I've been doing some study for my CWAP (wireless analysis) exam recently, so I've been spending quite some time staring at Wirehsark traces trying to figure out precisely what all of those 802.11 fields actually mean.
One thing I noticed whilst pouring over a few capture files is that some of them seemed to have some additional fields included in the trace, which seem to have nothing to do with fields defined in 802.11 frames at all. They are in a section of the packet decode called 'RadioTap Headers'. I wasn't too sure what they were and why they are available in some captures, whilst in others they were missing.
After a little bit of research, I found out a bit more information and thought it might be worth sharing in a quick blog post.
In summary, radiotap headers provide additional information that is added to each 802.11 frame when capturing frames with an analysis application. Just to be clear, these are not part of the standard 802.11 frame format, but are additional information added at the time of capture to provide supplementary data about the frames captured. For instance, in a standard 802.11 traffic capture, there is no information regarding the receive signal level for the frame at the time of capture, which could be very useful. As another example, there is no information about which channel is being used by a station that has generated the frame, which again could be very useful.
Radiotap headers provide additional information to supplement the raw frame capture data that can be derived by analyzing the 802.11 frames.
The next logical question is: "how to I get radiotap headers in my captures?". The headers are added by the wireless adapter (or its driver) that is being used to perform the frame capture. If the adapter does not inject the additional information as it captures frames, then no radiotap headers will be added.
I guess the best way to verify this is simply to perform a capture and see if they appear in your capture using Wireshark. In my own particular case, I was performing a capture with an AirPcap NX card, which provides radiotap information. However, I also performed captures with the internal WLAN NIC of my laptop, which does not provide radiotap data.
Here are a couple of examples so that you can see the difference between captures with and without radiotap headers:
|Fig 1 - Beacon frame, no radiotap data|
The capture above shows a standard capture of a beacon frame with no radio tap headers (taken with my laptop WLAN NIC card). Next, we see a beacon frame again, but this time with radiotap information included (taken with the AirPcap card):
|Fig 2 - Beacon frame, with radiotap data|
You can see the additional radiotap section in the frame decode highlighted with a red circle above.
Next, we'll snap open the radio tap headers and take a look at the information available:
|Fig 3 - Radiotap headers detail|
Right away, you can see fields which give us great supplementary information about the RF environment that the capture was taken in. Looking at the trace above, we can see that this frame was captured on channel 6 on the 2.4GHz band and that it was received by the wireless NIC capturing the frame at a level of -44dBm.
You can see that there are also additional snap-open sections to view even more information (i.e. Present flags, Flags and Channel type), so we'll take a brief look at each of those too.
The 'Present flags' section is a matrix of the information that is available in subsequent sections of the radiotap headers. The flags will vary depending on the information that can be provided by the NIC card that is performing the capture. (To see a comprehensive list of all possible fields and their meaning, take a look at the following page: http://www.radiotap.org/defined-fields )
For each field that is marked as 'true', there will be information for that section appearing in the radiotap headers that follow the 'Present Flags' matrix. For instance, in the matrix shown above we see that the flags for 'Channel' and 'dBm Antenna Signal' are true. We've already seen in a previous screen-shot that these field are included in the headers shown in the frame decode.
|Fig 5 - Radiotap 'Flags' section|
The flags section provides us even more information about the captured frame itself, giving details about preamble type, whether a short guard interval was used and whether the frame has a valid frame checksum.
|Fig 6 - Radiotap 'Channel Type' section|
In the 'Channel Type' section, we gain even more information about characteristics of the channel itself, such as which band is in use, the modulation type used and channel width information.
The examples shown here represent a subset of the fields that may be included in radiotap headers. To view all possible fields, and to understand their meaning, take a look at the following page: http://www.radiotap.org/defined-fields
To find out more about radiotap headers, take a look at http://www.radiotap.org which has comprehensive information on all apsects of radiotap headers.