Assigning String to Hostent in C

Question

I am coding something to find web servers. It is in C.

I have this

sprintf(pre_ip, "%d.%d.%d.%d", num1, num2, num3, num4);

num1-4 is an ip like 1.1.1.1. This part works..

he = gethostbyname(pre_ip);

This should assign struct hostent *he; to the ip..

but this doesnt work..

server_info.sin_addr = *((struct in_addr *)he->h_addr);
connect(socket_fd, (struct sockaddr *)&server_info, sizeof(struct sockaddr));

Here is the whole code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
//lol220
int main(int argc, char *argv[])
{
    struct sockaddr_in server_info;
    struct hostent *he;
    int socket_fd,num;
    char buffer[1024];

    char buff[1024];

    if (argc != 1) {
        fprintf(stderr, "Usage: client hostname\n");
        exit(1);
    }

    if ((socket_fd = socket(AF_INET, SOCK_STREAM, 0))== -1) {
        fprintf(stderr, "Socket Failure!!\n");
        exit(1);
    }

    memset(&server_info, 0, sizeof(server_info));
    server_info.sin_family = AF_INET;
    server_info.sin_port = htons(80);

//-------------------------------looooop------------------------
int num1 = 1;
int num2 = 1;
int num3 = 1;
int num4 = 1;
int done = 1;
char ip;
char pre_ip[256];
while(done){
        if(num4 == 256){
        num4 = 1;
        num3++;
        }
        if(num3 == 256){
        num3 = 1;
        num2++;
        }
        if(num2 == 256){
        num2 = 1;
        num1++;
        }
        if(num1 == 255 && num2 == 255 && num3 == 255 && num4 == 255){
        done = 0;
        }
        // MOST LIKELY NON WORKING PART 
        sprintf(pre_ip, "%d.%d.%d.%d", num1, num2, num3, num4);
        printf("%s\n", pre_ip);
        he = gethostbyname(pre_ip);
        server_info.sin_addr = *((struct in_addr *)he->h_addr);
            if (connect(socket_fd, (struct sockaddr *)&server_info, sizeof(struct sockaddr))<0) {
                printf("Could not connect to %s", he);
            }
            else{
                printf("Could connect to %s", he);
            }
        // MOST LIKELY NON WORKING PART 

num4++;
}
return 0;
}

It compiles fine.

prints "1.1.1.1" then it says "Could not connect to 8T1.1.1.2" so that means he == "8T1.1.1.2.

A `hostent*` is not a `char*` so you cannot print the content of a `hostent` using `%s`. — Remy Lebeau, Sep 01 '14 at 03:35

score 0 · Answer 1 · answered Sep 01 '14 at 02:46

0

When you print the ip in this line:

printf("Could not connect to %s", he);

you cast struct hostent to char *, this leads the odd chars before the ip. Actually, it's fine to use the ip to connect the server directly, no need for gethostbyname. If you really want to try this function, just like this:

bzero(&server_info, sizeof(server_info));
const char *ip = inet_ntoa(server_info.sin_addr);
...
printf("Could not connect to %s", ip);

answered Sep 01 '14 at 02:46

jfly

7,715
3
35
65

Or, instead of calling `inet_ntoa()`, just use the `pre_ip` buffer that was already formatted with the IP address beforehand – Remy Lebeau Sep 01 '14 at 03:31
Indeed so. Maybe op just copied some code from somewhere and tried it:) – jfly Sep 01 '14 at 03:44
Please consider [not using `bzero()`](http://stackoverflow.com/a/18331555/2171689). The [`bzero()`](http://pubs.opengroup.org/onlinepubs/009695399/functions/bzero.html) function was deprecated in POSIX.1-2001 and removed in POSIX.1-2008. Use [`memset()`](http://pubs.opengroup.org/onlinepubs/9699919799/functions/memset.html) instead. – This isn't my real name Sep 02 '14 at 18:47

Remy Lebeau · Answer 2 · 2014-09-02T15:04:08.247

Try something more like this instead:

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

int main(int argc, char *argv[])
{
    struct sockaddr_in server_info;
    int socket_fd;

    if (argc != 1) {
        fprintf(stderr, "Usage: client hostname\n");
        exit(1);
    }

    if ((socket_fd = socket(AF_INET, SOCK_STREAM, 0))== -1) {
        fprintf(stderr, "Socket Failure!!\n");
        exit(1);
    } 

    memset(&server_info, 0, sizeof(server_info));
    server_info.sin_family = AF_INET;
    server_info.sin_port = htons(80);

    for (uint32_t ip = 0x01010101; ip != 0xFFFFFFFF; ++ip) {
        server_info.sin_addr.s_addr = htonl(ip);
        printf("Connect to %s: ", inet_ntoa(server_info.sin_addr));
        if (connect(socket_fd, (struct sockaddr *)&server_info, sizeof(server_info)) == -1) {
            printf("Failed\n");
        }
        else {
            printf("Success\n");
            close(socket_fd);
            if ((socket_fd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
                fprintf(stderr, "Socket Failure!!\n");
                exit(1);
            } 
        }
    }
    return 0;
}

Because it was in the original code, so I left it in, assuming the poster will use it later. The only thing I changed was the connect loop. — Remy Lebeau, Sep 02 '14 at 15:03

Assigning String to Hostent in C

2 Answers2