Why pass the pad!=0 for radiotap

[johnnylin110]

Hello,
thanks for provide the parser for Radiotap
I have a question that in radiotap.py line 320-321
https://github.com/radiotap/python-radiotap/blob/master/radiotap/radiotap.py#L320
why pass the pad!=0 case for radiotap and return the 0,{}
In Wire-Shark , the packet with pad!=0 still has radiotap field and can parse the information the same as your function do.
So I wonder why pad!=0 can't parse in this case?

Thanks!

[bcopeland]

On Wed, Oct 13, 2021 at 10:51:34PM -0700, johnnylin110 wrote: Hello, thanks for provide the parser for Radiotap I have a question that in radiotap.py line 320-321 [https://github.com/radiotap/python-radiotap/blob/master/radiotap/radiotap.py#L320](url) why pass the pad!=0 case for radiotap and return the 0,{} In Wire-Shark , the packet with pad!=0 still has radiotap field and can parse the information the same as your function do. So I wonder why pad!=0 can't parse in this case?

As I recall (been a while) the reason to check the pad was to avoid parsing corrupted radiotap headers as there's no perfect way to tell whether we have an actual header or just random garbage in front of a packet. Do you have an example capture file where the pad is routinely non-zero?

…

-- Bob Copeland %% https://bobcopeland.com/

[johnnylin110]

@bcopeland
Thanks for the prompt reply, I record by 802.11 Wi-Fi packet, for the packet I record, about half of the packets have pad.
I want to parse radiotap to fetch the data rate information. (normal data rate or VHT data rate), is there any other way to get the data rate information for the packet if we just pass the packet?
Sorry I can't give you the packet due to personal reason.

However, I also notice that, for the python code, the A-MPDU, and VHT part didn't have alignment,
https://github.com/radiotap/python-radiotap/blob/master/radiotap/radiotap.py#L157
https://github.com/radiotap/python-radiotap/blob/master/radiotap/radiotap.py#L170
in the introduction web https://www.radiotap.org/fields/defined, it says that VHT and A-MPDU need the alignment (2 for VHT, 4 for A-MPDU)
So Is the code right or the website right? because i also notice that the result parse by this code generate different results from Wire-Sharks ( it seems to have alignment on this two field)

[bcopeland]

On Thu, Oct 14, 2021 at 07:16:08AM -0700, johnnylin110 wrote: However, I also notice that, for the python code, the A-MPDU, and VHT part didn't have alignment,

This is probably an omission and when I originally wrote it the fields were aligned by happenstance on my hardware. You can just add the appropriate align() call in the parse method, feel free to send a PR to fix.

…

-- Bob Copeland %% https://bobcopeland.com/