for frequency-hopping radios only.
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The first byte is the hop set.
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The second byte is the pattern in use.
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.It Dv IEEE80211_RADIOTAP_DBM_ANTSIGNAL
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This field contains a single signed 8-bit value, which indicates the
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RF signal power at the antenna, in decibels difference from 1mW.
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.It Dv IEEE80211_RADIOTAP_DBM_ANTNOISE
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This field contains a single signed 8-bit value, which indicates the
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RF noise power at the antenna, in decibels difference from 1mW.
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.It Dv IEEE80211_RADIOTAP_LOCK_QUALITY
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This field contains a single unsigned 16-bit value, indicating the
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quality of the Barker Code lock.
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No unit is specified for this field.
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There does not appear to be a standard way of measuring this at this time;
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this quantity is often referred to as
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.Dq "Signal Quality"
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in some datasheets.
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.It Dv IEEE80211_RADIOTAP_TX_ATTENUATION
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This field contains a single unsigned 16-bit value, expressing transmit
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power as unitless distance from maximum power set at factory calibration.
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0 indicates maximum transmit power.
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Monotonically nondecreasing with lower power levels.
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.It Dv IEEE80211_RADIOTAP_DB_TX_ATTENUATION
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This field contains a single unsigned 16-bit value, expressing transmit
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power as decibel distance from maximum power set at factory calibration.
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0 indicates maximum transmit power.
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Monotonically nondecreasing with lower power levels.
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.It Dv IEEE80211_RADIOTAP_DBM_TX_POWER
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Transmit power expressed as decibels from a 1mW reference.
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This field is a single signed 8-bit value.
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This is the absolute power level measured at the antenna port.
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.It Dv IEEE80211_RADIOTAP_ANTENNA
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For radios which support antenna diversity, this field contains a single
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unsigned 8-bit value specifying which antenna is being used to transmit
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or receive this frame.
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The first antenna is antenna 0.
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.It Dv IEEE80211_RADIOTAP_DB_ANTSIGNAL
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This field contains a single unsigned 8-bit value, which indicates the
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RF signal power at the antenna, in decibels difference from an
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arbitrary, fixed reference.
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.It Dv IEEE80211_RADIOTAP_DB_ANTNOISE
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This field contains a single unsigned 8-bit value, which indicates the
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RF noise power at the antenna, in decibels difference from an
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arbitrary, fixed reference.
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.It Dv IEEE80211_RADIOTAP_EXT
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This bit is reserved for any future extensions to the
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.Vt radiotap
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structure.
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It should not be used at this time.
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.El
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.Sh EXAMPLES
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Radiotap header for the Cisco Aironet driver:
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.Bd -literal -offset indent
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struct an_rx_radiotap_header {
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struct ieee80211_radiotap_header ar_ihdr;
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u_int8_t ar_flags;
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u_int8_t ar_rate;
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u_int16_t ar_chan_freq;
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u_int16_t ar_chan_flags;
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u_int8_t ar_antsignal;
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u_int8_t ar_antnoise;
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} __attribute__((__packed__));
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.Ed
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.Pp
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Bitmap indicating which fields are present in the above structure:
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.Bd -literal -offset indent
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#define AN_RX_RADIOTAP_PRESENT \\
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((1 << IEEE80211_RADIOTAP_FLAGS) | \\
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(1 << IEEE80211_RADIOTAP_RATE) | \\
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(1 << IEEE80211_RADIOTAP_CHANNEL) | \\
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(1 << IEEE80211_RADIOTAP_DBM_ANTSIGNAL) | \\
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(1 << IEEE80211_RADIOTAP_DBM_ANTNOISE))
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.Ed
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.Sh SEE ALSO
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.Xr bpf 4 ,
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.Xr ieee80211 9
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.Sh HISTORY
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The
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.Nm
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definitions first appeared in
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.Nx 1.5 ,
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and were later ported to
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.Fx 4.6 .
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.\"
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.Sh AUTHORS
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.An -nosplit
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The
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.Nm
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interface was designed and implemented by
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.An David Young Aq dyoung@pobox.com .
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.Pp
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This manual page was written by
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.An Bruce M. Simpson Aq bms@FreeBSD.org
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and
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.An Darron Broad Aq darron@kewl.org .
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