.\" .\" 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. .\" .Dd March 11, 2019 .Dt IEEE80211_RADIOTAP 9 .Os .Sh NAME .Nm ieee80211_radiotap .Nd 802.11 device packet capture support .Sh SYNOPSIS .In net80211/ieee80211_var.h .\" .Pp .Ft void .Fo ieee80211_radiotap_attach .Fa "struct ieee80211com *" .Fa "struct ieee80211_radiotap_header *th" .Fa "int tlen" .Fa "uint32_t tx_radiotap" .Fa "struct ieee80211_radiotap_header *rh" .Fa "int rlen" .Fa "uint32_t rx_radiotap" .Fc .\" .Ft int .Fn ieee80211_radiotap_active_vap "struct ieee80211vap *" .\" .Ft int .Fn ieee80211_radiotap_active "struct ieee80211com *" .\" .Ft void .Fn ieee80211_radiotap_tx "struct ieee80211vap *" "struct mbuf *" .Sh DESCRIPTION The .Nm net80211 layer used by 802.11 drivers includes support for a device-independent packet capture format called .Nm radiotap that is understood by tools such as .Xr tcpdump 1 . This facility is designed for capturing 802.11 traffic, including information that is not part of the normal 802.11 frame structure. .Pp Radiotap was designed to balance the desire for a hardware-independent, extensible capture format against the need to conserve CPU and memory bandwidth on embedded systems. These considerations led to a format consisting of a standard preamble followed by an extensible bitmap indicating the presence of optional capture fields. A .Nm net80211 device driver supporting .Vt radiotap defines two packed structures that it shares with .Nm net80211 . These structures embed an instance of a .Vt ieee80211_radiotap_header structure at the beginning, with subsequent fields in the appropriate order, and macros to set the bits of the .Va it_present bitmap to indicate which fields exist and are filled in by the driver. This information is then supplied through the .Fn ieee80211_radiotap_attach call after a successful .Fn ieee80211_ifattach request. .Pp With radiotap setup, drivers just need to fill in per-packet capture state for frames sent/received and dispatch capture state in the transmit path (since control is not returned to the .Nm net80211 layer before the packet is handed to the device). To minimize overhead this work should be done only when one or more processes are actively capturing data; this is checked with one of .Fn ieee80211_radiotap_active_vap and .Fn ieee80211_radiotap_active . In the transmit path capture work looks like this: .Bd -literal -offset indent if (ieee80211_radiotap_active_vap(vap)) { ... /* record transmit state */ ieee80211_radiotap_tx(vap, m); /* capture transmit event */ } .Ed .Pp While in the receive path capture is handled in .Nm net80211 but state must be captured before dispatching a frame: .Bd -literal -offset indent if (ieee80211_radiotap_active(ic)) { ... /* record receive state */ } \&... ieee80211_input(...); /* packet capture handled in net80211 */ .Ed .Pp .\" The following fields are defined for .Vt radiotap , in the order in which they should appear in the buffer supplied to .Nm net80211 . .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 (TSF). In theory, for each received frame, this value is recorded when the first bit of the MPDU arrived at the MAC. In practice, hardware snapshots the TSF otherwise and one cannot assume this data is accurate without driver adjustment. .It Dv IEEE80211_RADIOTAP_FLAGS This field contains a single unsigned 8-bit value, containing one or more of these bit flags: .Bl -tag -width indent .It Dv IEEE80211_RADIOTAP_F_CFP Frame was sent/received during the Contention Free Period (CFP). .It Dv IEEE80211_RADIOTAP_F_SHORTPRE Frame was sent/received with short preamble. .It Dv IEEE80211_RADIOTAP_F_WEP Frame was encrypted. .It Dv IEEE80211_RADIOTAP_F_FRAG Frame was an 802.11 fragment. .It Dv IEEE80211_RADIOTAP_F_FCS Frame contents includes the FCS. .It Dv IEEE80211_RADIOTAP_F_DATAPAD Frame contents potentially has padding between the 802.11 header