net: emit dns 'lookup' event before connect

net.connect() and net.createConnection() now emit a 'lookup' event
after resolving the hostname but before connecting.

Fixes #5418.

doc/api/net.markdown

@@ -450,6 +450,15 @@ The amount of bytes sent.
 
 `net.Socket` instances are [EventEmitter][] with the following events:
 
+### Event: 'lookup'
+
+Emitted after resolving the hostname but before connecting.
+Not applicable to UNIX sockets.
+
+* `err` {Error | Null} The error object.  See [dns.lookup()][].
+* `address` {String} The IP address.
+* `family` {String | Null} The address type.  See [dns.lookup()][].
+
 ### Event: 'connect'
 
 Emitted when a socket connection is successfully established.

lib/net.js

@@ -860,6 +860,8 @@ Socket.prototype.connect = function(options, cb) {
     var host = options.host;
     debug('connect: find host ' + host);
     require('dns').lookup(host, function(err, ip, addressType) {
+      self.emit('lookup', err, ip, addressType);
+
       // It's possible we were destroyed while looking this up.
       // XXX it would be great if we could cancel the promise returned by
       // the look up.

test/simple/test-net-dns-error.js

@@ -43,6 +43,12 @@ socket.on('error', function(err) {
   assert.equal(err.code, 'ENOTFOUND');
   actual_bad_connections++;
 });
+socket.on('lookup', function(err, ip, type) {
+  assert(err instanceof Error);
+  assert.equal(err.code, 'ENOTFOUND');
+  assert.equal(ip, undefined);
+  assert.equal(type, undefined);
+});
 
 process.on('exit', function() {
   assert.equal(actual_bad_connections, expected_bad_connections);

test/simple/test-net-dns-lookup.js

@@ -0,0 +1,43 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+var net = require('net');
+var ok = false;
+
+var server = net.createServer(function(client) {
+  client.end();
+  server.close();
+});
+
+server.listen(common.PORT, '127.0.0.1', function() {
+  net.connect(common.PORT, '127.0.0.1').on('lookup', function(err, ip, type) {
+    assert.equal(err, null);
+    assert.equal(ip, '127.0.0.1');
+    assert.equal(type, '4');
+    ok = true;
+  });
+});
+
+process.on('exit', function() {
+  assert(ok);
+});