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:

Quote:

---

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? :confused:

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 ?

You can use the inet_ntoa function to convert an IPv4 address into a string. This function takes a struct in_addr so we will need to create one:

Code:

---

                if (pRecord->wType == DNS_TYPE_A)
                {
                        struct in_addr ipAddr;
                        ipAddr.S_un.S_addr = pRecord2->Data.A.IpAddress;

                        printf("  IP Address: %s\n", inet_ntoa(ipAddr));
                }

---

There is no guarantee that there will be any A records in an MX lookup.

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.