bool
TLSTicketKeyManager::setTLSTicketKeySeeds(
    const std::vector<std::string>& oldSeeds,
    const std::vector<std::string>& currentSeeds,
    const std::vector<std::string>& newSeeds) {

  recordTlsTicketRotation(oldSeeds, currentSeeds, newSeeds);

  bool result = true;

  activeKeys_.clear();
  ticketKeys_.clear();
  ticketSeeds_.clear();
  const std::vector<string> *seedList = &oldSeeds;
  for (uint32_t i = 0; i < 3; i++) {
    TLSTicketSeedType type = (TLSTicketSeedType)i;
    if (type == SEED_CURRENT) {
      seedList = &currentSeeds;
    } else if (type == SEED_NEW) {
      seedList = &newSeeds;
    }

    for (const auto& seedInput: *seedList) {
      TLSTicketSeed* seed = insertSeed(seedInput, type);
      if (seed == nullptr) {
        result = false;
        continue;
      }
      insertNewKey(seed, 1, nullptr);
    }
  }
  if (!result) {
    VLOG(2) << "One or more seeds failed to decode";
  }

  if (ticketKeys_.size() == 0 || activeKeys_.size() == 0) {
    LOG(WARNING) << "No keys configured, falling back to default";
    SSL_CTX_set_tlsext_ticket_key_cb(ctx_->getSSLCtx(), nullptr);
    return false;
  }
  SSL_CTX_set_tlsext_ticket_key_cb(ctx_->getSSLCtx(),
                                   TLSTicketKeyManager::callback);

  return true;
}
示例#2
0
/*
 * Configure callbacks and other properties that can't be set directly
 * in the server/client CONF.
 */
static void configure_handshake_ctx(SSL_CTX *server_ctx, SSL_CTX *client_ctx,
                                    const SSL_TEST_CTX *test_ctx)
{
    switch (test_ctx->client_verify_callback) {
    case SSL_TEST_VERIFY_ACCEPT_ALL:
        SSL_CTX_set_cert_verify_callback(client_ctx, &verify_accept_callback,
                                         NULL);
        break;
    case SSL_TEST_VERIFY_REJECT_ALL:
        SSL_CTX_set_cert_verify_callback(client_ctx, &verify_reject_callback,
                                         NULL);
        break;
    default:
        break;
    }
    if (test_ctx->session_ticket_expected == SSL_TEST_SESSION_TICKET_BROKEN) {
        SSL_CTX_set_tlsext_ticket_key_cb(server_ctx, broken_session_ticket_callback);
    }
}
示例#3
0
/*
 * Configure callbacks and other properties that can't be set directly
 * in the server/client CONF.
 */
static void configure_handshake_ctx(SSL_CTX *server_ctx, SSL_CTX *server2_ctx,
                                    SSL_CTX *client_ctx,
                                    const SSL_TEST_CTX *test_ctx,
                                    CTX_DATA *server_ctx_data,
                                    CTX_DATA *server2_ctx_data,
                                    CTX_DATA *client_ctx_data)
{
    unsigned char *ticket_keys;
    size_t ticket_key_len;

    switch (test_ctx->client_verify_callback) {
    case SSL_TEST_VERIFY_ACCEPT_ALL:
        SSL_CTX_set_cert_verify_callback(client_ctx, &verify_accept_cb,
                                         NULL);
        break;
    case SSL_TEST_VERIFY_REJECT_ALL:
        SSL_CTX_set_cert_verify_callback(client_ctx, &verify_reject_cb,
                                         NULL);
        break;
    default:
        break;
    }

    /* link the two contexts for SNI purposes */
    switch (test_ctx->servername_callback) {
    case SSL_TEST_SERVERNAME_IGNORE_MISMATCH:
        SSL_CTX_set_tlsext_servername_callback(server_ctx, servername_ignore_cb);
        SSL_CTX_set_tlsext_servername_arg(server_ctx, server2_ctx);
        break;
    case SSL_TEST_SERVERNAME_REJECT_MISMATCH:
        SSL_CTX_set_tlsext_servername_callback(server_ctx, servername_reject_cb);
        SSL_CTX_set_tlsext_servername_arg(server_ctx, server2_ctx);
        break;
    default:
        break;
    }

