remove native-dns and use dig directly

native-dns module is unmaintained and we keep getting sporadic
errors from that module

Fixes #220

src/dig.js

@@ -0,0 +1,46 @@
+'use strict';
+
+exports = module.exports = {
+    resolve: resolve
+};
+
+var assert = require('assert'),
+    child_process = require('child_process'),
+    debug = require('debug')('box:dig');
+
+function resolve(domain, type, options, callback) {
+    assert.strictEqual(typeof domain, 'string');
+    assert.strictEqual(typeof type, 'string');
+    assert.strictEqual(typeof options, 'object');
+    assert.strictEqual(typeof callback, 'function');
+
+    // dig @server cloudron.io TXT +short
+    var args = [ ];
+    if (options.server) args.push('@' + options.server);
+    if (type === 'PTR') {
+        args.push('-x', domain);
+    } else {
+        args.push(domain, type);
+    }
+    args.push('+short');
+
+    child_process.execFile('/usr/bin/dig', args, { encoding: 'utf8', killSignal: 'SIGKILL', timeout: options.timeout || 0 }, function (error, stdout, stderr) {
+        if (error && error.killed) error.code = 'ETIMEDOUT';
+
+        if (error || stderr) debug('resolve error (%j): %j %s %s', args, error, stdout, stderr);
+        if (error) return callback(error);
+
+        debug('resolve (%j): %s', args, stdout);
+
+        if (!stdout) return callback(); // timeout or no result
+
+        var lines = stdout.trim().split('\n');
+        if (type === 'MX') {
+            lines = lines.map(function (line) {
+                var parts = line.split(' ');
+                return { priority: parts[0], exchange: parts[1] };
+            });
+        }
+        return callback(null, lines);
+    });
+}