hostlab.c

find the information about a host with the DNS retrieving system calls, such as gethostbyname() and

#include <stdio.h>  /*for printf() and fprintf() */
#include <arpa/inet.h> /*for inet_addr() */
#include <stdlib.h>  /*for exit() */
#include <netdb.h>  /*for gethostbyname() and gethostbyaddr() */

int main(int argc, char *argv[] ) {
         
     /*convert IP address in numbers-and-dots notation into unsigned long in network byte order*/
   if ( inet_addr(argv[1]) != INADDR_NONE ) {
                
                char **ptr; // pointer to store the host aliases
                int i ;   //variable used in the for loop
                struct in_addr addr;
                char ip[INET_ADDRSTRLEN];
                struct hostent *host;
                 
                    
                 addr.s_addr = inet_addr(argv[1]);  

         
                 /* retrive host entry from DNS and query key is an IP address */
                 host = gethostbyaddr( (const char*)&addr, sizeof(ip),AF_INET);//cast to generic address
              
                 if ( host == NULL ) {
                       printf("failed\n");
                       exit(1);
                    }

                 printf("h_name\t = %s\n", host->h_name);
                 printf("h_addrtype\t=%d\n",host->h_addrtype);
                 printf("h_length\t=%d \n",host->h_length);
   
                 ptr = host->h_aliases;

                  while(*ptr)  {
                        printf("host alias\t =%s\n",*ptr);
                        ptr++;
                    }

                  for ( i=0; host->h_addr_list[i] != NULL; i++ ) {

                           //mapping the IP address in network byte order to dotted decimal format
                           if ( inet_ntop(AF_INET,host->h_addr_list[i],ip,sizeof(ip) ) != NULL)
                                 printf("IP address: %s\n",ip);
                           else
                                 printf("error\n");
                     }
                            return 0;
                 }
   // retrieve host entry from DNS and query key is domain name        
   else {
                             char **ptr;
                             int i;
                             char ip[INET_ADDRSTRLEN];
                             struct hostent *host;
                           
                             host = gethostbyname(argv[1]);
                             if ( host== NULL) 
                                {
                                     printf("failed\n");
                                     exit(1);
                                }

                              printf("h_name\t = %s\n", host->h_name);
                              printf("h_addrtype\t= %d\n",host->h_addrtype);
                              printf("h_length\t= %d\n",host->h_length);
                              
                              ptr = host->h_aliases;
                         
                                  while(*ptr) {
                                        printf("host alias\t=%s\n", *ptr);
                                          ptr++;
                                     }  // end while
                       
                            for ( i=0; host->h_addr_list[i]!= NULL; i++ )  {
                                    if(inet_ntop(AF_INET, host->h_addr_list[i],ip,sizeof(ip) ) != NULL )
                                             printf("IP  address: %s\n",ip);
                                    else
                                        printf("error");
                             }  // end for
                               return 0;
             } // end else
                   return 0;
       }  // end main