    /*
     * The initial_ctx/session_ctx always handles the encrypt/decrypt of the
     * session ticket. This ticket_key callback is assigned to the second
     * session (assigned via SNI), and should never be invoked
     */
    if (server2_ctx != NULL)
        SSL_CTX_set_tlsext_ticket_key_cb(server2_ctx,
                                         do_not_call_session_ticket_cb);

    if (test_ctx->session_ticket_expected == SSL_TEST_SESSION_TICKET_BROKEN) {
        SSL_CTX_set_tlsext_ticket_key_cb(server_ctx, broken_session_ticket_cb);
    }
    if (test_ctx->server_npn_protocols != NULL) {
        parse_protos(test_ctx->server_npn_protocols,
                     &server_ctx_data->npn_protocols,
                     &server_ctx_data->npn_protocols_len);
        SSL_CTX_set_next_protos_advertised_cb(server_ctx, server_npn_cb,
                                              server_ctx_data);
    }
    if (test_ctx->server2_npn_protocols != NULL) {
        parse_protos(test_ctx->server2_npn_protocols,
                     &server2_ctx_data->npn_protocols,
                     &server2_ctx_data->npn_protocols_len);
        OPENSSL_assert(server2_ctx != NULL);
        SSL_CTX_set_next_protos_advertised_cb(server2_ctx, server_npn_cb,
                                              server2_ctx_data);
    }
    if (test_ctx->client_npn_protocols != NULL) {
        parse_protos(test_ctx->client_npn_protocols,
                     &client_ctx_data->npn_protocols,
                     &client_ctx_data->npn_protocols_len);
        SSL_CTX_set_next_proto_select_cb(client_ctx, client_npn_cb,
                                         client_ctx_data);
    }
    if (test_ctx->server_alpn_protocols != NULL) {
        parse_protos(test_ctx->server_alpn_protocols,
                     &server_ctx_data->alpn_protocols,
                     &server_ctx_data->alpn_protocols_len);
        SSL_CTX_set_alpn_select_cb(server_ctx, server_alpn_cb, server_ctx_data);
    }
    if (test_ctx->server2_alpn_protocols != NULL) {
        OPENSSL_assert(server2_ctx != NULL);
        parse_protos(test_ctx->server2_alpn_protocols,
                     &server2_ctx_data->alpn_protocols,
                     &server2_ctx_data->alpn_protocols_len);
        SSL_CTX_set_alpn_select_cb(server2_ctx, server_alpn_cb, server2_ctx_data);
    }
    if (test_ctx->client_alpn_protocols != NULL) {
        unsigned char *alpn_protos = NULL;
        size_t alpn_protos_len;
        parse_protos(test_ctx->client_alpn_protocols,
                     &alpn_protos, &alpn_protos_len);
        /* Reversed return value convention... */
        OPENSSL_assert(SSL_CTX_set_alpn_protos(client_ctx, alpn_protos,
                                               alpn_protos_len) == 0);
        OPENSSL_free(alpn_protos);
    }
    /*
     * Use fixed session ticket keys so that we can decrypt a ticket created with
     * one CTX in another CTX. Don't address server2 for the moment.
     */
    ticket_key_len = SSL_CTX_set_tlsext_ticket_keys(server_ctx, NULL, 0);
    ticket_keys = OPENSSL_zalloc(ticket_key_len);
    OPENSSL_assert(ticket_keys != NULL);
    OPENSSL_assert(SSL_CTX_set_tlsext_ticket_keys(server_ctx, ticket_keys,
                                                  ticket_key_len) == 1);
    OPENSSL_free(ticket_keys);
}
示例#4
0
/*
 * Configure callbacks and other properties that can't be set directly
 * in the server/client CONF.
 */
static void configure_handshake_ctx(SSL_CTX *server_ctx, SSL_CTX *server2_ctx,
                                    SSL_CTX *client_ctx,
                                    const SSL_TEST_CTX *test,
                                    const SSL_TEST_EXTRA_CONF *extra,
                                    CTX_DATA *server_ctx_data,
                                    CTX_DATA *server2_ctx_data,
                                    CTX_DATA *client_ctx_data)
{
    unsigned char *ticket_keys;
    size_t ticket_key_len;

