dgram: do not emit error for failed DNS lookups in send()

doc/api/dgram.markdown

@@ -114,7 +114,9 @@ assigned a random port number and is bound to the "all interfaces" address
 An optional callback may be specified to detect DNS errors or for determining
 when it's safe to reuse the `buf` object.  Note that DNS lookups delay the time
 to send for at least one tick.  The only way to know for sure that the datagram
-has been sent is by using a callback.
+has been sent is by using a callback. If an error occurs and a callback is given,
+the error will be the first argument to the callback. If a callback is not given,
+the error is emitted as an `'error'` event on the `socket` object.
 
 With consideration for multi-byte characters, `offset` and `length` will
 be calculated with respect to

lib/dgram.js

@@ -299,7 +299,11 @@ Socket.prototype.send = function(buffer,
 
   self._handle.lookup(address, function(ex, ip) {
     if (ex) {
-      if (callback) callback(ex);
+      if (typeof callback === 'function') {
+        callback(ex);
+        return;
+      }
+
       self.emit('error', ex);
     } else if (self._handle) {
       var req = new SendWrap();

test/internet/test-dgram-send-cb-quelches-error.js

@@ -0,0 +1,37 @@
+'use strict';
+var common = require('../common');
+var mustCall = common.mustCall;
+var assert = require('assert');
+var dgram = require('dgram');
+var dns = require('dns');
+
+var socket = dgram.createSocket('udp4');
+var buffer = new Buffer('gary busey');
+
+dns.setServers([]);
+
+socket.once('error', onEvent);
+
+// assert that:
+// * callbacks act as "error" listeners if given.
+// * error is never emitter for missing dns entries
+//   if a callback that handles error is present
+// * error is emitted if a callback with no argument is passed
+socket.send(buffer, 0, buffer.length, 100,
+            'dne.example.com', mustCall(callbackOnly));
+
+function callbackOnly(err) {
+  assert.ok(err);
+  socket.removeListener('error', onEvent);
+  socket.on('error', mustCall(onError));
+  socket.send(buffer, 0, buffer.length, 100, 'dne.example.com');
+}
+
+function onEvent(err) {
+  assert.fail('Error should not be emitted if there is callback');
+}
+
+function onError(err) {
+  assert.ok(err);
+  socket.close();
+}