Cannot get correct signal strength from RadioTap about libtins HOT 7 CLOSED

Can you provide an example on how you are instantiating a RadioTap object? In case you are seeing this on a pcap file, can you attach it?

It's not possible to add a pointer to the raw buffer in the PDU class because the buffer is constructed when sending the packet. There's no buffer that contains the whole packet attached to the PDU. You could call PDU::serialize which generates that buffer for you (see the docs).

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I use find_pdu in a sniffer loop.

bool SSISampler::OnPacketRecv(const PDU& pdu)
{
    if (stop_sampling) {
        return false;
    }

    const RadioTap* tap = pdu.find_pdu<RadioTap>();

    if (tap) {
        const Dot11* dot11 = tap->find_pdu<Dot11>();

        cout << "From: " << dot11->addr1() << ", SSI: " << (int)tap->dbm_signal() << " dBm" << endl;

        if (callback) {
            callback(dot11->addr1(), tap->dbm_signal());
        }
    }

    return true;
}

The tap->dbm_signal() always returns 0.

I hacked RadioTap::RadioTap() directly to see if the packet has a SSI value.

RadioTap::RadioTap(const uint8_t *buffer, uint32_t total_sz) 
{
    check_size(total_sz, sizeof(_radio));
    const uint8_t *buffer_start = buffer;
    std::memcpy(&_radio, buffer, sizeof(_radio));
    uint32_t radiotap_hdr_size = length();
    check_size(total_sz, radiotap_hdr_size);
    buffer += sizeof(_radio);
    radiotap_hdr_size -= sizeof(_radio);

    for (int i = 0; i < 0x20; i++) {
      std::cout << std::hex << (int)buffer[i] << " ";
    }
    std::cout << std::endl;
    std::cout << "SSI: " << std::dec << (int)buffer[0x16] - 256 << std::endl;    // SSI is here!

    if(_radio.flags.tsft) 
        read_field(buffer, radiotap_hdr_size, _tsft);

    if(_radio.flags.flags) 
        read_field(buffer, radiotap_hdr_size, _flags);
....

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If you could use tcpdump to capture at least one packet that includes a RadioTap header and attaching it here, I could fix the bug easily. Also, which platform are you running this on?

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Test pcap file attached:
https://www.dropbox.com/s/vtlj3n80p4dtpca/cap.pcap?dl=0

I'm running it on a x86_64 intel machine.

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Thanks for the pcap. I've found the problem. Apparently it's a completely undocumented field, the last one on the headers flags, which (and this is just based on what I see on your packets) allows the RadioTap header to be extended with another set of flag fields right after the previous one.

I don't know what's the rationale on that flag, but it completely breaks parsing because optional fields start 32 bits after they normally do. I'm working on fixing it, will let you know when it's done.

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I've just pushed a fix. It's working on your pcap file. Could you check if it works for you please? Thanks!

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Now it works like a charm.
Thank you very much!

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