Question

I have a a variable that contains an IP address. I am trying to do an nslookup on that by instead of the DNS name being returned I get 0. I am in a Linux environment. Destination IP comes from a vector (string dest_ip = vector[2]).

#include <stdio.h>
#include <iostream>
#include <string.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <vector>
#include <algorithm>
#include <iterator>
#include <sstream>

using namespace std;

void split(const std::string& str, std::vector<std::string>& v) {
    std::stringstream ss(str);
    ss >> std::noskipws;
    std::string field;
    char ws_delim;
    while(1) {
        if( ss >> field )
            v.push_back(field);
        else if (ss.eof())
            break;
        else
            v.push_back(std::string());
        ss.clear();
        ss >> ws_delim;
    }
}

int main()
{

  string input_line;

  while(cin){

  getline(cin, input_line);

   for(int i=0; input_line[i]; i++)
                      if(input_line[i] == ':') input_line[i] = ' ';
                     for(int i=0; input_line[i]; i++)
                      if(input_line[i] == '/') input_line[i] = ' ';

  std::vector<std::string> v;
  split(input_line, v);

  string dest_ip = v[4];

  struct hostent *he;
  int i,len,type;
  len = dest_ip.length();
  type=AF_INET;

  he = gethostbyaddr(dest_ip.c_str(),len,type);

  cout<<"Hostname: "<<he<<"\n";

  return 0;
}

Again, instead of receiving the host name I get 0.

Answer 1

You cannot pass a c-style-string (ie. null-terminated) directly to gethostbyaddr.

You’ll need to create a struct in_addr and pass a pointer to the created struct as first parameter to gethostbyaddr. To generate a struct in_addr from a char const* use inet_aton.

The below example is taken from man gethostbyaddr:

---

EXAMPLES

• Print out the hostname associated with a specific IP address:

  const char *ipstr = "127.0.0.1";
  struct in_addr ip;
  struct hostent *hp;
  
  if (!inet_aton(ipstr, &ip))
          errx(1, "can't parse IP address %s", ipstr);
  
  if ((hp = gethostbyaddr((const void *)&ip, sizeof ip, AF_INET)) == NULL)
          errx(1, "no name associated with %s", ipstr);
  
   printf("name associated with %s is %s\n", ipstr, hp->h_name);
  

---

How do I do further checks to pin-point what went wrong?

If your use of gethostbyaddr returns NULL you should check what went wrong by looking at the variable h_errno.

h_errno can have one of the below defined values:

1. HOST_NOT_FOUND
2. TRY_AGAIN
3. NO_RECOVERY
4. NO_DATA

Please consult your manual for more details regarding the issue.

---

^{Your snippet is completely wrong..}

^{The snippet provided by you doesn’t even compile, but you are in a way showing what you are trying to accomplish but I cannot know this for certain.
This post contains details that should be considered to be “educated guesses“.}

^{OP changed his post..}