Get all IP-Addresses in C

Question

I have the following problem. I have to write a Plugin for Pidgin in the Language C. I am completely new to C. I found the following Code.

  WORD wVersionRequested;
  WSADATA wsaData;
  char name[255];
  char* ip;
  PHOSTENT hostinfo;
  wVersionRequested = MAKEWORD( 2, 0 );

  if ( WSAStartup( wVersionRequested, &wsaData ) == 0 )  {
        if( gethostname ( name, sizeof(name)) == 0) {
              if((hostinfo = gethostbyname(name)) != NULL) {
                    ip = inet_ntoa (*(struct in_addr *)*hostinfo->h_addr_list);
              }
        }
        WSACleanup( );
  } 

I have the IP-Address 172.28.52.220 But because of my VMWare I also have the IP 10.0.1.3. In my Plugin now the IP 10.0.1.3 is assigned to my variable. i need the IP to find out in which location of my company I am. I need hte 172...

Now I could find in the winsock2.h that *hostinfo->h_addr_list contains the list of Ip addresses. How can I assign the 172. Address to my IP_Variable?

Thank you in advance for your help!

---

Edit: Just to clarify: I don´t want to have my external IP address. I need my internals.

Answer 1

Here is an example I tested on on linux. I dont have access to a Windows system until tomorrow, but can test and update the answer if required.

It is comparable to the Windows version only without the WSAStartup call at the beginning.

#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <netdb.h>

int main()
{
  char hostnamebuff[100];

  if(gethostname(hostnamebuff, 100) == 0)
  {
    struct hostent* hostinfo = gethostbyname(hostnamebuff);
    printf("host name is %s\n", hostnamebuff);

    if(hostinfo != NULL)
    {
      char** paddrlist = hostinfo->h_addr_list;

      printf("host list is\n");      
      while(*paddrlist != NULL)
      {
         char addrbuff[INET6_ADDRSTRLEN];
         if(inet_ntop(hostinfo->h_addrtype, *paddrlist, addrbuff, hostinfo->h_addrtype == AF_INET ? INET_ADDRSTRLEN : INET6_ADDRSTRLEN))
         {
           printf("%s\n", addrbuff);

           if(strncmp(addrbuff, "172.", 4) == 0)
           {
             printf("its a match\n");
             break;
           }
         } else
         {
           printf("failed to convert an address\n");
         }
         paddrlist++;
      }
    } else
    {
      printf("failed on gethostbyname\n");
    }
  } else
  {
    printf("failed on gethostname errno=%d\n", errno);
  }
}

The h_addr_list member of hostent is a NULL terminated array of pointers to char* (so its double pointer). My example shows how to traverse this. Hope it helps.

As for this distinctly smelly bit of code

ip = inet_ntoa (*(struct in_addr *)*hostinfo->h_addr_list);

This is a highly unreadable way of getting the first entry in the address list.