and the data payload to align the payload to a 32-bit boundary. .It Dv IEEE80211_RADIOTAP_F_BADFCS Frame was received with an invalid FCS. .It Dv IEEE80211_RADIOTAP_F_SHORTGI Frame was sent/received with Short Guard Interval. .El .It Dv IEEE80211_RADIOTAP_RATE This field contains a single unsigned 8-bit value that is the data rate. Legacy rates are in units of 500Kbps. MCS rates (used on 802.11n/HT channels) have the high bit set and the MCS in the low 7 bits. .It Dv IEEE80211_RADIOTAP_CHANNEL This field contains two unsigned 16-bit values. The first value is the center frequency for the channel the frame was sent/received on. The second value is a bitmap containing flags that specify channel properties. .Pp This field is deprecated in favor of .Dv IEEE80211_RADIOTAP_XCHANNEL but may be used to save space in the capture file for legacy devices. .\".It Dv IEEE80211_RADIOTAP_FHSS .\"This field contains two 8-bit values. .\"This field should be present only for frequency-hopping radios. .\"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 that 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 that 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 This field contains a single unsigned 8-bit value that specifies which antenna was used to transmit or receive the frame. Antenna numbering is device-specific but typically the primary antenna has the lowest number. On transmit a value of zero may be seen which typically means antenna selection is left to the device. .It Dv IEEE80211_RADIOTAP_DB_ANTSIGNAL This field contains a single unsigned 8-bit value that 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 that indicates the RF noise power at the antenna, in decibels difference from an arbitrary, fixed reference. .It Dv IEEE80211_RADIOTAP_XCHANNEL This field contains four values: a 32-bit unsigned bitmap of flags that describe the channel attributes, a 16-bit unsigned frequency in MHz (typically the channel center), an 8-bit unsigned IEEE channel number, and a signed 8-bit value that holds the maximum regulatory transmit power cap in .5 dBm (8 bytes total). Channel flags are defined in: .In net80211/_ieee80211.h (only a subset are found in .In net80211/ieee80211_radiotap.h ). This property supersedes .Dv IEEE80211_RADIOTAP_CHANNEL and is the only way to completely express all channel attributes and the mapping between channel frequency and IEEE channel number. .El .Sh EXAMPLES Radiotap receive definitions for the Intersil Prism driver: .Bd -literal -offset indent #define WI_RX_RADIOTAP_PRESENT \\ ((1 << IEEE80211_RADIOTAP_TSFT) \\ (1 << IEEE80211_RADIOTAP_FLAGS) | \\ (1 << IEEE80211_RADIOTAP_RATE) | \\ (1 << IEEE80211_RADIOTAP_CHANNEL) | \\ (1 << IEEE80211_RADIOTAP_DB_ANTSIGNAL) | \\ (1 << IEEE80211_RADIOTAP_DB_ANTNOISE)) struct wi_rx_radiotap_header { struct ieee80211_radiotap_header wr_ihdr; uint64_t wr_tsf; uint8_t wr_flags; uint8_t wr_rate; uint16_t wr_chan_freq; uint16_t wr_chan_flags; uint8_t wr_antsignal; uint8_t wr_antnoise; } __packed __aligned(8); .Ed .Pp and transmit definitions for the Atheros driver: .Bd -literal -offset indent #define ATH_TX_RADIOTAP_PRESENT ( \\ (1 << IEEE80211_RADIOTAP_FLAGS) | \\ (1 << IEEE80211_RADIOTAP_RATE) | \\ (1 << IEEE80211_RADIOTAP_DBM_TX_POWER) | \\ (1 << IEEE80211_RADIOTAP_ANTENNA) | \\ (1 << IEEE80211_RADIOTAP_XCHANNEL) | \\ 0) struct ath_tx_radiotap_header { struct ieee80211_radiotap_header wt_ihdr; uint8_t wt_flags; uint8_t wt_rate; uint8_t wt_txpower; uint8_t wt_antenna; uint32_t wt_chan_flags; uint16_t wt_chan_freq; uint8_t wt_chan_ieee; int8_t wt_chan_maxpow; } __packed; .Ed .Sh SEE ALSO .Xr tcpdump 1 , .Xr bpf 4 , .Xr ieee80211 9 .Sh HISTORY The .Nm definitions first appeared in .Nx 1.5 . .\" .Sh AUTHORS .An -nosplit The original version of this manual page was written by .An Bruce M. Simpson Aq Mt bms@FreeBSD.org and .An Darron Broad Aq Mt darron@kewl.org .