1. getname.c, get the host information, mainly the IP address if you know the host name.
[code]
/* As usual,
make the appropriate includes and declare the variables. */
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <netdb.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
char *host, **names, **addrs;
struct hostent *hostinfo;
/* Set the host in question to the argument supplied with the getname call,
or default to the user's machine. */
if(argc == 1) {
char myname[256];
gethostname(myname, 255);
host = myname;
}
else
host = argv[1];
/* Make the call to gethostbyname and report an error if no information is found. */
hostinfo = gethostbyname(host);
if(!hostinfo) {
fprintf(stderr, "cannot get info for host: %s\n", host);
exit(1);
}
/* Display the hostname and any aliases it may have. */
printf("results for host %s:\n", host);
printf("Name: %s\n", hostinfo -> h_name);
printf("Aliases:");
names = hostinfo -> h_aliases;
while(*names) {
printf(" %s", *names);
names++;
}
printf("\n");
/* Warn and exit if the host in question isn't an IP host. */
if(hostinfo -> h_addrtype != AF_INET) {
fprintf(stderr, "not an IP host!\n");
exit(1);
}
/* Otherwise, display the IP address(es). */
addrs = hostinfo -> h_addr_list;
while(*addrs) {
printf(" %s", inet_ntoa(*(struct in_addr *)*addrs));
addrs++;
}
printf("\n");
exit(0);
}
[code end]
Note: this example code is extracted from book, Beginning
Linux Programming, the 3rd edition.
2. getaddress.c, get the host name supposing you know its IP address.
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <netdb.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
struct hostent *hostinfo;
struct in_addr *addr,address;
if(argc==1)
{
address.s_addr= inet_addr("127.0.0.1");
addr=&address;
}
else
{
address.s_addr= inet_addr(argv[1]);
addr=&address;
}
hostinfo = gethostbyaddr((char *)addr,sizeof(addr),AF_INET);
if(!hostinfo)
{
fprintf(stderr, "Invalid IP address!\n");
exit(1);
}
else
printf("Name: %s\n", hostinfo ->h_name);
exit(0);
}
Note: This example code is written by myself, and have been tested on Redhat Linux system. While I can not guarantee it works on other Linux operating system.
