[dgram] only look up hostname for the requested address family

- [lib/dns.js] dns.lookup takes a new optional argument "family" which
  should be the integer 4, 6, dns.AF_INET or dns.AF_INET6. Passing a
  non-false "family" argument makes c-ares explicitly look up addresses for
  the specified family.

- [test/simple/test-c-ares.js] test explicit address family lookups

1 files changed, 73 insertions, 26 deletions

diff --git a/lib/dns.js b/lib/dns.js
index afd5cb1fcc5..84d5d3b5210 100644
--- a/lib/dns.js
+++ b/lib/dns.js
@@ -100,46 +100,93 @@ exports.getHostByName = function (domain, callback) {
 var net;
 
 // Easy DNS A/AAAA look up
-exports.lookup = function (domain, callback) {
-  var addressType = dns.isIP(domain);
-  if (addressType) {
-    process.nextTick(function () {
-      callback(null, domain, addressType);
-    });
+// lookup(domain, [family,] callback)
+exports.lookup = function (domain, family, callback) {
+  if (arguments.length === 2) {
+    callback = family;
+    family = undefined;
+  } else if (family && family !== 4 && family !== 6) {
+    family = parseInt(family);
+    if (family === dns.AF_INET) {
+      family = 4;
+    } else if (family === dns.AF_INET6) {
+      family = 6;
+    } else if (family !== 4 && family !== 6) {
+      throw new Error('invalid argument: "family" must be 4 or 6');
+    }
+  }
+  if (!domain) {
+    callback(null, null, family === 6 ? 6 : 4);
+    return;
+  }
+  var matchedFamily = dns.isIP(domain);
+  if (matchedFamily) {
+    callback(null, domain, matchedFamily);
   } else {
-    if (/\w\.local\.?$/.test(domain) ) {
+    if (/\w\.local\.?$/.test(domain)) {
       // ANNOYING: In the case of mDNS domains use NSS in the thread pool.
       // I wish c-ares had better support.
       process.binding('net').getaddrinfo(domain, 4, function (err, domains4) {
         callback(err, domains4[0], 4);
       });
     } else {
-      channel.getHostByName(domain, dns.AF_INET, function (err, domains4) {
-        if (domains4 && domains4.length) {
-          callback(null, domains4[0], 4);
-        } else {
-          channel.getHostByName(domain, dns.AF_INET6, function (err, domains6) {
-            if (domains6 && domains6.length) {
-              callback(null, domains6[0], 6);
+      if (family) {
+        // resolve names for explicit address family
+        var af = (family === 4) ? dns.AF_INET : dns.AF_INET6;
+        channel.getHostByName(domain, af, function (err, domains) {
+          if (!err && domains && domains.length) {
+            if (family !== dns.isIP(domains[0])) {
+              callback(new Error('not found'), []);
             } else {
-              callback(err, []);
+              callback(null, domains[0], family);
             }
-          });
-        }
-      });
+          } else {
+            callback(err, []);
+          }
+        });
+      } else {
+        // first resolve names for v4 and if that fails, try v6
+        channel.getHostByName(domain, dns.AF_INET, function (err, domains4) {
+          if (domains4 && domains4.length) {
+            callback(null, domains4[0], 4);
+          } else {
+            channel.getHostByName(domain, dns.AF_INET6,
+                                  function (err, domains6) {
+              if (domains6 && domains6.length) {
+                callback(null, domains6[0], 6);
+              } else {
+                callback(err, []);
+              }
+            });
+          }
+        });
+      }
     }
   }
 };
 
 
-exports.resolve4    = function(domain, callback) { channel.query(domain, dns.A, callback) };
-exports.resolve6    = function(domain, callback) { channel.query(domain, dns.AAAA, callback) };
-exports.resolveMx   = function(domain, callback) { channel.query(domain, dns.MX, callback) };
-exports.resolveTxt  = function(domain, callback) { channel.query(domain, dns.TXT, callback) };
-exports.resolveSrv  = function(domain, callback) { channel.query(domain, dns.SRV, callback) };
-exports.reverse     = function(domain, callback) { channel.query(domain, dns.PTR, callback) };
-exports.resolveNs   = function(domain, callback) { channel.query(domain, dns.NS, callback) };
-
+exports.resolve4    = function(domain, callback) {
+  channel.query(domain, dns.A, callback);
+};
+exports.resolve6    = function(domain, callback) {
+  channel.query(domain, dns.AAAA, callback);
+};
+exports.resolveMx   = function(domain, callback) {
+  channel.query(domain, dns.MX, callback);
+};
+exports.resolveTxt  = function(domain, callback) {
+  channel.query(domain, dns.TXT, callback);
+};
+exports.resolveSrv  = function(domain, callback) {
+  channel.query(domain, dns.SRV, callback);
+};
+exports.reverse     = function(domain, callback) {
+  channel.query(domain, dns.PTR, callback);
+};
+exports.resolveNs   = function(domain, callback) {
+  channel.query(domain, dns.NS, callback);
+};
 
 var resolveMap = {
   'A'   : exports.resolve4,