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Dissector: how to get wlan radiotap data from upper layers?

asked 2022-02-14 12:51:47 +0000

doragasu gravatar image

updated 2022-02-14 13:33:09 +0000

grahamb gravatar image

I am creating a custom dissector that filters a specific ethertype and dissects the encapsulated data:

custom_handle = create_dissector_handle(dissect_custom, proto_custom);
dissector_add_uint("ethertype", CUSTOM_PROTO_ETHERTYPE, custom_handle);

The dissector works, but among other information, I have a requirement to add the packet RSSI to the Info column. The RSSI is in the radiotap header, so the question is how can I get it?

I have tried the signal fields in the packet_info struct, like: gint8 rssi = pinfo->pseudo_header->ieee_802_11.signal_dbm;, but they are always zero (why?). I have also searched how to get data from upper layers, but I have not found any clear information either (replies to questions asking this always show a workaround to avoid doing it, so I suspect there is a good reason to avoid this approach).

After a lot of reading, I do not know if I have to create my own radiotap dissector variant and use the p_add_proto_data() function, or if I have to register a tap interface, or maybe I can somehow get the info from pinfo->rec, or if I have to somehow register the dissector at a lower level and manually call the dissectors for the intermediate levels... I am really confused, how can I get the radiotap info?

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answered 2022-02-15 07:46:39 +0000

doragasu gravatar image

I have found that I can just add the RSSI as a new column in the packet view, and that should work. Unfortunately, I would like to get the RSSI from both antennas present in the device, and it seems this is not possible with this trick, so I still need to access data from upper layers. But for anyone just needing the better value, this should work.

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Asked: 2022-02-14 12:51:47 +0000

Seen: 150 times

Last updated: Feb 15 '22