Question

I know I can choose to verify the client with --ssl-verify, but how can I specify which CA chain that I want to use? I’m used to providing a file (like with curl’s --cacert or WEBrick’s :SSLCACertificateFile), so I’ve got one ready, but I can’t seem to find documentation on how to pass it to thin.

Answer 1

Short answer: You can’t.

Long answer: You could, but you’d have to update EventMachine’s C++ extension that builds the ssl connection, and update the call stack up through EventMachine and Thin to pass the certificate authority file along.

How I found this out: Source Code! It’s all on github

• thin’s command line opts are parsed in thin:lib/thin/runner.rb

  opts.separator "SSL options:"
  
  opts.on(      "--ssl", "Enables SSL")                                           { @options[:ssl] = true }
  opts.on(      "--ssl-key-file PATH", "Path to private key")                     { |path| @options[:ssl_key_file] = path }
  opts.on(      "--ssl-cert-file PATH", "Path to certificate")                    { |path| @options[:ssl_cert_file] = path }
  opts.on(      "--ssl-verify", "Enables SSL certificate verification")           { @options[:ssl_verify] = true }
  

• and then used to create a controller

  controller = case
  when cluster? then Controllers::Cluster.new(@options)
  when service? then Controllers::Service.new(@options)
  else               Controllers::Controller.new(@options)
  end
  

• In thin:lib/controllers/controller.rb the ssl options are pulled back out to be stored with the server object

  # ssl support
  if @options[:ssl]
    server.ssl = true
    server.ssl_options = { :private_key_file => @options[:ssl_key_file], :cert_chain_file => @options[:ssl_cert_file], :verify_peer => @options[:ssl_verify] }
  end
  

• and are finally used to initialize the connection to the client

  def initialize_connection(connection)
    connection.backend                 = self
    connection.app                     = @server.app
    connection.comm_inactivity_timeout = @timeout
    connection.threaded                = @threaded
  
    if @ssl
      connection.start_tls(@ssl_options)
    end
  

• This connection is an EventMachine::Connection, defined in eventmachine:lib/em/connection.rb. EventMachine::Connection#start_tls passes the parameters along to EventMachine::set_tls_parms.

  def start_tls args={}
    priv_key, cert_chain, verify_peer = args.values_at(:private_key_file, :cert_chain_file, :verify_peer)
  
    [priv_key, cert_chain].each do |file|
      next if file.nil? or file.empty?
      raise FileNotFoundException,
      "Could not find #{file} for start_tls" unless File.exists? file
    end 
  
    EventMachine::set_tls_parms(@signature, priv_key || '', cert_chain || '', verify_peer)
    EventMachine::start_tls @signature
  end 
  

• EventMachine::set_tls_parms is part of the C++ extension and is defined in eventmachine:ext/rubymain.cpp as the five argument C function t_set_tls_parms

  rb_define_module_function (EmModule, "set_tls_parms", (VALUE(*)(...))t_set_tls_parms, 4);
  

• And t_set_tls_parms defined elsewhere in the same file just passes the ssl options on to evma_set_tls_parms.

  static VALUE t_set_tls_parms (VALUE self, VALUE signature, VALUE privkeyfile, VALUE certchainfile, VALUE verify_peer)
  {
    /* set_tls_parms takes a series of positional arguments for specifying such things
     * as private keys and certificate chains.
     * It's expected that the parameter list will grow as we add more supported features.
     * ALL of these parameters are optional, and can be specified as empty or NULL strings.
     */
    evma_set_tls_parms (NUM2ULONG (signature), StringValuePtr (privkeyfile), StringValuePtr (certchainfile), (verify_peer == Qtrue ? 1 : 0));
    return Qnil;
  }
  

• The vanilla C function evma_set_tls_parms is defined in eventmachine:ext/cmain.cpp. It passes the ssl options on to EventableDescriptor‘s SetTlsParms method:

  extern "C" void evma_set_tls_parms (const unsigned long binding, const char *privatekey_filename, const char *certchain_filename, int verify_peer)
  {
    ensure_eventmachine("evma_set_tls_parms");
    EventableDescriptor *ed = dynamic_cast <EventableDescriptor*> (Bindable_t::GetObject (binding));
    if (ed)
      ed->SetTlsParms (privatekey_filename, certchain_filename, (verify_peer == 1 ? true : false));
  } 
  

• That SetTlsParms instance method is defined in eventmachine:ed.cpp, and all it really does is cache the ssl options in some instance variables.

  void ConnectionDescriptor::SetTlsParms (const char *privkey_filename, const char *certchain_filename, bool verify_peer)
  {
    #ifdef WITH_SSL
    if (SslBox)
      throw std::runtime_error ("call SetTlsParms before calling StartTls");
    if (privkey_filename && *privkey_filename)
      PrivateKeyFilename = privkey_filename;
    if (certchain_filename && *certchain_filename)
      CertChainFilename = certchain_filename;
    bSslVerifyPeer = verify_peer;
    #endif
  
    #ifdef WITHOUT_SSL
    throw std::runtime_error ("Encryption not available on this event-machine");
    #endif
  }
  

• Those instance variables are used later in the StartTls instance method (defined in the same file), and passed on to initialize a new SslBox_t

  void ConnectionDescriptor::StartTls()
  {
    #ifdef WITH_SSL
    if (SslBox)
      throw std::runtime_error ("SSL/TLS already running on connection");
  
    SslBox = new SslBox_t (bIsServer, PrivateKeyFilename, CertChainFilename, bSslVerifyPeer, GetBinding());
    _DispatchCiphertext();
    #endif
  

• The SslBox_t constructor is defined in eventmachine:ext/ssl.cpp, where it uses the ssl options to initialize a new SslContext_t.

  SslBox_t::SslBox_t (bool is_server, const string &privkeyfile, const string &certchainfile, bool verify_peer, const unsigned long binding):
    bIsServer (is_server),
    bHandshakeCompleted (false),
    bVerifyPeer (verify_peer),
    pSSL (NULL),
    pbioRead (NULL),
    pbioWrite (NULL)
  {
    /* TODO someday: make it possible to re-use SSL contexts so we don't have to create
     * a new one every time we come here.
     */
  
    Context = new SslContext_t (bIsServer, privkeyfile, certchainfile);
    assert (Context);
  

• The SslContext_t constructor is defined in the same file where it uses those options with the standard OpenSSL C bindings:

  // The SSL_CTX calls here do NOT allocate memory.
  int e;
  if (privkeyfile.length() > 0)
    e = SSL_CTX_use_PrivateKey_file (pCtx, privkeyfile.c_str(), SSL_FILETYPE_PEM);
  else
    e = SSL_CTX_use_PrivateKey (pCtx, DefaultPrivateKey);
  if (e <= 0) ERR_print_errors_fp(stderr);
  assert (e > 0);
  
  if (certchainfile.length() > 0)
    e = SSL_CTX_use_certificate_chain_file (pCtx, certchainfile.c_str());
  else
    e = SSL_CTX_use_certificate (pCtx, DefaultCertificate);
  if (e <= 0) ERR_print_errors_fp(stderr);
  assert (e > 0);
  

So now we know how the ssl options are used. If the call chain were modified to pass a CA file name along with the rest down to this point, say as const string &certauthfile, we could use just a couple more OpenSSL calls to add the authority file:

if (certauthfile.length() > 0)
  e = SSL_CTX_load_verify_locations(pCtx, certauthfile.c_str(), NULL);
else
  ;// no default necessary
if (e <= 0) ERR_print_errors_fp(stderr);
assert (e > 0);

Submitting a patch to do this is left as an exercise for the sufficiently motivated.