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Prototype SSL Socket Server

Reading time: 4 minutes

This write-up was inspired by this stack overflow question. While I've worked with sockets before, I wanted to prototype a Haxe HXCPP socket server leveraging secure SSL connections.

OS Note

I use Linux, so this write-up is written from that perspective. Commands shown will likely be similar on OSX. Windows users will have to modify some commands. I used Haxe 4.0.0-rc1 and hxcpp 4.0.19 for this test.

Getting Started

The first thing you'll need for playing with SSL is a certificate and a key file.

Test Certificates

The OpenSSL project has some test certificates that you can use. You can download the certificate and key files directly from their github repo:

These are not bound to any particular domain, so the server can use any setHostname(), and the client can access the server simply by IP address. The above files worked in the example code below.

Real Certificates

Often in the real world, you use certificates to prove domain ownership. You can get certificates for your domain for free from letsencrypt.org -- you'll have to provide domain ownership verification.

Notes on using cert files: - You may need to convert your files to PEM format (Google or openssl CLI tool may help you). - You may sometimes need to concatenate multiple intermediate / CA pem files into an overall chain.pem file.

In the below example, I'm using foo.example.com as the hostname. Your hostname will vary and your cert files are bound to a specific hostname, such as myhost.mydomain.com. Some certs are valid for multiple hosts via a wildcard cert (e.g. *.mydomain.com).

Testing your certs on localhost

If you want to test a cert for mydomain.com on your local computer (without pushing your code to your live mydomain.com server), you can point mydomain.com at localhost by inserting a line into your /etc/hosts file:   mydomain.com

Now requests to mydomain.com will be redirected to (aka, localhost, your computer). You may need to restart your browser or reboot for this change to take effect.

You might generate a certificate for test.mydomain.com or localhost.mydomain.com, specifically for testing or development of SSL / HTTPS connections. You could use /etc/hosts entries on developers computers, or CI servers.

cpp.Lib.stringReference note

Note: until this issue is fixed in Haxe, you may need to alter haxe/std/cpp/_std/sys/ssl/Key.hx line 17 to use toString() instead of cpp.Lib.stringReference() as follows:

var str = data.toString(); // cpp.Lib.stringReference(data);

About the Server

This example server uses Threads, where the main thread accepts connections, and then passes each connected client socket off to a reader thread. The reader thread simply calls Sys.print() with any data received, and gracefully exits if an end-of-file (Eof) is received.

Using threads, thread messages, and blocking sockets is an efficient design. That way, a thread can simply sleep until the socket wakes it up. Which is what you want, as opposed to the CPU spinning in a while loop, endlessly checking if a socket is unblocked.

The server loads my private key and certificate chain files, and listens on port 8000.

The Code

import sys.net.Host;
import sys.net.Socket;
import sys.ssl.Socket as SocketSSL; // aliased to avoid conflict with sys.net.Socket
import sys.ssl.Certificate;
import sys.ssl.Key;
import cpp.vm.Thread;

class Main {
  public static function main() {
    var listener_socket = new SocketSSL();
    var cert = Certificate.loadFile('my_chain.pem');
    listener_socket.setCertificate(cert, Key.loadFile('my_private.key'));

    // e.g. for an application like an HTTPs server, the client
    // doesn't need to provide a certificate. Otherwise we get:
    // Error: SSL - No client certification received from the client, 
    // but required by the authentication mode
    listener_socket.verifyCert = false;

    // Binding means, listen on "any / all IP addresses on this host"
    listener_socket.bind(new Host(''), 8000);
    listener_socket.listen(9999); // big max connections

    while (true) {
      // Accepting socket
      trace('waiting to accept...');
      var peer_connection:SocketSSL = listener_socket.accept();
      if (peer_connection != null) {
        trace('got connection from : ' + peer_connection.peer());
        peer_connection.handshake(); // This may not be necessary, if !verifyCert

        // Spawn a reader thread for this connection:
        var thrd = Thread.create(reader);
        trace('sending socket...');

  static function reader() {
    var peer_connection:Socket = cast Thread.readMessage(true);
    trace('new reader thread...');

    while (true) {
      try {
      } catch (e:haxe.io.Eof) {
        trace('Eof, reader thread exiting...');
      } catch (e:Any) {
        trace('Uncaught: ${e}'); // throw e;

Testing the Server

So, let's see it in action!

The project directory contains Main.hx, as well as my cert files, my_chain.pem and my_private.key.

Compile and start the server in one terminal:

> haxe -main Main -debug -cpp out && ./out/Main-debug
...compiling info removed...
Main.hx:37: waiting to accept...

Now connect from another terminal with a client that's a command line utility for testing ssl connections:

> openssl s_client -connect foo.example.com:8000
...lots of info about the cert...
SSL handshake has read 3374 bytes and written 370 bytes
Verification: OK

And it hangs there, waiting for you to type input. On the server side we see:

Main.hx:38: got connection from : { host => Host, port => 57394 }
Main.hx:43: sending socket...
Main.hx:45: ok...
Main.hx:35: waiting to accept...
Main.hx:54: new reader thread...

Typing messages into the client terminal, those messages are echoed on the server side as expected.

We can open many clients in separate terminals, they each get their own reader thread.

In the client terminal, hitting CTRL+C exits the client, and on the server we see:

Main.hx:61: Eof, reader thread exiting...

The reader thread exits gracefully, while the server is still running.

Everything is working as expected!

Next Steps

When you're ready to take your server past the prototype phase, you'll want to pass some "data processor" thread to the reader threads (the same way the socket is passed, via Thread.sendMessage().) Then when the reader receives data from a client, it would send that data to the data processor thread.

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