WLAN Scan with NDIS Miniport and Much More

Introduction

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.

Background

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;
public:
    airctl(void);
        // 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;};
    
public:
    ~airctl(void);
private:
    // 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 !");
    }else{
        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(m_airctl.open(dv->name)!= true)
        {
            MessageBox("Cant open selected device","fehler",0);
        }else
        {
            ::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,
                                GENERIC_READ,
                                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;
        }
        else
        {
               // ... 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,
                                IOCTL_NDIS_QUERY_GLOBAL_STATS,
                                &oidcode,
                                sizeof( oidcode),
                                ( ULONG *) NULL,
                                0,
                                &bytesreturned,
                                NULL) ;

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

        memset( m_pBSSIDList, 0, sizeof( NDIS_802_11_BSSID_LIST) * 
            NUMBEROF_BSSIDS) ;
        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
                                &oidcode,
                                sizeof( oidcode),//  input buffer
                                ( ULONG *) m_pBSSIDList,// output buffer
                                sizeof( NDIS_802_11_BSSID_LIST) * 
                                NUMBEROF_BSSIDS,
                                &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;
} NDIS_WLAN_BSSID, *PNDIS_WLAN_BSSID;

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);
            temp--;
        }  

        //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.

History

First release version 0.1

License

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