tests: do not hardcode service name in test-dns.

Instead of hard-coding http service name in test-dns, retrieve it from
/etc/services. This is not ideal, but it's still better than hard-coding
it.

Fixes nodejs#8047.

test/common.js

@@ -219,3 +219,35 @@ exports.checkSpawnSyncRet = function(ret) {
   assert.strictEqual(ret.status, 0);
   assert.strictEqual(ret.error, undefined);
 };
+
+var etcServicesFileName = path.join('/etc', 'services');
+if (process.platform === 'win32') {
+  etcServicesFileName = path.join('/Windows', 'System32', 'drivers', 'etc',
+    'services');
+}
+
+exports.getServiceName = function getServiceName(port, protocol, callback) {
+  /*
+   * I'm not a big fan of readFileSync, but reading /etc/services asynchronously
+   * here would require implementing a simple line parser, which seems overkill
+   * for a simple utility function that is not running concurrently with any
+   * other one.
+   */
+  var servicesContent = fs.readFileSync(etcServicesFileName,
+    { encoding: 'utf8'});
+  var regexp = util.format('^(\\w+)\\s+\\s%d/%s\\s', port, protocol);
+  var re = new RegExp(regexp, 'm');
+
+  /*
+   * By default, if a service can't be found in /etc/services,
+   * its name is considered to be its port number.
+   */
+  var serviceName = String(port);
+
+  var matches = re.exec(servicesContent);
+  if (matches && matches.length > 1) {
+    serviceName = matches[1];
+  }
+
+  return serviceName;
+}

test/internet/test-dns.js

@@ -495,7 +495,15 @@ TEST(function test_lookupservice_ip_ipv4(done) {
   var req = dns.lookupService('127.0.0.1', 80, function(err, host, service) {
     if (err) throw err;
     assert.strictEqual(host, 'localhost');
-    assert.strictEqual(service, 'http');
+
+    /*
+     * Retrieve the actual HTTP service name as setup on the host currently
+     * running the test by reading it from /etc/services. This is not ideal,
+     * as the service name lookup could use another mechanism (e.g nscd), but
+     * it's already better than hardcoding it.
+     */
+    var httpServiceName = common.getServiceName(80, 'tcp');
+    assert.strictEqual(service, httpServiceName);
 
     done();
   });
@@ -515,7 +523,15 @@ TEST(function test_lookupservice_ip_ipv6(done) {
      * that most sane platforms use either one of these two by default.
      */
     assert(host === 'localhost' || host === 'ip6-localhost');
-    assert.strictEqual(service, 'http');
+
+    /*
+     * Retrieve the actual HTTP service name as setup on the host currently
+     * running the test by reading it from /etc/services. This is not ideal,
+     * as the service name lookup could use another mechanism (e.g nscd), but
+     * it's already better than hardcoding it.
+     */
+    var httpServiceName = common.getServiceName(80, 'tcp');
+    assert.strictEqual(service, httpServiceName);
 
     done();
   });