Category: Dev

I’ve previously written about using node.js as a simple UDP listener, now I thought I’d expand on this and give an example of a node.js application that works as both a UDP and TCP server.

A simple service that offers both UDP and TCP connections would be the Daytime Protocol, as defined in RFC 867. It listens to port 13 for both TCP and UDP connections, and returns the current date and time. Your local computer may well have this function enabled, to test the TCP version, you can simply telnet to port 13 and see what is returned.

telnet localhost 13

If you find you don’t have a local Daytime service running, try calling the one at time.ien.it, you should see something like this…

$ telnet time.ien.it 13
Trying 193.204.114.105...
Connected to ntp.ien.it.
Escape character is '^]'.
12 APR 2011 18:51:11 CEST
Connection closed by foreign host.

As you can see it simply returns the date and time before closing the connection.

We could write something like this very easily in node.js. In this case we’ll return the date in JavaScript’s UTC string format instead to keep it simple.

var net = require('net');
var port = 1300;
var now = function() {
  var date = new Date();
  return new Buffer(date.toUTCString() + "rn");
};
var tcpserver = net.createServer(function(c) {
  c.write(now());
  c.end();
});
tcpserver.listen(port);

We can test this by telneting to localhost on port 13

$ telnet localhost 1300
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
Tue, 12 Apr 2011 17:17:55 GMT
Connection closed by foreign host.

Let’s take a quick detour into what the code is doing if you’ve not used node.js for any network programming before.

We require('net') so we can use node.js’s network socket support.

The now() function simply returns a Buffer with the current UTC date and time string in. A Buffer in node.js refers to the raw memory used behind the data, in this case our string with the date and time in.

Finally, we create the server, and add a callback function that is called when something connects to the socket. In this case we’re just writing out the contents of the now() function and ending the connection. Last but not least, we tell the socket which port to listen on.

We can see that this code works, but the Daytime protocol also offers a UDP service on the same port that can return the date and time string to a client.

Let’s add in some code to support this. We’ll assume our now() function and port variable are available already.

var dgram = require('dgram');
var udpserver = dgram.createSocket("udp4", function(msg, rinfo) {
  var daytime = now();
  udpserver.send(daytime, 0, daytime.length, rinfo.port, rinfo.address);
});
udpserver.bind(port);

So what is going on here? Well like the TCP code, we have to require('dgram'); to have access to node.js’s UDP support.

Next we create the socket and create a callback function that returns the date and time string back to the IP address and port of the calling computer. Finally we bind this to the port so we start listening for messages.

It’s harder to test a UDP server as we can’t telnet in, we will have to write a simple UDP client to send a blank message to the server and print out any replies to the console.

var dgram = require('dgram');
var message = new Buffer(" ");
var server_ip = '127.0.0.1';
var server_port = 43278;
var client = dgram.createSocket("udp4");
client.on('message', function (msg) {
  console.log(msg.toString());
  client.close();
});
client.send(message, 0, message.length, server_port, server_ip);

When you run this client on the same machine as your server you should get back the current date and time.

Here’s the final code for our Daytime server.

var net = require('net');
var dgram = require('dgram');
var port = 1300;
var now = function() {
  var date = new Date();
  return new Buffer(date.toUTCString() + "rn");
}
var tcpserver = net.createServer(function(c) {
  c.write(now());
  c.end();
});
tcpserver.listen(port);
var udpserver = dgram.createSocket("udp4", function(msg, rinfo) {
  var daytime = now();
  udpserver.send(daytime, 0, daytime.length, rinfo.port, rinfo.address);
});
udpserver.bind(port);