    TEST_check(SSL_CTX_set_max_send_fragment(server_ctx,
                                             test->max_fragment_size) == 1);
    if (server2_ctx != NULL) {
        TEST_check(SSL_CTX_set_max_send_fragment(server2_ctx,
                                                 test->max_fragment_size) == 1);
    }
    TEST_check(SSL_CTX_set_max_send_fragment(client_ctx,
                                             test->max_fragment_size) == 1);

    switch (extra->client.verify_callback) {
    case SSL_TEST_VERIFY_ACCEPT_ALL:
        SSL_CTX_set_cert_verify_callback(client_ctx, &verify_accept_cb,
                                         NULL);
        break;
    case SSL_TEST_VERIFY_REJECT_ALL:
        SSL_CTX_set_cert_verify_callback(client_ctx, &verify_reject_cb,
                                         NULL);
        break;
    default:
        break;
    }

    /* link the two contexts for SNI purposes */
    switch (extra->server.servername_callback) {
    case SSL_TEST_SERVERNAME_IGNORE_MISMATCH:
        SSL_CTX_set_tlsext_servername_callback(server_ctx, servername_ignore_cb);
        SSL_CTX_set_tlsext_servername_arg(server_ctx, server2_ctx);
        break;
    case SSL_TEST_SERVERNAME_REJECT_MISMATCH:
        SSL_CTX_set_tlsext_servername_callback(server_ctx, servername_reject_cb);
        SSL_CTX_set_tlsext_servername_arg(server_ctx, server2_ctx);
        break;
    default:
        break;
    }

    /*
     * The initial_ctx/session_ctx always handles the encrypt/decrypt of the
     * session ticket. This ticket_key callback is assigned to the second
     * session (assigned via SNI), and should never be invoked
     */
    if (server2_ctx != NULL)
        SSL_CTX_set_tlsext_ticket_key_cb(server2_ctx,
                                         do_not_call_session_ticket_cb);

    if (extra->server.broken_session_ticket) {
        SSL_CTX_set_tlsext_ticket_key_cb(server_ctx, broken_session_ticket_cb);
    }
#ifndef OPENSSL_NO_NEXTPROTONEG
    if (extra->server.npn_protocols != NULL) {
        parse_protos(extra->server.npn_protocols,
                     &server_ctx_data->npn_protocols,
                     &server_ctx_data->npn_protocols_len);
        SSL_CTX_set_next_protos_advertised_cb(server_ctx, server_npn_cb,
                                              server_ctx_data);
    }
    if (extra->server2.npn_protocols != NULL) {
        parse_protos(extra->server2.npn_protocols,
                     &server2_ctx_data->npn_protocols,
                     &server2_ctx_data->npn_protocols_len);
        TEST_check(server2_ctx != NULL);
        SSL_CTX_set_next_protos_advertised_cb(server2_ctx, server_npn_cb,
                                              server2_ctx_data);
    }
    if (extra->client.npn_protocols != NULL) {
        parse_protos(extra->client.npn_protocols,
                     &client_ctx_data->npn_protocols,
                     &client_ctx_data->npn_protocols_len);
        SSL_CTX_set_next_proto_select_cb(client_ctx, client_npn_cb,
                                         client_ctx_data);
    }
#endif
    if (extra->server.alpn_protocols != NULL) {
        parse_protos(extra->server.alpn_protocols,
                     &server_ctx_data->alpn_protocols,
                     &server_ctx_data->alpn_protocols_len);
        SSL_CTX_set_alpn_select_cb(server_ctx, server_alpn_cb, server_ctx_data);
    }
    if (extra->server2.alpn_protocols != NULL) {
        TEST_check(server2_ctx != NULL);
        parse_protos(extra->server2.alpn_protocols,
                     &server2_ctx_data->alpn_protocols,
                     &server2_ctx_data->alpn_protocols_len);
        SSL_CTX_set_alpn_select_cb(server2_ctx, server_alpn_cb, server2_ctx_data);
    }
    if (extra->client.alpn_protocols != NULL) {
        unsigned char *alpn_protos = NULL;
        size_t alpn_protos_len;
        parse_protos(extra->client.alpn_protocols,
                     &alpn_protos, &alpn_protos_len);
        /* Reversed return value convention... */
        TEST_check(SSL_CTX_set_alpn_protos(client_ctx, alpn_protos,
                                           alpn_protos_len) == 0);
        OPENSSL_free(alpn_protos);
    }

