CODEWITHSUNDEEP
c++linux
user2950356
user2950356

Reputation:

gethostbyaddr() returns null

I know this question was asked here before and I read all the answers, but wasn't able to resolve this on my own. Here's the deal: I have to make a program which takes a bunch of internet addresses (like google.com, www.root.cz) and prints out an official name, alternative names and IP adresses and domain names of aliases. We've got a sample code for 1 address without the aliases domain name, so I used it, then made a few changes to fit my needs. The code looks like this:

#include <iostream>
#include <netdb.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/socket.h>

int main (int argc, char *argv[])
{

for (int i = 1; i < argc; ++i){
    hostent *H = gethostbyname(argv[i]);
    if (H == NULL)
    {
        std::cerr << "Nepodařilo se zjistit adresu" << std::endl;
            return -1;
    }
    std::cout << "Oficiální jméno: " << H->h_name << std::endl;
    std::cout << "Alternativy: " << std::endl;
    /* Budeme procházet pole H->h_aliases. Pole je ukončené NULL.*/
        char **alternativy = H->h_aliases;
    while(*alternativy != NULL)
{
    std::cout << "\t" << *(alternativy++) << std::endl;
}
/* Budeme procházet pole H->h_addr_list. Pole je ukončené NULL. 
       Zvolíme jiný postup než v předchozím případě.*/
std::cout << "Adresy: " << std::endl;
    for(int i = 0; H->h_addr_list[i] != NULL; ++i)
    {
        /* H->h_addr_list je binární forma IP adresy. Pro převod na
           tečkovou konvenci použijeme inet_ntoa.*/
const char* addr = inet_ntoa(*(in_addr *)H->h_addr_list[i]);
hostent *name = gethostbyaddr(addr, sizeof(addr), AF_INET);
std::cout<<addr<<"->"<<name<<std::endl;
}
std::cout<<std::endl;
}
    return 0;
}

Everything works fine. Only name is always null. I tried running this on ubuntu 12.04 I have on my laptop and on virtual Debian Squeeze(don't remember number). I checked some of the config files as stated in previous questions. Also tried configuring more DNS servers. Nothing works. I also tried using getnameinfo, but for some reason it always returned only given IP address. Any help will be greatly appreciated.

Upvotes: 0

Views: 1194

Answers (2)

Jeribo
Jeribo

Reputation: 465

You are passing wrong format of IP that you want to translate back to hostname.

Change your for like this. 

for(int i = 0; H->h_addr_list[i] != NULL; ++i)
{
    /* H->h_addr_list je binární forma IP adresy. Pro převod na tečkovou konvenci použijeme inet_ntoa.*/
    const char* addr = inet_ntoa(*(in_addr *)H->h_addr_list[i]);
    hostent *name = gethostbyaddr((in_addr *)H->h_addr_list[i], sizeof(in_addr), AF_INET);
    std::cout<<addr<<"->"<<name->h_name<<std::endl;
}

also look for getaddrinfo() because gethotbyname(),gethostbyaddr() are obsolete functions

Upvotes: 1

akrog
akrog

Reputation: 326

There is an error calling gethostbyaddr() with incorrect parameters (it expects a binary address, not a string), and another error displaying the returned name (which is a field of hostent). I have tried the following and seems to work fine:

hostent *name = gethostbyaddr(H->h_addr_list[i], sizeof (struct in_addr), AF_INET);
std::cout << addr << "->" << name->h_name << std::endl;

Upvotes: 0

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