NETWORK AND SYSTEM PROGRAMMING

Transcription

1 NETWORK AND SYSTEM PROGRAMMING LAB 09 Network Byte Ordering, inet_aton, inet_addr, inet_ntoa Functions Objectives: To learn byte order conversion To understand inet-aton, inet_addr, inet_ntoa Functions Network Bye Ordering: Different kinds of computers use different conventions for the ordering of bytes within a word. Some computers put the most significant byte within a word first (this is called bigendian order), and others put it last ( little-endian order). So, what one computer thinks is the number 1, can be the number 256 for the other computer, and vice-versa.

2 So that machines with different byte order conventions can communicate, the Internet protocols specify a canonical byte order convention for data transmitted over the network. This is known as network byte When establishing an Internet socket connection, you must make sure that the data in the sin_port and sin_addr members of the sockaddr_in structure are represented in network byte If you are encoding integer data in the messages sent through the socket, you should convert this to network byte order too. If you don't do this, your program may fail when running on or talking to other kinds of machines. Byte Ordering Functions: Routines for converting data between a host's internal representation and Network Byte Order are: Function htons() htonl() ntohl() ntohs() Description Host to Network Short Host to Network Long Network to Host Long Network to Host Short Here is more detail of these functions: unsigned short htons(unsigned short hostshort) This function converts 16-bit (2-byte) quantities from host byte order to network byte unsigned long htonl(unsigned long hostlong) This function converts 32-bit (4-byte) quantities from host byte order to network byte unsigned short ntohs(unsigned short netshort) This function converts 16-bit (2-byte) quantities from network byte order to host byte

3 unsigned long ntohl(unsigned long netlong) This function converts 32-bit quantities from network byte order to host byte These functions are macros and result in the insertion of conversion source code into the calling program. On little-endian machines the code will change the values around to network byte On big-endian machines no code is inserted since none is needed; the functions are defined as null. Program to determine host byte order: In this example we store the two-byte value 0x0102 in the short integer and then look at the two consecutive bytes, c[0] and c[1] to determine the byte int main(int argc, char **argv) union short s; char c[sizeof(short)]; un; un.s = 0x0102; if (sizeof(short) == 2) if (un.c[0] == 1 && un.c[1] == 2) printf("big-endian\n"); else if (un.c[0] == 2 && un.c[1] == 1) printf("little-endian\n"); else printf("unknown\n"); else printf("sizeof(short) = %d\n", sizeof(short)); exit(0); Program9-1: Determining Byte Order.

4 Byte Order Conversion: I. Function: uint16_t htons (uint16_t hostshort) This function converts the uint16_t integer hostshort from host byte order to network byte II. Function: uint16_t ntohs (uint16_t netshort) This function converts the uint16_t integer netshort from network byte order to host byte #include <string.h> #include <stdint.h> void main() uint16_t seq; char buffer[2]; buffer[0]= 0x00; buffer[1]= 0x10; memcpy(&seq, buffer,sizeof(uint16_t)); printf("\n Value = %d, Converted ntohs = %d \n", seq, ntohs(seq)); TASK 1: Program9-2: Example of ntohs Function. Send any other value to network and examine the output. III. Function: uint32_t htonl (uint32_t hostlong) This function converts the uint32_t integer hostlong from host byte order to network byte

5 IV. Function: uint32_t ntohl (uint32_t netlong) This function converts the uint32_t integer netlong from network byte order to host byte #include <string.h> #include <stdint.h> void main() uint32_t network, host; char buffer[4]; buffer[0]= 0x00; buffer[1]= 0x00; buffer[2]= 0x00; buffer[3]= 0x01; memcpy(&host, buffer,sizeof(uint32_t)); network= htonl(host); printf("\n Value = %d, Converted ntohl = %d \n", host, network); TASK 2: Program9-3: Example of ntohl Function. Send any other value to network and examine the output. Write a code to determine that input value remain same on applying both the functions i.e. ntohl & htonl. SYNOPSIS #include <sys/socket.h> #include <netinet/in.h> inet_addr, inet_aton, inet_ntoa Functions

6 int inet_aton(const char *cp, struct in_addr *inp); in_addr_t inet_addr(const char *cp); char *inet_ntoa(struct in_addr in); DESCRIPTION inet_aton() converts the Internet host address cp from the standard numbers-and-dots notation into binary data and stores it in the structure that inp points to. inet_aton returns nonzero if the address is valid, zero if not. The inet_addr() function converts the Internet host address cp from numbers-and-dots notation into binary data in network byte If the input is invalid, INADDR_NONE (usually -1) is returned. This is an obsolete interface to inet_aton, described immediately above; it is obsolete because -1 is a valid address ( ), and inet_aton provides a cleaner way to indicate error return. The inet_ntoa() function converts the Internet host address in given in network byte order to a string in standard numbers-and-dots notation. The string is returned in a statically allocated buffer, which subsequent calls will overwrite. #include <stdlib.h> int main(int argc, char *argv[]) struct in_addr addr; if (argc!= 2) fprintf(stderr, "%s <dotted-address>\n", argv[0]); exit(exit_failure); if (inet_aton(argv[1], &addr) == 0) perror("inet_aton"); exit(exit_failure); printf("%s\n", inet_ntoa(addr)); exit(exit_success); Program9-3: Example of inet_aton & inet_ntoa Function.

7 TASK 3: Examine the output of Program9-3 by providing different dotted addresses.