    /*
     * Use fixed session ticket keys so that we can decrypt a ticket created with
     * one CTX in another CTX. Don't address server2 for the moment.
     */
    ticket_key_len = SSL_CTX_set_tlsext_ticket_keys(server_ctx, NULL, 0);
    ticket_keys = OPENSSL_zalloc(ticket_key_len);
    TEST_check(ticket_keys != NULL);
    TEST_check(SSL_CTX_set_tlsext_ticket_keys(server_ctx, ticket_keys,
                                              ticket_key_len) == 1);
    OPENSSL_free(ticket_keys);

    /* The default log list includes EC keys, so CT can't work without EC. */
#if !defined(OPENSSL_NO_CT) && !defined(OPENSSL_NO_EC)
    TEST_check(SSL_CTX_set_default_ctlog_list_file(client_ctx));
    switch (extra->client.ct_validation) {
    case SSL_TEST_CT_VALIDATION_PERMISSIVE:
        TEST_check(SSL_CTX_enable_ct(client_ctx, SSL_CT_VALIDATION_PERMISSIVE));
        break;
    case SSL_TEST_CT_VALIDATION_STRICT:
        TEST_check(SSL_CTX_enable_ct(client_ctx, SSL_CT_VALIDATION_STRICT));
        break;
    case SSL_TEST_CT_VALIDATION_NONE:
        break;
    }
#endif
}
示例#5
0
文件: tls_server.c 项目: aosm/postfix
TLS_APPL_STATE *tls_server_init(const TLS_SERVER_INIT_PROPS *props)
{
    SSL_CTX *server_ctx;
    long    off = 0;
    int     verify_flags = SSL_VERIFY_NONE;
    int     cachable;
    int     scache_timeout;
    int     ticketable = 0;
    int     protomask;
    TLS_APPL_STATE *app_ctx;
    int     log_mask;

    /*
     * Convert user loglevel to internal logmask.
     */
    log_mask = tls_log_mask(props->log_param, props->log_level);

    if (log_mask & TLS_LOG_VERBOSE)
	msg_info("initializing the server-side TLS engine");

    /*
     * Load (mostly cipher related) TLS-library internal main.cf parameters.
     */
    tls_param_init();

    /*
     * Detect mismatch between compile-time headers and run-time library.
     */
    tls_check_version();

    /*
     * Initialize the OpenSSL library by the book! To start with, we must
     * initialize the algorithms. We want cleartext error messages instead of
     * just error codes, so we load the error_strings.
     */
    SSL_load_error_strings();
    OpenSSL_add_ssl_algorithms();

    /*
     * First validate the protocols. If these are invalid, we can't continue.
     */
    protomask = tls_protocol_mask(props->protocols);
    if (protomask == TLS_PROTOCOL_INVALID) {
	/* tls_protocol_mask() logs no warning. */
	msg_warn("Invalid TLS protocol list \"%s\": disabling TLS support",
		 props->protocols);
	return (0);
    }

