Network Security Tools

Building the Protocol Blocks

Once we have created a libnet context, we can start building the protocol blocks to be sent. Remember that we must create the protocol blocks in order, from the highest-level protocol to the lowest-level protocol we are required to build. Because we are using the LIBNET_LINK injection type, we are required to create the link layer packet as well as any higher-level packets. Therefore, we need to start by creating the ARP packet header, as shown in Example 11-2.

Example 11-2. Creating the ARP header

in_addr_t ipaddr;                     /* source ip address */
in_addr_t destaddr;                   /* destination ip address */
u_int8_t *macaddr;                    /* destination mac address */
struct libnet_ether_addr *hwaddr;     /* source MAC address */
libnet_ptag_t arp = 0;                /* ARP protocol tag */

/* get the hardware address for the card we are using */
hwaddr = libnet_get_hwaddr (l);

/* build the ARP header */
arp = libnet_autobuild_arp (ARPOP_REPLY,              /* operation */
                          (u_int8_t *) hwaddr,        /* source hardware addr */
                          (u_int8_t *) &ipaddr,       /* source protocol addr */
                          macaddr,                    /* target hardware addr */
                          (u_int8_t *) &destaddr,     /* target protocol addr */
                          l);                         /* libnet context */

if (arp == -1)
    {
      fprintf (stderr,
           "Unable to build ARP header: %s\n", libnet_geterror (l));
      exit (1);
    }

Example 11-2 uses the libnet_autobuild_arp() function, which has the following prototype:

libnet_ptag_t libnet_autobuild_arp (u_int16_t op, u_int8_t *sha, u_int8_t *spa,
             u_int8_t *tha, u_int8_t *tpa, libnet_t *l)

The build and autobuild functions ...