How do I choose a certificate authority file when using thin?

Question

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.