NETWORKING - Hacking: The Art of Exploitation - page 231

reader@hacking:~/booksrc $
$ grep -R "typedef.*__be16" /usr/include
/usr/include/linux/types.h:typedef __u16 __bitwise __be16;
$ grep -R "typedef.*__u16" /usr/include | grep short
/usr/include/linux/i2o-dev.h:typedef unsigned short __u16;
/usr/include/linux/cramfs_fs.h:typedef unsigned short __u16;
/usr/include/asm/types.h:typedef unsigned short __u16;
$
The include file also defines the Ethernet header length in ETH_HLEN as
14 bytes. This adds up, since the source and destination MAC addresses use
6 bytes each, and the packet type field is a 16-bit short integer that takes up
2 bytes. However, many compilers will pad structures along 4-byte boundaries
for alignment, which means that sizeof(struct ethhdr) would return an
incorrect size. To avoid this, ETH_HLEN or a fixed value of 14 bytes should
be used for the Ethernet header length.
By including <linux/if_ether.h> , these other include files containing
the required __be16 type definition are also included. Since we want to make
our own structures for hacking-network.h, we should strip out references to
unknown type definitions. While we're at it, let's give these fields better names.
Added to hacking-network.h
#define ETHER_ADDR_LEN 6
#define ETHER_HDR_LEN 14
struct ether_hdr {
unsigned char ether_dest_addr[ETHER_ADDR_LEN]; // Destination MAC address
unsigned char ether_src_addr[ETHER_ADDR_LEN]; // Source MAC address
unsigned short ether_type; // Type of Ethernet packet
} ;
We can do the same thing with the IP and TCP structures, using the
corresponding structures and RFC diagrams as a reference.
From /usr/include/netinet/ip.h
struct iphdr
{
#if __BYTE_ORDER == __LITTLE_ENDIAN
unsigned int ihl:4;
unsigned int version:4;
#elif __BYTE_ORDER == __BIG_ENDIAN
unsigned int version:4;
unsigned int ihl:4;
#else
# error "Please fix <bits/endian.h>"
#endif
u_int8_t tos;
u_int16_t tot_len;
u_int16_t id;