Hacking - The Art of Exploitation, 2nd Edition

Networking 203

htons(short value) Host-to-Network Short

Converts a 16-bit integer from the host’s byte order to network byte order

ntohl(long value) Network-to-Host Long

Converts a 32-bit integer from network byte order to the host’s byte order

ntohs(long value) Network-to-Host Short

Converts a 16-bit integer from network byte order to the host’s byte order

For compatibility with all architectures, these conversion functions should

still be used even if the host is using a processor with big-endian byte ordering.

0x424 Internet Address Conversion

When you see 12.110.110.204, you probably recognize this as an Internet

address (IP version 4). This familiar dotted-number notation is a common

way to specify Internet addresses, and there are functions to convert this

notation to and from a 32-bit integer in network byte order. These functions

are defined in the arpa/inet.h include file, and the two most useful con-

version functions are:

inet_aton(char ascii_addr, struct in_addr network_addr)

ASCII to Network

This function converts an ASCII string containing an IP address in dotted-

number format into an in_addr structure, which, as you remember, only

contains a 32-bit integer representing the IP address in network byte

order.

inet_ntoa(struct in_addr *network_addr)

Network to ASCII

This function converts the other way. It is passed a pointer to an in_addr

structure containing an IP address, and the function returns a character

pointer to an ASCII string containing the IP address in dotted-number

format. This string is held in a statically allocated memory buffer in the

function, so it can be accessed until the next call to inet_ntoa(), when the

string will be overwritten.

0x425 A Simple Server Example

The best way to show how these functions are used is by example. The following

server code listens for TCP connections on port 7890. When a client connects,

it sends the message Hello, world! and then receives data until the connection

is closed. This is done using socket functions and structures from the include

files mentioned earlier, so these files are included at the beginning of the

program. A useful memory dump function has been added to hacking.h,

which is shown on the following page.