.\" $OpenBSD: ieee80211_radiotap.9,v 1.5 2005/07/14 13:51:58 jmc Exp $ .\" .\" Copyright (c) 2004 Bruce M. Simpson , .\" Darron Broad , .\" David Young . .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" $FreeBSD: src/share/man/man9/ieee80211_radiotap.9,v 1.3 2004/07/07 12:59:39 ru Exp $ .\" $Id: ieee80211_radiotap.9,v 1.5 2005/07/14 13:51:58 jmc Exp $ .\" .Dd March 2, 2004 .Dt IEEE80211_RADIOTAP 9 .Os .Sh NAME .Nm ieee80211_radiotap .Nd software 802.11 stack packet capture definitions .Sh SYNOPSIS .In net80211/ieee80211_var.h .In net80211/ieee80211_ioctl.h .In net80211/ieee80211_radiotap.h .In net/bpf.h .\" .Sh DESCRIPTION The .Nm definitions provide a device-independent .Xr bpf 4 attachment for the capture of information about 802.11 traffic which is not part of the 802.11 frame structure. .Pp Radiotap was designed to balance the desire for a capture format that conserved CPU and memory bandwidth on embedded systems, with the desire for a hardware-independent, extensible format that would support the diverse capabilities of virtually all 802.11 radios. .Pp These considerations led radiotap to settle on a format consisting of a standard preamble followed by an extensible bitmap indicating the presence of optional capture fields. .Pp The capture fields were packed into the header as compactly as possible, modulo the requirements that they had to be packed swiftly, with suitable alignment, in the same order as the bits indicating their presence. .Pp This typically includes information such as signal quality and timestamps. This information may be used by a variety of user agents, including .Xr tcpdump 8 . It is requested by using the .Xr bpf 4 data-link type .Dv DLT_IEEE_80211_RADIO . .Pp .\" Each frame using this attachment has the following header prepended to it: .Bd -literal -offset indent struct ieee80211_radiotap_header { u_int8_t it_version; /* set to 0 */ u_int8_t it_pad; u_int16_t it_len; /* entire length */ u_int32_t it_present; /* fields present */ } __attribute__((__packed__)); .Ed .Pp .\" A device driver implementing .Vt radiotap typically defines a packed structure embedding an instance of .Vt "struct ieee80211_radiotap_header" at the beginning, with subsequent fields in the appropriate order, and a macro to set the bits of the .Va it_present bitmap to indicate which fields exist and are filled in by the driver. .\" .Pp Radiotap headers are copied to userland via a separate bpf attachment. It is necessary for the driver to create this attachment after calling .Xr ieee80211_ifattach 9 by calling .Fn bpfattach2 with the data-link type set to .Dv DLT_IEEE_80211_RADIO . .Pp .\" When the information is available, usually immediately before a link-layer transmission or after a receive, the driver copies it to the bpf layer using the .Fn bpf_mtap2 function. .Pp .\" The following extension fields are defined for .Vt radiotap , in the order in which they should appear in the buffer copied to userland: .Bl -tag -width indent .It Dv IEEE80211_RADIOTAP_TSFT This field contains the unsigned 64-bit value, in microseconds, of the MAC's 802.11 Time Synchronization Function timer, when the first bit of the MPDU arrived at the MAC. This field should be present for received frames only. .It Dv IEEE80211_RADIOTAP_FLAGS This field contains a single unsigned 8-bit value, containing a bitmap of flags specifying properties of the frame being transmitted or received. .It Dv IEEE80211_RADIOTAP_RATE This field contains a single unsigned 8-bit value, which is the data rate in use in units of 500Kbps. .It Dv IEEE80211_RADIOTAP_CHANNEL This field contains two unsigned 16-bit values. The first value is the frequency upon which this PDU was transmitted or received. The second value is a bitmap containing flags which specify properties of the channel in use. These are documented within the header file .Aq Pa net80211/ieee80211_radiotap.h . .It Dv IEEE80211_RADIOTAP_FHSS This field contains two 8-bit values. This field should be present for frequency-hopping radios only. The first byte is the hop set. The second byte is the pattern in use. .It Dv IEEE80211_RADIOTAP_DBM_ANTSIGNAL This field contains a single signed 8-bit value, which indicates the RF signal power at the antenna, in decibels difference from 1mW. .It Dv IEEE80211_RADIOTAP_DBM_ANTNOISE This field contains a single signed 8-bit value, which indicates the RF noise power at the antenna, in decibels difference from 1mW. .It Dv IEEE80211_RADIOTAP_LOCK_QUALITY This field contains a single unsigned 16-bit value, indicating the quality of the Barker Code lock. No unit is specified for this field. There does not appear to be a standard way of measuring this at this time; this quantity is often referred to as .Dq "Signal Quality" in some datasheets. .It Dv IEEE80211_RADIOTAP_TX_ATTENUATION This field contains a single unsigned 16-bit value, expressing transmit power as unitless distance from maximum power set at factory calibration. 0 indicates maximum transmit power. Monotonically nondecreasing with lower power levels. .It Dv IEEE80211_RADIOTAP_DB_TX_ATTENUATION This field contains a single unsigned 16-bit value, expressing transmit power as decibel distance from maximum power set at factory calibration. 0 indicates maximum transmit power. Monotonically nondecreasing with lower power levels. .It Dv IEEE80211_RADIOTAP_DBM_TX_POWER Transmit power expressed as decibels from a 1mW reference. This field is a single signed 8-bit value. This is the absolute power level measured at the antenna port. .It Dv IEEE80211_RADIOTAP_ANTENNA For radios which support antenna diversity, this field contains a single unsigned 8-bit value specifying which antenna is being used to transmit or receive this frame. The first antenna is antenna 0. .It Dv IEEE80211_RADIOTAP_DB_ANTSIGNAL This field contains a single unsigned 8-bit value, which indicates the RF signal power at the antenna, in decibels difference from an arbitrary, fixed reference. .It Dv IEEE80211_RADIOTAP_DB_ANTNOISE This field contains a single unsigned 8-bit value, which indicates the RF noise power at the antenna, in decibels difference from an arbitrary, fixed reference. .It Dv IEEE80211_RADIOTAP_EXT This bit is reserved for any future extensions to the .Vt radiotap structure. A driver can set .Dv IEEE80211_RADIOTAP_EXT to extend the it_present bitmap by another 64 bits. The bitmap can be extended by multiples of 32 bits to 96, 128, 160 bits, or longer, by setting .Dv IEEE80211_RADIOTAP_EXT in the extensions. The bitmap ends at the first extension field where .Dv IEEE80211_RADIOTAP_EXT is not set. .El .Sh EXAMPLES Radiotap header for the Realtek RTL8180L driver .Xr rtw 4 : .Bd -literal -offset indent struct rtw_rx_radiotap_header { struct ieee80211_radiotap_header rr_ihdr; u_int64_t rr_tsft; u_int8_t rr_flags; u_int8_t rr_rate; u_int16_t rr_chan_freq; u_int16_t rr_chan_flags; u_int16_t rr_barker_lock; u_int8_t rr_antsignal; } __attribute__((__packed__)); .Ed .Pp Bitmap indicating which fields are present in the above structure: .Bd -literal -offset indent #define RTW_RX_RADIOTAP_PRESENT \\ ((1 << IEEE80211_RADIOTAP_TSFT) | \\ (1 << IEEE80211_RADIOTAP_FLAGS) | \\ (1 << IEEE80211_RADIOTAP_RATE) | \\ (1 << IEEE80211_RADIOTAP_CHANNEL) | \\ (1 << IEEE80211_RADIOTAP_LOCK_QUALITY) | \\ (1 << IEEE80211_RADIOTAP_DB_ANTSIGNAL) | \\ 0) .Ed .Sh SEE ALSO .Xr bpf 4 , .Xr ieee80211 9 .Sh HISTORY The .Nm definitions first appeared in .Nx 1.5 , and were later ported to .Fx 4.6 and .Ox 3.6 . .\" .Sh AUTHORS The .Nm interface was designed and implemented by .An David Young Aq dyoung@pobox.com . .Pp This manual page was written by .An Bruce M. Simpson Aq bms@FreeBSD.org and .An Darron Broad Aq darron@kewl.org .