    /*
     * Create an application data index for SSL objects, so that we can
     * attach TLScontext information; this information is needed inside
     * tls_verify_certificate_callback().
     */
    if (TLScontext_index < 0) {
	if ((TLScontext_index = SSL_get_ex_new_index(0, 0, 0, 0, 0)) < 0) {
	    msg_warn("Cannot allocate SSL application data index: "
		     "disabling TLS support");
	    return (0);
	}
    }

    /*
     * If the administrator specifies an unsupported digest algorithm, fail
     * now, rather than in the middle of a TLS handshake.
     */
    if (!tls_validate_digest(props->mdalg)) {
	msg_warn("disabling TLS support");
	return (0);
    }

    /*
     * Initialize the PRNG (Pseudo Random Number Generator) with some seed
     * from external and internal sources. Don't enable TLS without some real
     * entropy.
     */
    if (tls_ext_seed(var_tls_daemon_rand_bytes) < 0) {
	msg_warn("no entropy for TLS key generation: disabling TLS support");
	return (0);
    }
    tls_int_seed();

    /*
     * The SSL/TLS specifications require the client to send a message in the
     * oldest specification it understands with the highest level it
     * understands in the message. Netscape communicator can still
     * communicate with SSLv2 servers, so it sends out a SSLv2 client hello.
     * To deal with it, our server must be SSLv2 aware (even if we don't like
     * SSLv2), so we need to have the SSLv23 server here. If we want to limit
     * the protocol level, we can add an option to not use SSLv2/v3/TLSv1
     * later.
     */
    ERR_clear_error();
    if ((server_ctx = SSL_CTX_new(SSLv23_server_method())) == 0) {
	msg_warn("cannot allocate server SSL_CTX: disabling TLS support");
	tls_print_errors();
	return (0);
    }

    /*
     * See the verify callback in tls_verify.c
     */
    SSL_CTX_set_verify_depth(server_ctx, props->verifydepth + 1);

    /*
     * The session cache is implemented by the tlsmgr(8) server.
     * 
     * XXX 200502 Surprise: when OpenSSL purges an entry from the in-memory
     * cache, it also attempts to purge the entry from the on-disk cache.
     * This is undesirable, especially when we set the in-memory cache size
     * to 1. For this reason we don't allow OpenSSL to purge on-disk cache
     * entries, and leave it up to the tlsmgr process instead. Found by
     * Victor Duchovni.
     */
    if (tls_mgr_policy(props->cache_type, &cachable,
		       &scache_timeout) != TLS_MGR_STAT_OK)
	scache_timeout = 0;
    if (scache_timeout <= 0)
	cachable = 0;

    /*
     * Protocol work-arounds, OpenSSL version dependent.
     */
    off |= tls_bug_bits();

    /*
     * Add SSL_OP_NO_TICKET when the timeout is zero or library support is
     * incomplete.  The SSL_CTX_set_tlsext_ticket_key_cb feature was added in
     * OpenSSL 0.9.8h, while SSL_NO_TICKET was added in 0.9.8f.
     */
#ifdef SSL_OP_NO_TICKET
#if !defined(OPENSSL_NO_TLSEXT) && OPENSSL_VERSION_NUMBER >= 0x0090808fL
    ticketable = (scache_timeout > 0 && !(off & SSL_OP_NO_TICKET));
    if (ticketable)
	SSL_CTX_set_tlsext_ticket_key_cb(server_ctx, ticket_cb);
#endif
    if (!ticketable)
	off |= SSL_OP_NO_TICKET;
#endif

    SSL_CTX_set_options(server_ctx, off);

    /*
     * Global protocol selection.
     */
    if (protomask != 0)
	SSL_CTX_set_options(server_ctx,
		   ((protomask & TLS_PROTOCOL_TLSv1) ? SSL_OP_NO_TLSv1 : 0L)
	     | ((protomask & TLS_PROTOCOL_TLSv1_1) ? SSL_OP_NO_TLSv1_1 : 0L)
	     | ((protomask & TLS_PROTOCOL_TLSv1_2) ? SSL_OP_NO_TLSv1_2 : 0L)
		 | ((protomask & TLS_PROTOCOL_SSLv3) ? SSL_OP_NO_SSLv3 : 0L)
	       | ((protomask & TLS_PROTOCOL_SSLv2) ? SSL_OP_NO_SSLv2 : 0L));

