Is there a way to make separate DNS requests for IPv4 and IPv6 in swift or ObjC

Question

What I have so far is:

void startQueryIPv4(const char *hostName){
printf("startQueryIPv4");
DNSServiceRef serviceRef;
DNSServiceGetAddrInfo(&serviceRef, kDNSServiceFlagsForceMulticast, 0, kDNSServiceProtocol_IPv4, hostName, queryIPv4Callback, NULL);
DNSServiceProcessResult(serviceRef);
DNSServiceRefDeallocate(serviceRef);
}


static void queryIPv4Callback(DNSServiceRef sdRef, DNSServiceFlags flags, uint32_t interfaceIndex, DNSServiceErrorType errorCode, const char *hostname, const struct sockaddr *address, uint32_t ttl, void *context){
printf("queryIPv4Callback");

if (errorCode == kDNSServiceErr_NoError) {

    printf("no error");
    char *theAddress = NULL;

    switch(address->sa_family) {
        case AF_INET: {
            struct sockaddr_in *addr_in = (struct sockaddr_in *)address;
            theAddress = malloc(INET_ADDRSTRLEN);
            inet_ntop(AF_INET, &(addr_in->sin_addr), theAddress, INET_ADDRSTRLEN);
            break;
        }
        case AF_INET6: {
            struct sockaddr_in6 *addr_in6 = (struct sockaddr_in6 *)address;
            theAddress = malloc(INET6_ADDRSTRLEN);
            inet_ntop(AF_INET6, &(addr_in6->sin6_addr), theAddress, INET6_ADDRSTRLEN);
            break;
        }
        default:
            break;
    }
    printf("IP address: %s\n", theAddress);
    free(theAddress);

} else {
    printf("%d", errorCode);
}

}

But the callback is never called. In the console I get this error: TIC Read Status [9:0x0]: 1:57 ObjectiveC is not my power but I had to mess with it. Any help will be appreciated.

Answer 1

Inasmuch as Objective-C is a true superset of C and Darwin is certified compatible with SUS 3, your Objective-C program for iOS should be able to use the C interface to the system's name resolver: getaddrinfo(). You can use the third argument to this function to specify that you want only IPv4 results (or only IPv6 results).

Things of which you should be aware:

• this is of course a synchronous interface; if you want asynchronous then you'll need to arrange for that yourself.

• getaddrinfo() allocates and returns a linked list of addresses, so

  • in principle, you might need to check more than one

  • you need to free the list after you're done with it via freeaddrinfo()

Answer 2

The reason you aren't getting a callback is that you aren't using a dispatch queue. The DNSServiceGetAddrInfo API is asynchronous. If you are doing this on an Apple device, you want something more like this:

void startQueryIPv4(const char *hostName) {
  printf("startQueryIPv4");
  DNSServiceRef serviceRef;
  DNSServiceGetAddrInfo(&serviceRef, kDNSServiceFlagsForceMulticast, 0, kDNSServiceProtocol_IPv4, hostName, queryIPv4Callback, NULL);
  main_queue = dispatch_get_main_queue();
  DNSServiceSetDispatchQueue(sdref, main_queue);
  dispatch_main();
}

Note that dispatch_main() is the main event loop for libdispatch: if you want to do other stuff, you need to schedule it in the dispatch loop, because dispatch_main() will not return.

In your original code you called DNSServiceRefDeallocate(), but you can't do that until you want to stop the query. If you call it right after you start the query, it will cancel the query. So e.g. you could call it from the callback.

However, a better flow would be to do a long-lived query (kDNSServiceFlagsLongLivedQuery) so that you get an update whenever the information changes. Of course you'd then need to change the host you're connecting to, so only do this if your application will be connected for an extended period.

Additionally, you may get more than one answer. If you do, it may be that some answers work to connect, and others don't. So you might like to accumulate answers and try each one, rather than giving up if the first answer you get doesn't work. The callback will include the kDNSServiceFlagsMoreComing flag if there is more data coming immediately. Each time the callback is called, it will get one answer (or an indication that the query has failed in some way).

Of course, this is a fairly low-level API. If you want to make your life a bit easier, you should use Network Framework. Network Framework does the "happy eyeballs" part for you—trying each response until it gets a connection, and returning you the connection it gets, canceling the others.

But you didn't ask about that, so I won't go into details here.