Re: [tcpdump-workers] What's the difference between NdisMediumBare80211 (DLT_IEEE802_11) and NdisMediumRadio80211 (DLT_IEEE802_11_RADIO)

[Yang Luo <hsluoyb () gmail com>]

Hi Guy,

Very informative about the radiotap header. It helps a lot:)

BTW, should adding radiotap header to a 802.11 packet be an option which
can be selected by the user? If yes, which is by default? with radiotap or
without it?
In fact, I want to know how Linux implements this? I tend to keep the alike
manner as other systems.


Cheers,
Yang

On Thu, Apr 7, 2016 at 9:37 AM, Guy Harris <guy () alum mit edu> wrote:

> On Apr 6, 2016, at 5:41 PM, Yang Luo <hsluoyb () gmail com> wrote:
>
> > I wonder why this mail went to my spam.. I don't know anything about
> radiotap header so I'm afraid i'm not supplying it.
>
> It's a way to provide "radio metadata" for packets; see
>
>         http://www.radiotap.org
>
> for a description of it.
>
> If you were to implement that in the future, you'd get the "Media-Specific
> OOB Data for Received 802.11 Packets":
>
>
> https://msdn.microsoft.com/en-us/library/windows/hardware/ff559169(v=vs.85).aspx
>
> in a DOT11_EXTSTA_RECV_CONTEXT structure:
>
>
> https://msdn.microsoft.com/en-us/library/windows/hardware/ff548626(v=vs.85).aspx
>
> when you receive a packet.  Then you'd provide a link-layer header type of
> DLT_IEEE802_11_RADIO, and synthesize a radiotap header.  When you open the
> device, you'd have to fetch the device's data rate mapping table with the
> OID_DOT11_DATA_RATE_MAPPING_TABLE OID:
>
>
> https://msdn.microsoft.com/en-us/library/windows/hardware/ff569139(v=vs.85).aspx
>
>
> https://msdn.microsoft.com/en-us/library/windows/hardware/ff547679(v=vs.85).aspx
>
> and associate that with the private data for the pcap_t.
>
> Then, for each received packet:
>
>    if DOT11_RECV_FLAG_RAW_PACKET_TIMESTAMP is set in uReceiveFlags,
> provide a radiotap TSFT field with the value from the ullTimestamp field of
> the structure;
>
>    provide a radiotap Flags field with 0x10 set if the frame includes the
> FCS (you'll probably have to experiment a bit to see whether you get the
> FCS or not - the answer might differ for data and management frames, based
> on Network Monitor's behavior) and with 0x40 set if
> DOT11_RECV_FLAG_RAW_PACKET_FCS_FAILURE is set in uReceiveFlags;
>
>    provide a radiotap Rate field whose value is the result of looking up
> the ucDataRate field's value in the data rate mapping table and returning
> the usDataRateValue value from that table - if it's not found, don't
> provide the Rate field;
>
>    provide a radiotap Channel field where the frequency value is the
> uChCenterFrequency field of the structure and the flags are derived from
> the uChCenterFrequency and uPhyId fields of the structure - assuming that
> the uPhyId value is one of the ones from
>
>
> https://msdn.microsoft.com/en-us/library/windows/hardware/ff548741(v=vs.85).aspx
>
>    then the mapping would be:
>
>         dot11_phy_type_fhss - set 0x0800 in the flags (11 legacy FHSS);
>
>         dot11_phy_type_ofdm - set 0x0040 in the flags (11a);
>
>         dot11_phy_type_hrdsss - set 0x0020 in the flags (11b);
>
>         dot11_phy_type_erp - set 0x0040 in the flags (11g, unknown whether
> it's pure or not);
>
>    and, unless it's dot11_phy_type_irbaseband, set 0x0100 if the frequency
> is in the 5 GHz range or set 0x0080 if it's in the 2.4 GHz range;
>
>    provide a radiotap Antenna signal field whose value is the value of the
> lRSSI field in the structure;
>
>    if the phy is dot11_phy_type_ht, provide a radiotap MCS field where the
> known field is 0 and the other fields are also zeroed out (i.e., it's 11n,
> but we don't know anything else about it);
>
>    if the phy is dot11_phy_type_vht, provide a radiotap VHT field where
> the known field is 0 and the other fields are also zeroed out (i.e., it's
> 11ac, but we don't know anything else about it).
>
> > And I have confirmed that my captured packets are parsed well using
> NdisMediumBare80211. In Wireshark it shows "IEEE 802.11 Data".
>
> That means that you're just supplying packets that begin with an 802.11
> header, with no form of radio information preceding it, so...
>
> > So I think I will just use this value.
>
> ...that is exactly what you should be doing.
___________________________________________________________________________
Sent via:    Wireshark-dev mailing list <wireshark-dev () wireshark org>
Archives:    https://www.wireshark.org/lists/wireshark-dev
Unsubscribe: https://wireshark.org/mailman/options/wireshark-dev
             mailto:wireshark-dev-request () wireshark org?subject=unsubscribe