Using the gethostbyname command

mdlister · 03-08-2005, 08:54 AM

Hi, i have been trying to create a program that will connect to a server using its DNS name and have found out that i need to use the gethostbyname function. I also require it to read off a txt file that is stored there without the http headers and have not got a clue how to do it. When i compile my code using -gcc -Wall -ansi -pedantic -c client.c i get a load of errors. Would anyone be able to set me on the right path to makeing this work? the code that i am using is an example taken from a clint server program that i have tried to change.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

/* Explicit declaration of an imported function. We could use a .h
for this. */
void readloop (int our_socket);

int main
char address*
{
int our_socket;
struct sockaddr_in our_address;
struct hostent* host_info;

/*Resolve the host*/

host_info = gethostbyname("host address");

if h_name == NULL
printf("/nError Connecting To Host");
else
{
printf("/nConnection Made");
}

printf("Client starting...\n");
if ((our_socket = socket(PF_INET, SOCK_STREAM, 0)) > 0)
printf("The socket has been created\n");
else
{
perror("Could not create socket");
exit(1);
}
our_address.sin_family = AF_INET;
if (argc < 2)
{
printf("Need one argument: the numeric IP address of the server!\n");
exit(3);
}
printf("Hopefully, %s is the numeric IP address of the server...\n",
argv[1]);
/* No error checking on the next line -- hence the previous message! */
inet_pton(AF_INET, argv[1], &our_address.sin_addr);
our_address.sin_port = htons(23456);
if (connect(our_socket,
(struct sockaddr *) & our_address,
sizeof(our_address)) == 0)
printf("Connected to host %s\n",
inet_ntoa(our_address.sin_addr));
else
{
perror("Could not connect");
exit(2);
}
/* Insert read text file here */

if (close(our_socket) != 0)
herror("Warning: problem closing our_socket");
printf("Exiting.\n");
exit(EXIT_SUCCESS);
}

Thanx again if any one can help!!!

perfect_circle · 03-08-2005, 09:50 AM

You have many syntax mistakes like not closing }, forgetting to put () in functions. Those are really easy to correct and you should do this by yourself, since you are a C newbie.

A newbie C network programming guide is here: http://www.ecst.csuchico.edu/~beej/guide/net/html/
It contains an example of a simple server client model. Also use spaces and tabs when nesting something .It will be a lot easier to correct syntax mistakes. it's easier to see if a branket is missing, when writing like this:

Code:

if(.....)
{
    if(...)
    {
         .................
     }
     .....................
}

than when writing like this:
if(.....)
{
if(...)
{
.................
}
.....................
}
Use the code tag when u post code, because otherwise the spaces are ignored (default HTML behaviour).

foo_bar_foo · 03-08-2005, 02:12 PM

since this is a daunting task for a beginner i will try to get you started
this will take the host name as an argument from the command line
i will not do any error checking so you can add that in

Code:

int main (int argc, char* const argv[]) {
   int our_socket;
   struct sockaddr_in our_address;
   struct hostent* host_info;
   our_socket = socket(PF_INET, SOCK_STREAM, 0);
   our_address.sin_family = AF_INET;
   host_info = gethostbyname(argv[1]);
   our_address.sin_addr = *((struct in_addr *) host_info->h_addr);
   // web servers generally listen to port 80
   our_address.sin_port = htons(80);
   connect(our_socket,(struct sockaddr *) &our_address, sizeof(our_address));
   // here call a function to get your page
   get_page (our_socket);
    return 0;
}

void get_page (int our_socket) {
    
}

mdlister · 03-12-2005, 06:22 PM

Hey thanx for the help, i'll give that a go in the morning to c how it turns out!

foo_bar_foo · 03-12-2005, 09:50 PM

c_ool,

try to figure out how to get the page on you own
later if you can't, write back and someone can show

one note is that some web servers nowdays requirwe browser verification/authentication or whatever it is before they will serve up stuff --

mdlister · 03-14-2005, 07:50 AM

hey, foo_bar_foo

thanx for the connecting code, i managed to get it to connect to a host defined in the the code now and have it reading a txt file from the webserver. I'll work on the validation now!

Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

int main (int argc, char* const argv[]) {
char buffer[1000];
int our_socket;
struct sockaddr_in our_address;
struct hostent* host_info;
our_socket = socket(PF_INET, SOCK_STREAM, 0);
our_address.sin_family = AF_INET;
host_info = gethostbyname("server_address");
our_address.sin_addr = *((struct in_addr *) host_info->h_addr);
// web servers generally listen to port 80
our_address.sin_port = htons(80);
connect(our_socket,(struct sockaddr *) &our_address, sizeof(our_address));
// here call a function to get your page
write(our_socket, "GET /~pjb/tmp/cnet221file.txt\n", 30);
recv(our_socket, buffer, 1000, 0);
printf("%s", buffer);
return 0;
}

Thanx again