Network Security Tools

Initializing the Session

libnet enables you to build arbitrary network packets using three main concepts: contexts, protocol blocks, and protocol tags. A context is an opaque handle used to maintain the session state for building the complete packet. A context is referred to by a variable of type libnet_t. A protocol block is the libnet internal data built for each network layer you have created. You refer to these via protocol tags of type libnet_ptag_t. As when using libpcap, you should never need to know precisely what is in either the libnet context or the protocol blocks. When the packet is sent, the libnet_t context is provided, libnet creates the packet from the protocol blocks created, and the packet is sent.

Therefore, the first thing our libnet tool requires is a libnet context of type libnet_t. We create a context using the libnet_init( ) function, which has the following prototype:

libnet_t *libnet_init (int injection_type, char *device, char *err_buf)

The example program uses this function to open its session, as shown in Example 11-1.

Example 11-1. Using libnet_init( )

#include <libnet.h>

libnet_t *l;            /* libnet context */
char errbuf[LIBNET_ERRBUF_SIZE];    /* error messages */

/* open handle */
l = libnet_init (LIBNET_LINK, device, errbuf);

if (l == NULL)
  {
    fprintf (stderr, "Error opening context: %s", errbuf);
    exit (1);
  }

Because the I am tool is creating ARP packets, and because ARP is a link layer protocol, we cannot use one of the LIBNET_RAW injection types for this ...