[Yang Luo <hsluoyb@xxxxxxxxx>]

Hi Guy,

On Thu, Apr 7, 2016 at 9:37 AM, Guy Harris <guy@xxxxxxxxxxxx> wrote:

On Apr 6, 2016, at 5:41 PM, Yang Luo <hsluoyb@xxxxxxxxx> 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;

FCS is the suffix 4 bytes of a packet. Actually I didn't see those 4 bytes in the packets (only 802.11 data for now) using Wireshark (captured by npcap-nmap-0.06-r13-wifi.exe). So I'm afraid  it's the case that I can't get FCS for data frames? As I didn't start the work of capturing management frames yet. it will not be considered for now.

So I think I will just leave FLags field with 0x00 then.

 

   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;

How to determine if uChCenterFrequency is 5GHz or 2.4GHz? What about the following conditions?

if (pwInfo->uChCenterFrequency == 5000)

and

if (pwInfo->uChCenterFrequency == 2400)

Also I found the frequency of Radiotap "Channel" field is 16bit, but uChCenterFrequency is a ULONG type (32bit). They are both MHz unit. I hope a direct assignment is OK?

 

   provide a radiotap Antenna signal field whose value is the value of the lRSSI field in the structure;

Again a size truncating issue. I found radiotap Antenna signal field is 8bit. And pwInfo->lRSSI is a LONG type (32bit). is a direct assignment OK?

 

   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).

So simply speaking, if phy is dot11_phy_type_ht, then MCS field (3 bytes) all zero.

If phy is dot11_phy_type_vht, then VHT field (12 bytes) all zero.

Also, another question is, how large buffer is needed at most for the whole Radiotap header (including Radiotap data)?

Like the below code, I allocated a buffer of 256 bytes. I don't know if this is enough big. Or if there are any better handle ways?

For now, all the code about the Radiotap is here:

---------------------------------------------------------------

PDOT11_EXTSTA_RECV_CONTEXT  pwInfo;

IEEE80211_RADIOTAP_HEADER RadiotapHeader = { 0 };

CHAR buf[256];

int cur = 0;

pwInfo = NET_BUFFER_LIST_INFO(pNetBufList, MediaSpecificInformation);

// [Radiotap] "TSFT" field.

if ((pwInfo->uReceiveFlags & DOT11_RECV_FLAG_RAW_PACKET_TIMESTAMP) == DOT11_RECV_FLAG_RAW_PACKET_TIMESTAMP)

{

RadiotapHeader.it_present |= IEEE80211_RADIOTAP_TSFT;

RtlCopyMemory(buf + cur, &pwInfo->ullTimestamp, sizeof(INT64) / sizeof(UCHAR));

cur += sizeof(INT64) / sizeof(UCHAR);

}

// [Radiotap] "Flags" field.

if (TRUE) // The packet doesn't have FCS. We always have no FCS for all packets currently.

{

RadiotapHeader.it_present |= IEEE80211_RADIOTAP_FLAGS;

*((UCHAR*)buf + cur) = 0x0; // 0x0: none

cur += sizeof(UCHAR) / sizeof(UCHAR);

}

else // The packet has FCS.

{

RadiotapHeader.it_present |= IEEE80211_RADIOTAP_FLAGS;

*((UCHAR*)buf + cur) = 0x10; // 0x10: frame includes FCS

// FCS check fails.

if ((pwInfo->uReceiveFlags & DOT11_RECV_FLAG_RAW_PACKET_FCS_FAILURE) == DOT11_RECV_FLAG_RAW_PACKET_FCS_FAILURE)

{

*((UCHAR*)buf + cur) |= 0x40; // 0x40: frame failed FCS check

}

cur += sizeof(UCHAR) / sizeof(UCHAR);

}

// [Radiotap] "Rate" field.

// Not finished.

if (TRUE) // looking up the ucDataRate field's value in the data rate mapping table

{

RadiotapHeader.it_present |= IEEE80211_RADIOTAP_RATE;

//RtlCopyMemory(buf + cur, &pwInfo->ullTimestamp, sizeof(INT64) / sizeof(UCHAR));

//cur += sizeof(INT64) / sizeof(UCHAR);

}

// [Radiotap] "Channel" field.

if (TRUE)

{

USHORT flags = 0;

if (pwInfo->uPhyId == dot11_phy_type_fhss)

{

flags = IEEE80211_CHAN_GFSK; // 0x0800

}

else if (pwInfo->uPhyId == dot11_phy_type_ofdm)

{

flags = IEEE80211_CHAN_OFDM; // 0x0040

}

else if (pwInfo->uPhyId == dot11_phy_type_hrdsss)

{

flags = IEEE80211_CHAN_CCK; // 0x0020

}

else if (pwInfo->uPhyId == dot11_phy_type_erp)

{

flags = IEEE80211_CHAN_OFDM; // 0x0040

}

else if (pwInfo->uPhyId != dot11_phy_type_irbaseband)

{

if (pwInfo->uChCenterFrequency == 5000) // 5 GHz

{

flags = IEEE80211_CHAN_5GHZ; // 0x0100

}

else if (pwInfo->uChCenterFrequency == 2400) // 2.4 GHz

{

flags = IEEE80211_CHAN_2GHZ; // 0x0080

}

else

{

// should be a else here?

}

}

RadiotapHeader.it_present |= IEEE80211_RADIOTAP_CHANNEL;

*((USHORT*)buf + cur) = flags;

cur += sizeof(USHORT) / sizeof(UCHAR);

*((USHORT*)buf + cur) = pwInfo->uChCenterFrequency;

cur += sizeof(USHORT) / sizeof(UCHAR);

}

// [Radiotap] "Antenna signal" field.

if (TRUE)

{

RadiotapHeader.it_present |= IEEE80211_RADIOTAP_DBM_ANTSIGNAL;

RtlCopyMemory(buf + cur, &pwInfo->lRSSI, sizeof(UCHAR) / sizeof(UCHAR));

cur += sizeof(UCHAR) / sizeof(UCHAR);

}

// [Radiotap] "MCS" field.

if (pwInfo->uPhyId == dot11_phy_type_ht)

{

RadiotapHeader.it_present |= IEEE80211_RADIOTAP_MCS;

RtlZeroMemory(buf + cur, 3 * sizeof(UCHAR) / sizeof(UCHAR));

cur += 3 * sizeof(UCHAR) / sizeof(UCHAR);

}

// [Radiotap] "VHT" field.

else if (pwInfo->uPhyId == dot11_phy_type_vht)

{

RadiotapHeader.it_present |= IEEE80211_RADIOTAP_VHT;

RtlZeroMemory(buf + cur, 12 * sizeof(UCHAR) / sizeof(UCHAR));

cur += 12 * sizeof(UCHAR) / sizeof(UCHAR);

}

---------------------------------------------------------------

Cheers,

Yang

 

> 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.