    /*
     * Some sites may want to give the client less rope. On the other hand,
     * this could trigger inter-operability issues, the client should not
     * offer ciphers it implements poorly, but this hasn't stopped some
     * vendors from getting it wrong.
     * 
     * XXX: Given OpenSSL's security history, nobody should still be using
     * 0.9.7, let alone 0.9.6 or earlier. Warning added to TLS_README.html.
     */
    if (var_tls_preempt_clist)
	SSL_CTX_set_options(server_ctx, SSL_OP_CIPHER_SERVER_PREFERENCE);

    /*
     * Set the call-back routine to debug handshake progress.
     */
    if (log_mask & TLS_LOG_DEBUG)
	SSL_CTX_set_info_callback(server_ctx, tls_info_callback);

    /*
     * Load the CA public key certificates for both the server cert and for
     * the verification of client certificates. As provided by OpenSSL we
     * support two types of CA certificate handling: One possibility is to
     * add all CA certificates to one large CAfile, the other possibility is
     * a directory pointed to by CApath, containing separate files for each
     * CA with softlinks named after the hash values of the certificate. The
     * first alternative has the advantage that the file is opened and read
     * at startup time, so that you don't have the hassle to maintain another
     * copy of the CApath directory for chroot-jail.
     */
    if (tls_set_ca_certificate_info(server_ctx,
				    props->CAfile, props->CApath) < 0) {
	/* tls_set_ca_certificate_info() already logs a warning. */
	SSL_CTX_free(server_ctx);		/* 200411 */
	return (0);
    }

    /*
     * Load the server public key certificate and private key from file and
     * check whether the cert matches the key. We can use RSA certificates
     * ("cert") DSA certificates ("dcert") or ECDSA certificates ("eccert").
     * All three can be made available at the same time. The CA certificates
     * for all three are handled in the same setup already finished. Which
     * one is used depends on the cipher negotiated (that is: the first
     * cipher listed by the client which does match the server). A client
     * with RSA only (e.g. Netscape) will use the RSA certificate only. A
     * client with openssl-library will use RSA first if not especially
     * changed in the cipher setup.
     */
    if (tls_set_my_certificate_key_info(server_ctx,
					props->cert_file,
					props->key_file,
					props->dcert_file,
					props->dkey_file,
					props->eccert_file,
					props->eckey_file) < 0) {
	/* tls_set_my_certificate_key_info() already logs a warning. */
	SSL_CTX_free(server_ctx);		/* 200411 */
	return (0);
    }

    /*
     * According to OpenSSL documentation, a temporary RSA key is needed when
     * export ciphers are in use, because the certified key cannot be
     * directly used.
     */
    SSL_CTX_set_tmp_rsa_callback(server_ctx, tls_tmp_rsa_cb);

    /*
     * Diffie-Hellman key generation parameters can either be loaded from
     * files (preferred) or taken from compiled in values. First, set the
     * callback that will select the values when requested, then load the
     * (possibly) available DH parameters from files. We are generous with
     * the error handling, since we do have default values compiled in, so we
     * will not abort but just log the error message.
     */
    SSL_CTX_set_tmp_dh_callback(server_ctx, tls_tmp_dh_cb);
    if (*props->dh1024_param_file != 0)
	tls_set_dh_from_file(props->dh1024_param_file, 1024);
    if (*props->dh512_param_file != 0)
	tls_set_dh_from_file(props->dh512_param_file, 512);

    /*
     * Enable EECDH if available, errors are not fatal, we just keep going
     * with any remaining key-exchange algorithms.
     */
    (void) tls_set_eecdh_curve(server_ctx, props->eecdh_grade);

