How to get DNS TXT record in iOS app

Here's something that I threw together a bit quickly. I am not very familiar with TXT records, so you can test it with a few scenarios, but this demonstrates the basic concept. You can change it to return TTL values ​​if you need. You want to add -lresolv

to your linker flags and import headers resolv.h

.

#include <resolv.h>

static NSArray *fetchTXTRecords(NSString *domain)
{
    // declare buffers / return array
    NSMutableArray *answers = [NSMutableArray new];
    u_char answer[1024];
    ns_msg msg;
    ns_rr rr;

    // initialize resolver
    res_init();

    // send query. res_query returns the length of the answer, or -1 if the query failed
    int rlen = res_query([domain cStringUsingEncoding:NSUTF8StringEncoding], ns_c_in, ns_t_txt, answer, sizeof(answer));

    if(rlen == -1)
    {
        return nil;
    }

    // parse the entire message
    if(ns_initparse(answer, rlen, &msg) < 0)
    {
        return nil;
    }

    // get the number of messages returned
    int rrmax = rrmax = ns_msg_count(msg, ns_s_an);

    // iterate over each message
    for(int i = 0; i < rrmax; i++)
    {
        // parse the answer section of the message
        if(ns_parserr(&msg, ns_s_an, i, &rr))
        {
            return nil;
        }

        // obtain the record data
        const u_char *rd = ns_rr_rdata(rr);

        // the first byte is the length of the data
        size_t length = rd[0];

        // create and save a string from the C string
        NSString *record = [[NSString alloc] initWithBytes:(rd + 1) length:length encoding:NSUTF8StringEncoding];
        [answers addObject:record];
    }

    return answers;
}

      

        
        
        
      

    

The usage is pretty straight forward:

NSArray *records = fetchTXTRecords(@"google.com");
NSLog(@"%@", records);

// outputs:
// (
//     "v=spf1 include:_spf.google.com ip4:216.73.93.70/31 ip4:216.73.93.72/31 ~all"
// )