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Network Security Tools by Justin Clarke, Nitesh Dhanjani

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Adapting Arpsniff to 802.11

To adapt Arpsniff to capture information from a wireless packet-capture source, we need to make a few changes to the application logic. We assume the wireless device used in this example supports the AVS wireless capture format.

First of all, we need to specify the sizes of some of the additional frames captured:

/* ugly shortcuts - Defining our header types */
#define ETH_HEADER_SIZE 14
#define AVS_HEADER_SIZE 64                 /* AVS capture header size */
#define DATA_80211_FRAME_SIZE 24           /* header for 802.11 data packet */
#define LLC_HEADER_SIZE 8                  /* LLC frame for encapsulation */

We are specifying additional header sizes because of the additional headers our ARP packet has when capturing from a wireless source due to RFC 1042 IP encapsulation, as shown in Figure 10-4.

ARP packet format on 802.11 from an AVS capture source

Figure 10-4. ARP packet format on 802.11 from an AVS capture source

To determine the type of packet embedded in the 802.11 packet, we need to have a definition for the LLC header so that we can extract the ether_type value:

/* SNAP LLC header format */
struct snap_header           
{
  u_int8_t dsap; 
  u_int8_t ssap;
  u_int8_t ctl;
  u_int16_t org; 
  u_int8_t org2;
  u_int16_t ether_type;          /* ethernet type */              
} _ _attribute_ _ ((_ _packed_  _));

Now we can alter the process_packet function to work with a captured 802.11 packet from an AVS wireless source:

/* callback function to process a packet when captured */ void process_packet ...

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