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NAME
ieee80211_radiotap — 802.11 device packet capture support
SYNOPSIS
#include <net80211/ieee80211_var.h>
void
ieee80211_radiotap_attach(struct ieee80211com *, struct ieee80211_radiotap_header *th,
int tlen, uint32_t tx_radiotap, struct ieee80211_radiotap_header *rh, int rlen,
uint32_t rx_radiotap);
int
ieee80211_radiotap_active_vap(struct ieee80211vap *);
int
ieee80211_radiotap_active(struct ieee80211com *);
void
ieee80211_radiotap_tx(struct ieee80211vap *, struct mbuf *);
DESCRIPTION
The net80211 layer used by 802.11 drivers includes support for a device-independent packet
capture format called radiotap that is understood by tools such as tcpdump(1). This
facility is designed for capturing 802.11 traffic, including information that is not part of
the normal 802.11 frame structure.
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 net80211 device driver
supporting radiotap defines two packed structures that it shares with net80211. These
structures embed an instance of a ieee80211_radiotap_header structure at the beginning, with
subsequent fields in the appropriate order, and macros to set the bits of the it_present
bitmap to indicate which fields exist and are filled in by the driver. This information is
then supplied through the ieee80211_radiotap_attach() call after a successful
ieee80211_ifattach() request.
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 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 ieee80211_radiotap_active_vap() and ieee80211_radiotap_active(). In the
transmit path capture work looks like this:
if (ieee80211_radiotap_active_vap(vap)) {
... /* record transmit state */
ieee80211_radiotap_tx(vap, m); /* capture transmit event */
}
While in the receive path capture is handled in net80211 but state must be captured before
dispatching a frame:
if (ieee80211_radiotap_active(ic)) {
... /* record receive state */
}
...
ieee80211_input(...); /* packet capture handled in net80211 */
The following fields are defined for radiotap, in the order in which they should appear in
the buffer supplied to net80211.
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.
IEEE80211_RADIOTAP_FLAGS
This field contains a single unsigned 8-bit value, containing one or more of these
bit flags:
IEEE80211_RADIOTAP_F_CFP
Frame was sent/received during the Contention Free Period (CFP).
IEEE80211_RADIOTAP_F_SHORTPRE
Frame was sent/received with short preamble.
IEEE80211_RADIOTAP_F_WEP
Frame was encrypted.
IEEE80211_RADIOTAP_F_FRAG
Frame was an 802.11 fragment.
IEEE80211_RADIOTAP_F_FCS
Frame contents includes the FCS.
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.
IEEE80211_RADIOTAP_F_BADFCS
Frame was received with an invalid FCS.
IEEE80211_RADIOTAP_F_SHORTGI
Frame was sent/received with Short Guard Interval.
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.
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.
This field is deprecated in favor of IEEE80211_RADIOTAP_XCHANNEL but may be used to
save space in the capture file for legacy devices.
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.
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.
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.
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.
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.
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.
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: <net80211/_ieee80211.h> (only a subset are found in
<net80211/ieee80211_radiotap.h> ). This property supersedes
IEEE80211_RADIOTAP_CHANNEL and is the only way to completely express all channel
attributes and the mapping between channel frequency and IEEE channel number.
EXAMPLES
Radiotap receive definitions for the Intersil Prism driver:
#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);
and transmit definitions for the Atheros driver:
#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;
SEE ALSO
tcpdump(1), bpf(4), ieee80211(9)
HISTORY
The ieee80211_radiotap definitions first appeared in NetBSD 1.5.
AUTHORS
The original version of this manual page was written by Bruce M. Simpson <bms@FreeBSD.org>
and Darron Broad <darron@kewl.org>.