Note: this page is part of the 'hello-dns' documentation effort.

teaching DNS: C library

Based on tdns, a C++ project, a C API is also available.

This is meant as an easy gateway for C users. The tdns C-API is aimed to resolve simple queries, without having to import all the glory that is getdns. For any advanced work, including asynchronous queries, encryption and cryptography, please use getdns.

Basics

To start, initialize a TDNSContext object like this:

  struct TDNSContext* tdns = TDNSMakeContext("");
  if(!tdns) {
    fprintf(stderr, "Unable to initialize tdns\n");
    return EXIT_FAILURE;
  }

This will lift resolver addresses from the system default. To use a specific resolver, pass its address to TDNSMakeContext.

A context needs to be freed by calling freeTDNSContext.

Queries

Once a context is acquired, it can be used to perform queries:

  struct TDNSIPAddresses* ips;
  err = TDNSLookupIPs(tdns, "www.dns-oarc.net", 1000, 1, 1, &ips);
  if(err) {
    fprintf(stderr, "Error looking up domain name: %s\n", TDNSErrorMessage(err));
    return EXIT_FAILURE;
  }

This call specifies a timeout of 1000 milliseconds, and requests IPv4 and IPv6 addresses.

An error is indicated by a non-zero return value, in which case the error is available through TDNSErrorMessage.

Actual IP addresses, IPv4 or IPv6, are returned as struct sockaddr_storage pointers in TDNSIpAddressess::addresses:

  for(int n = 0; ips->addresses[n]; ++n) {
    struct sockaddr_storage* res = ips->addresses[n];
    char ip[INET6_ADDRSTRLEN];
    if(res->ss_family == AF_INET)
      inet_ntop(res->ss_family, &((struct sockaddr_in*)res)->sin_addr, ip, INET6_ADDRSTRLEN);
    else
      inet_ntop(res->ss_family, &((struct sockaddr_in6*)res)->sin6_addr, ip, INET6_ADDRSTRLEN);
    printf("IP address: %s\n", ip);
  }
  freeTDNSIPAddresses(ips);

The sequence is terminated by a 0 pointer in addressses. Note that the answer must also be freed with freeTDNSIPAddresses.

MX Records

Looking up MX records proceeds among similar lines:

  struct TDNSMXs* mxs;
  err = TDNSLookupMXs(tdns, "isc.org", 1000, &mxs);

  (...)

  for(int n = 0; mxs->mxs[n]; ++n) {
    struct TDNSMX* res = mxs->mxs[n];
    printf("MX %d %s\n", res->priority, res->name);
  }
  freeTDNSMXs(mxs);

TXT Records

And TXT records:

  struct TDNSTXTs* txts;
  err = TDNSLookupTXTs(tdns, "nl", 1000, &txts);

  (...)

  for(int n = 0; txts->txts[n]; ++n) {
    struct TDNSTXT* res = txts->txts[n];
    printf("TXT %s\n", res->content);
  }
  freeTDNSTXTs(txts);

Full code

The full code of these examples can be found on GitHub.