    /*
     * If we want to check client certificates, we have to indicate it in
     * advance. By now we only allow to decide on a global basis. If we want
     * to allow certificate based relaying, we must ask the client to provide
     * one with SSL_VERIFY_PEER. The client now can decide, whether it
     * provides one or not. We can enforce a failure of the negotiation with
     * SSL_VERIFY_FAIL_IF_NO_PEER_CERT, if we do not allow a connection
     * without one. In the "server hello" following the initialization by the
     * "client hello" the server must provide a list of CAs it is willing to
     * accept. Some clever clients will then select one from the list of
     * available certificates matching these CAs. Netscape Communicator will
     * present the list of certificates for selecting the one to be sent, or
     * it will issue a warning, if there is no certificate matching the
     * available CAs.
     * 
     * With regard to the purpose of the certificate for relaying, we might like
     * a later negotiation, maybe relaying would already be allowed for other
     * reasons, but this would involve severe changes in the internal postfix
     * logic, so we have to live with it the way it is.
     */
    if (props->ask_ccert)
	verify_flags = SSL_VERIFY_PEER | SSL_VERIFY_CLIENT_ONCE;
    SSL_CTX_set_verify(server_ctx, verify_flags,
		       tls_verify_certificate_callback);
    if (*props->CAfile)
	SSL_CTX_set_client_CA_list(server_ctx,
				   SSL_load_client_CA_file(props->CAfile));

    /*
     * Initialize our own TLS server handle, before diving into the details
     * of TLS session cache management.
     */
    app_ctx = tls_alloc_app_context(server_ctx, log_mask);

    if (cachable || ticketable || props->set_sessid) {

	/*
	 * Initialize the session cache.
	 * 
	 * With a large number of concurrent smtpd(8) processes, it is not a
	 * good idea to cache multiple large session objects in each process.
	 * We set the internal cache size to 1, and don't register a
	 * "remove_cb" so as to avoid deleting good sessions from the
	 * external cache prematurely (when the internal cache is full,
	 * OpenSSL removes sessions from the external cache also)!
	 * 
	 * This makes SSL_CTX_remove_session() not useful for flushing broken
	 * sessions from the external cache, so we must delete them directly
	 * (not via a callback).
	 * 
	 * Set a session id context to identify to what type of server process
	 * created a session. In our case, the context is simply the name of
	 * the mail system: "Postfix/TLS".
	 */
	SSL_CTX_sess_set_cache_size(server_ctx, 1);
	SSL_CTX_set_session_id_context(server_ctx,
				       (void *) &server_session_id_context,
				       sizeof(server_session_id_context));
	SSL_CTX_set_session_cache_mode(server_ctx,
				       SSL_SESS_CACHE_SERVER |
				       SSL_SESS_CACHE_NO_AUTO_CLEAR);
	if (cachable) {
	    app_ctx->cache_type = mystrdup(props->cache_type);

	    SSL_CTX_sess_set_get_cb(server_ctx, get_server_session_cb);
	    SSL_CTX_sess_set_new_cb(server_ctx, new_server_session_cb);
	}

	/*
	 * OpenSSL ignores timed-out sessions. We need to set the internal
	 * cache timeout at least as high as the external cache timeout. This
	 * applies even if no internal cache is used.  We set the session
	 * lifetime to twice the cache lifetime, which is also the issuing
	 * and retired key validation lifetime of session tickets keys. This
	 * way a session always lasts longer than the server's ability to
	 * decrypt its session ticket.  Otherwise, a bug in OpenSSL may fail
	 * to re-issue tickets when sessions decrypt, but are expired.
	 */
	SSL_CTX_set_timeout(server_ctx, 2 * scache_timeout);
    } else {

	/*
	 * If we have no external cache, disable all caching. No use wasting
	 * server memory resources with sessions they are unlikely to be able
	 * to reuse.
	 */
	SSL_CTX_set_session_cache_mode(server_ctx, SSL_SESS_CACHE_OFF);
    }

    return (app_ctx);
}