Thread: gethostbyname allways NULL

I'm calling gethostbyname() giving as parameter an MX record but it is allways NULL...

The code below, given a mail, checks the MX record of that provider and tryes to retrieve the IP address... but it fails while calling gethostbyname()

Code:

int getMX(char* email) {
    char* dominio;
    struct hostent *host;
    
	DNS_RECORD* pRecordList = NULL;
	DNS_RECORD* pRecord     = NULL;

    dominio = strtok (email, "@");
    dominio = strtok (NULL, "@");
 
	/* Send the DNS query... */
	if (NOERROR == DnsQuery_A(dominio,
                               DNS_TYPE_MX,
                               DNS_QUERY_STANDARD,
                               NULL,
                               &pRecordList,
                               NULL)) {
     	// get the first MX record
	    for (pRecord = pRecordList; pRecord != NULL; pRecord = pRecord->pNext) {
		    if (pRecord->wType == DNS_TYPE_MX) {
			    printf("-> MX Server:  %s\n", (char*)pRecord->Data.MX.pNameExchange);
			    // try to resolve it
			    if ((host = gethostbyname((char*)pRecord->Data.MX.pNameExchange)) != NULL){
			              printf("   Host: %s\n", host->h_name);
                } else printf("Unable to resolve the IP.\n");
		    } else printf("-> MX Server NOT FOUND\n");
      	    break;
        }
	DnsRecordListFree(pRecordList, DnsFreeRecordList);
	return 0;
    } else { printf("-> Unreachable.\n");
             return -1;
      }
}

the output is:

Mail: [email protected]
-> MX Server: gsmtp185.google.com
Unable to resolve the IP.

Start by determining what WSAGetLastError() returns after gethostbyname(). My call to gethostbyname() succeeds resolving to 64.233.185.27. Are you calling WSAStartup()?

oh great, I was missing the initialization step... now it works but I'm unable to retrieve the IP because host->h_name will give me the hostname... how can i retrieve it?

I'm confused about the Null-terminated list of addresses for the host in the http://msdn.microsoft.com/library/de.../hostent_2.asp struct. Is it here?

Download my multithreaded socket library on my library page (see sig for link) and take a look at the ResolveToIP() function in mtsocket.cpp for an example. You can also browse the code for other concepts regarding socket handling.

Thanx for ur code
I'm also trying to use an A query to retrieve the IP from the MX responce:

Code:

// A
DNS_RECORD* pRecordList2 = NULL;
DNS_RECORD* pRecord2     = NULL;

...
...

// resolve the IP
                if (NOERROR == DnsQuery_A((char*)pRecord->Data.MX.pNameExchange,
                                          DNS_TYPE_A,
                                          DNS_QUERY_STANDARD,
                                          NULL,
                                          &pRecordList2,
                                          NULL)) {
                  for (pRecord2 = pRecordList2; pRecord2 != NULL; pRecord2 = pRecord2->pNext) {
                      if (pRecord2->wType == DNS_TYPE_A) {
                          printf("-> IP:  %s\n", (char*)pRecord2->Data.A.IpAddress);
                      } else printf ("Unable to resolve the IP.\n");
                      
                      break;
                  }
                
                } else printf("Error, query A.\n");

but it crashes while trying to printf the IP.. how can I print an IPV4_ADDRESS ?

perfect, it works

ps: I'm not looking for an A record into an MX lookup in the additional info.. I'm getting the MX hostname from a domain and sending another dns packet containing an A request to the MX hostname, for a total of 2 packets.