Commit 08d1f215 authored by Andy Green's avatar Andy Green Committed by John W. Linville
Browse files

[PATCH] mac80211: Monitor mode radiotap injection docs

Add monitor mode radiotap injection docs.
Signed-off-by: default avatarAndy Green <>
Signed-off-by: default avatarJiri Benc <>
Signed-off-by: default avatarJohn W. Linville <>
parent 15028aad
How to use packet injection with mac80211
mac80211 now allows arbitrary packets to be injected down any Monitor Mode
interface from userland. The packet you inject needs to be composed in the
following format:
[ radiotap header ]
[ ieee80211 header ]
[ payload ]
The radiotap format is discussed in
Despite 13 radiotap argument types are currently defined, most only make sense
to appear on received packets. Currently three kinds of argument are used by
the injection code, although it knows to skip any other arguments that are
present (facilitating replay of captured radiotap headers directly):
- IEEE80211_RADIOTAP_RATE - u8 arg in 500kbps units (0x02 --> 1Mbps)
- IEEE80211_RADIOTAP_ANTENNA - u8 arg, 0x00 = ant1, 0x01 = ant2
- IEEE80211_RADIOTAP_DBM_TX_POWER - u8 arg, dBm
Here is an example valid radiotap header defining these three parameters
0x00, 0x00, // <-- radiotap version
0x0b, 0x00, // <- radiotap header length
0x04, 0x0c, 0x00, 0x00, // <-- bitmap
0x6c, // <-- rate
0x0c, //<-- tx power
0x01 //<-- antenna
The ieee80211 header follows immediately afterwards, looking for example like
0x08, 0x01, 0x00, 0x00,
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
0x13, 0x22, 0x33, 0x44, 0x55, 0x66,
0x13, 0x22, 0x33, 0x44, 0x55, 0x66,
0x10, 0x86
Then lastly there is the payload.
After composing the packet contents, it is sent by send()-ing it to a logical
mac80211 interface that is in Monitor mode. Libpcap can also be used,
(which is easier than doing the work to bind the socket to the right
interface), along the following lines:
ppcap = pcap_open_live(szInterfaceName, 800, 1, 20, szErrbuf);
r = pcap_inject(ppcap, u8aSendBuffer, nLength);
You can also find sources for a complete inject test applet here:
Andy Green <>
How to use radiotap headers
Pointer to the radiotap include file
Radiotap headers are variable-length and extensible, you can get most of the
information you need to know on them from:
This document gives an overview and warns on some corner cases.
Structure of the header
There is a fixed portion at the start which contains a u32 bitmap that defines
if the possible argument associated with that bit is present or not. So if b0
of the it_present member of ieee80211_radiotap_header is set, it means that
the header for argument index 0 (IEEE80211_RADIOTAP_TSFT) is present in the
argument area.
< 8-byte ieee80211_radiotap_header >
[ <possible argument bitmap extensions ... > ]
[ <argument> ... ]
At the moment there are only 13 possible argument indexes defined, but in case
we run out of space in the u32 it_present member, it is defined that b31 set
indicates that there is another u32 bitmap following (shown as "possible
argument bitmap extensions..." above), and the start of the arguments is moved
forward 4 bytes each time.
Note also that the it_len member __le16 is set to the total number of bytes
covered by the ieee80211_radiotap_header and any arguments following.
Requirements for arguments
After the fixed part of the header, the arguments follow for each argument
index whose matching bit is set in the it_present member of
- the arguments are all stored little-endian!
- the argument payload for a given argument index has a fixed size. So
IEEE80211_RADIOTAP_TSFT being present always indicates an 8-byte argument is
present. See the comments in ./include/net/ieee80211_radiotap.h for a nice
breakdown of all the argument sizes
- the arguments must be aligned to a boundary of the argument size using
padding. So a u16 argument must start on the next u16 boundary if it isn't
already on one, a u32 must start on the next u32 boundary and so on.
- "alignment" is relative to the start of the ieee80211_radiotap_header, ie,
the first byte of the radiotap header. The absolute alignment of that first
byte isn't defined. So even if the whole radiotap header is starting at, eg,
address 0x00000003, still the first byte of the radiotap header is treated as
0 for alignment purposes.
- the above point that there may be no absolute alignment for multibyte
entities in the fixed radiotap header or the argument region means that you
have to take special evasive action when trying to access these multibyte
entities. Some arches like Blackfin cannot deal with an attempt to
dereference, eg, a u16 pointer that is pointing to an odd address. Instead
you have to use a kernel API get_unaligned() to dereference the pointer,
which will do it bytewise on the arches that require that.
- The arguments for a given argument index can be a compound of multiple types
together. For example IEEE80211_RADIOTAP_CHANNEL has an argument payload
consisting of two u16s of total length 4. When this happens, the padding
rule is applied dealing with a u16, NOT dealing with a 4-byte single entity.
Example valid radiotap header
0x00, 0x00, // <-- radiotap version + pad byte
0x0b, 0x00, // <- radiotap header length
0x04, 0x0c, 0x00, 0x00, // <-- bitmap
0x6c, // <-- rate (in 500kHz units)
0x0c, //<-- tx power
0x01 //<-- antenna
Andy Green <>
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment