Jonathan Saggau's Blog

#define T_PTR		12		/* domain name pointer */
#define C_IN		1		/* the arpa internet */
static const int kBufLen = 1500;

//takes a string like 1.1.168.192.in-addr.arpa and turns it into the hostname
char *_reverseDns(const char* arpaPoint)
{
    int len = -1;
    char buffer[kBufLen];
    res_init();
    
    int queryType = T_PTR; /* domain name pointer */ 
    int arpanet = C_IN; /* the arpa internet = 1*/
    len = res_query(arpaPoint, arpanet, queryType, (u_char *)buffer, kBufLen);
    char *resStr;
    dns_reply_t *reply;
    if (len > 0)
    {
        reply = dns_parse_packet(buffer, len);
        resStr = ((*(reply->answer))->data).PTR->name;
    }
    else
    {
        if (errno)
            fprintf(stderr, "Error, res_query() error value: %d\n", errno);
        if (61 == errno) 
            resStr = "Could not connect to DNS";
        else
            resStr = "DNS entry Not Found";
    }    
    return resStr;
}

//makes 192.168.1.1 into 1.1.168.192.in-addr.arpa
NSString *makeDNSLookupString(NSString *inString)
{
    NSString *eachString = @"";
    NSScanner *scanner = [NSScanner scannerWithString:inString];
    NSUInteger scanLocation = 0;
    NSUInteger stringLen = [inString length];
    NSString *outStr = @"";
    while([scanner scanUpToString:@"." intoString:&eachString])
    {
        eachString = [eachString stringByAppendingString:@"."];
        outStr = [eachString stringByAppendingString:outStr];
        scanLocation = [scanner scanLocation];
        if (scanLocation < stringLen)
            [scanner setScanLocation:scanLocation + 1]; // hop the . character
    }
    outStr = [outStr stringByAppendingString:@"in-addr.arpa"];
    return outStr;
}

NSString *reverseDNS(NSString *ipAddress)
{
    NSString *arpaString = makeDNSLookupString(ipAddress); 
    //NSLog(@"arpaString = %@", arpaString);
    NSString *returnString = [NSString stringWithCString:_reverseDns([arpaString cStringUsingEncoding:NSASCIIStringEncoding]) encoding:NSASCIIStringEncoding];
    return returnString;
}

You can find a simple example app that uses this here.



It also uses a nice trick from The iPhone Developer's Cookbook. It's MIT licensed, so feel free to use it in your own app. Consider this a workaround until apple comes up with something simpler. This works for me installed on the iphone itself looking up addresses on the internet and on my local network (it loses the .local, but is otherwise aok). It doesn't validate the ip address string you give it or check to see if DNS is even available, so you'll want to wrap it in some reachability and validation calls, etc (which I did not do in the demo app, but it fails fairly gracefully when you give it garbage input).

Running reverseDNS(@"192.168.1.139") (which is the IP address of my imac) on the iPhone connected via wi-fi to my local network returns @"FreakinIMac" (which is the host name of my imac). It even returns the DD-WRT hostname of my router. It returns @"DNS entry Not Found if it can't find an entry or errs out and it will log errno if set by res_query to the console. You'll probably want to wrap some error handling code around this if you plan to use it in a real app.

Other useful stuff: You can find the source for libresolv here, which was quite helpful. The source to dns.c and dns_util.c proved useful as well. A post on an apple mailing list also pointed me in a direction. Having the source to the libraries you use is a great help. Thanks to Apple for making these available; keep 'em coming!

Note: If you want to log the whole parsed reply from the DNS server, use dns_print_reply (also in dns_utils) with DNS_PRINT_ANSWER as an arg. There is probably a lot more you can do with this method of DNS lookup. I'm guessing you can send pretty much any kind of DNS query and parse the result, just watch out for (and apply his genius patch, if necessary) that bug that Saurik found.

Note: Watching network traffic with a packet sniffer like Wireshark is quite informative.