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Posted 29 Oct 2007

WLAN Scan with NDIS Miniport and Much More

, 29 Oct 2007
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An article about using NDIS Miniport from userspace to access several functions of the WLANcard
Screenshot - screen.jpg


I spent some time with WLAN, and I wondered how NetStumbler performed this nice scan, but the source is not free. I programmed a lot of stuff but nothing worked. I tried to code my own driver and used WinPcap, but nothing worked very well. Then I read an article, "Scanning for Wireless Networks" in 29A magazine 8 from GriYo. My work is mainly based on this article and you can find a lot of sources on the internet that do the same thing.


It's possible via NDIS Miniport (from userspace) to access a lot of functions of the WLANcard. To get access to all these nice functions you need the Windows DDK.

Take a look at ntddndis.h and you will find a lot of actions you can perform. For example, you can get or set the WEP key. I only implemeted the scanning function, but you can improve it. To build your own application, you can take my article class or write our own.

Using the Code

Well, I put a lot of comments in the code so you will be able to understand it easily. All the functions you need are in the airctl class. Take a look at the header file.

class airctl
    HANDLE m_handle;
    deviceInfo* m_devices;
        // Frees the list of wlans
    void freeScanList(void);
    //scans for wlan and returns pointer to the list.
    NDIS_802_11_BSSID_LIST* scan(void);

bool open( char *device_name);
    //lists all devices and saves them in an internal list.
    BOOL list_devices( void);
    // return the pointer to the list
    inline deviceInfo* getDevList( ){return m_devices;};
    // adds one device to the list
    void AddDevice(char * desc, char * name);
    // clears the intern list
    void clearDeviceList(void);
    NDIS_802_11_BSSID_LIST* m_pBSSIDList;

    // intern used to get more informations about an interface
    BOOL get_device_info(   int Index,
                        char *key_name,
                        char *device_info,
                        char *device_description);


First we need a list of all devices. Call list_devices() and it will list everything in an single linked list. Then the demo program shows the user a list of all available devices. The structure for the list is as follows.

struct deviceInfo
    char *description;//information for the user
    char *name;// important to open the device
    deviceInfo* next;//single linked list
Look at the following piece of code. It is from the demo program.
//return me the pointer to the list
 deviceInfo* dv = win->m_airctl.getDevList();

 if (dv == NULL)
     List.AddString("No available interface found !");
     while (dv != NULL)//step through the linked list
         List.AddString(dv->description);// adds it to the list.
         dv = dv->next;

After the user selects a device, it will be opened.

To open the device with the aitctl class we only need to call open() with the device name in it.

if(>name)!= true)
        MessageBox("Cant open selected device","fehler",0);
        ::AfxBeginThread( threadFunc , (LPVOID) this);


But we are curious and will take a small look into that function.

//we open the physical device with CreateFileA
    m_handle = CreateFileA(   device_file,
                            FILE_SHARE_READ | FILE_SHARE_WRITE,// share
                                                               // mode
                            NULL, // default security attributes
                            OPEN_EXISTING,// disposition
                            0,// file attributes
                            NULL) ;// do not copy file attributes

    if( m_handle == INVALID_HANDLE_VALUE)
           return false;
           // ... open
        return true;


Now it's time to search the WLANs. We will call scan() from the aitctl class.

NDIS_802_11_BSSID_LIST * pBSSIDList = pDlg->m_airctl.scan();

How is this performed? It's done via DeviceIoControl. This is for direct input and output, or for retrieving information from a physical device. For more infomration read this article. It's a really nice series of articles.

First we force the WLAN device to scan for WLANs; we wait for a moment and then we ask the device again to tell us an answer. The source code for this looks like this:

oidcode = OID_802_11_BSSID_LIST_SCAN ;//action to perform

   DeviceIoControl(        m_handle,
                           sizeof( oidcode),
                           ( ULONG *) NULL,
                           NULL) ;

   Sleep( 2000) ;// we give him some time

   memset( m_pBSSIDList, 0, sizeof( NDIS_802_11_BSSID_LIST) *
   oidcode = OID_802_11_BSSID_LIST ;//action to perform

   if( DeviceIoControl(    m_handle,// device to be queried
                           IOCTL_NDIS_QUERY_GLOBAL_STATS,// operation to
                                                         // perform
                           sizeof( oidcode),//  input buffer
                           ( ULONG *) m_pBSSIDList,// output buffer
                           sizeof( NDIS_802_11_BSSID_LIST) *
                           &bytesreturned,// # bytes returned
                           NULL) == 0)// synchronous I/O
          // List failed
         return NULL;
   else {
         return m_pBSSIDList;

Now we have a pointer to the NDIS_802_11_BSSID_LIST take a look in ntddndis.h where it's defined.

typedef struct _NDIS_802_11_BSSID_LIST
    ULONG           NumberOfItems;      // in list below, at least 1
    NDIS_WLAN_BSSID Bssid[1];

So far so good, but what does NDIS_WLAN_BSSID look like?

typedef struct _NDIS_WLAN_BSSID
    ULONG                               Length;             // Length of this
                                                            // structure
    NDIS_802_11_MAC_ADDRESS             MacAddress;         // BSSID
    UCHAR                               Reserved[2];
    NDIS_802_11_SSID                    Ssid;               // SSID
    ULONG                               Privacy;            // WEP encryption
                                                            // requirement
    NDIS_802_11_RSSI                    Rssi;               // receive signal
                                                            // strength in dBm
    NDIS_802_11_NETWORK_TYPE            NetworkTypeInUse;
    NDIS_802_11_CONFIGURATION           Configuration;
    NDIS_802_11_NETWORK_INFRASTRUCTURE  InfrastructureMode;
    NDIS_802_11_RATES                   SupportedRates;

This looks much better. This is the information we would like to obtain. Now we can show them to the user.

But, we have one thing left to discuss: how to get the next item. For this we will use another pointer which points to NDIS_WLAN_BSSID. Now the variable ULONG Length tells us how long this entry is. After this a certain number of bytes the next entry "lies."

Well, the following is not a good solution, but it shows you what I mean and what it could look like.

//NumberOfItems indecates how many are now in the list
for(unsigned int i =0 ;i < pBSSIDList->NumberOfItems;i++){
    int temp=i;//used to build the differens
    //step to the next in list...
    PNDIS_WLAN_BSSID cpSsid=pBSSIDList->Bssid;//save

    //if we aren't in the first loop we have to set the pointer
    //to the next
    while(temp!=0 ){
        // go forward
        cpSsid=(PNDIS_WLAN_BSSID)((char*)cpSsid+ cpSsid->Length);

    //do something with your data

That's it man. Thank you for reading, now it's up to you.

Points of Interest

You can perform a lot of more functions to the WLAN device. Just take a look into ntddndis.h under 802.11 OIDs.


First release version 0.1


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

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Comments and Discussions

GeneralNo Scan Results Pin
masterluu21-Oct-08 23:26
membermasterluu21-Oct-08 23:26 

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