Hacking: the art of exploitation, 2nd edition by Jon Erickson

Author:Jon Erickson [Jon Erickson]
Language: eng
Format: epub
Tags: Sciences
ISBN: 9781593273385
Published: 2010-08-12T10:00:00+00:00

From /usr/include/if_ether.h

#define ETH_ALEN 6 /* Octets in one ethernet addr */ #define ETH_HLEN 14 /* Total octets in header */ /* * This is an Ethernet frame header. */ struct ethhdr { unsigned char h_dest[ETH_ALEN]; /* Destination eth addr */ unsigned char h_source[ETH_ALEN]; /* Source ether addr */ __be16 h_proto; /* Packet type ID field */ } __attribute__((packed));

This structure contains the three elements of an Ethernet header. The variable declaration of __be16 turns out to be a type definition for a 16-bit unsigned short integer. This can be determined by recursively grepping for the type definition in the include files.

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.

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