dns: add resolvePtr to query plain DNS PTR records

Resolving plain PTR records is used beyond reverse DNS, most
prominently with DNS-SD (RFC6763). This adds dns.resolvePtr(),
and uses it (instead of dns.reverse()) in dns.resolve().

PR-URL: nodejs#4921
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Brian White <mscdex@mscdex.net>

doc/api/dns.markdown

@@ -155,7 +155,7 @@ Valid values for `rrtype` are:
  * `'MX'` - mail exchange records
  * `'TXT'` - text records
  * `'SRV'` - SRV records
- * `'PTR'` - used for reverse IP lookups
+ * `'PTR'` - PTR records
  * `'NS'` - name server records
  * `'CNAME'` - canonical name records
  * `'SOA'` - start of authority record
@@ -248,6 +248,12 @@ be an array of objects with the following properties:
 }
 ```
 
+## dns.resolvePtr(hostname, callback)
+
+Uses the DNS protocol to resolve pointer records (`PTR` records) for the
+`hostname`. The `addresses` argument passed to the `callback` function will
+be an array of strings containing the reply records.
+
 ## dns.resolveTxt(hostname, callback)
 
 Uses the DNS protocol to resolve text queries (`TXT` records) for the

lib/dns.js

@@ -251,9 +251,10 @@ exports.resolveMx = resolveMap.MX = resolver('queryMx');
 exports.resolveNs = resolveMap.NS = resolver('queryNs');
 exports.resolveTxt = resolveMap.TXT = resolver('queryTxt');
 exports.resolveSrv = resolveMap.SRV = resolver('querySrv');
+exports.resolvePtr = resolveMap.PTR = resolver('queryPtr');
 exports.resolveNaptr = resolveMap.NAPTR = resolver('queryNaptr');
 exports.resolveSoa = resolveMap.SOA = resolver('querySoa');
-exports.reverse = resolveMap.PTR = resolver('getHostByAddr');
+exports.reverse = resolver('getHostByAddr');
 
 
 exports.resolve = function(hostname, type_, callback_) {

src/cares_wrap.cc

@@ -698,6 +698,49 @@ class QuerySrvWrap: public QueryWrap {
   }
 };
 
+class QueryPtrWrap: public QueryWrap {
+ public:
+  explicit QueryPtrWrap(Environment* env, Local<Object> req_wrap_obj)
+      : QueryWrap(env, req_wrap_obj) {
+  }
+
+  int Send(const char* name) override {
+    ares_query(env()->cares_channel(),
+               name,
+               ns_c_in,
+               ns_t_ptr,
+               Callback,
+               GetQueryArg());
+    return 0;
+  }
+
+  size_t self_size() const override { return sizeof(*this); }
+
+ protected:
+  void Parse(unsigned char* buf, int len) override {
+    HandleScope handle_scope(env()->isolate());
+    Context::Scope context_scope(env()->context());
+
+    struct hostent* host;
+
+    int status = ares_parse_ptr_reply(buf, len, NULL, 0, AF_INET, &host);
+    if (status != ARES_SUCCESS) {
+      ParseError(status);
+      return;
+    }
+
+    Local<Array> aliases = Array::New(env()->isolate());
+
+    for (uint32_t i = 0; host->h_aliases[i] != NULL; i++) {
+      aliases->Set(i, OneByteString(env()->isolate(), host->h_aliases[i]));
+    }
+
+    ares_free_hostent(host);
+
+    this->CallOnComplete(aliases);
+  }
+};
+
 class QueryNaptrWrap: public QueryWrap {
  public:
   explicit QueryNaptrWrap(Environment* env, Local<Object> req_wrap_obj)
@@ -1276,6 +1319,7 @@ static void Initialize(Local<Object> target,
   env->SetMethod(target, "queryNs", Query<QueryNsWrap>);
   env->SetMethod(target, "queryTxt", Query<QueryTxtWrap>);
   env->SetMethod(target, "querySrv", Query<QuerySrvWrap>);
+  env->SetMethod(target, "queryPtr", Query<QueryPtrWrap>);
   env->SetMethod(target, "queryNaptr", Query<QueryNaptrWrap>);
   env->SetMethod(target, "querySoa", Query<QuerySoaWrap>);
   env->SetMethod(target, "getHostByAddr", Query<GetHostByAddrWrap>);

test/internet/test-dns.js

@@ -166,6 +166,37 @@ TEST(function test_resolveSrv_failure(done) {
   checkWrap(req);
 });
 
+TEST(function test_resolvePtr(done) {
+  var req = dns.resolvePtr('8.8.8.8.in-addr.arpa', function(err, result) {
+    if (err) throw err;
+
+    assert.ok(result.length > 0);
+
+    for (var i = 0; i < result.length; i++) {
+      var item = result[i];
+      assert.ok(item);
+      assert.ok(typeof item === 'string');
+    }
+
+    done();
+  });
+
+  checkWrap(req);
+});
+
+TEST(function test_resolvePtr_failure(done) {
+  var req = dns.resolvePtr('something.invalid', function(err, result) {
+    assert.ok(err instanceof Error);
+    assert.strictEqual(err.errno, 'ENOTFOUND');
+
+    assert.ok(result == undefined);
+
+    done();
+  });
+
+  checkWrap(req);
+});
+
 TEST(function test_resolveNaptr(done) {
   var req = dns.resolveNaptr('sip2sip.info', function(err, result) {
     if (err) throw err;

test/parallel/test-c-ares.js

@@ -31,7 +31,7 @@ assert.throws(function() {
 // C:\Windows\System32\drivers\etc\hosts
 // so we disable this test on Windows.
 if (!common.isWindows) {
-  dns.resolve('127.0.0.1', 'PTR', function(error, domains) {
+  dns.reverse('127.0.0.1', function(error, domains) {
     if (error) throw error;
     assert.ok(Array.isArray(domains));
   });

test/parallel/test-dns-cares-domains.js

@@ -12,6 +12,7 @@ var methods = [
   'resolveNs',
   'resolveTxt',
   'resolveSrv',
+  'resolvePtr',
   'resolveNaptr',
   'resolveSoa'
 ];