text: /*++
  /* NAME
  /*	tls_server 3
  /* SUMMARY
  /*	server-side TLS engine
  /* SYNOPSIS
  /*	#include <tls.h>
  /*
  /*	TLS_APPL_STATE *tls_server_init(props)
  /*	const TLS_SERVER_INIT_PROPS *props;
  /*
  /*	TLS_SESS_STATE *tls_server_start(props)
  /*	const TLS_SERVER_START_PROPS *props;
  /*
  /*	TLS_SESS_STATE *tls_server_post_accept(TLScontext)
  /*	TLS_SESS_STATE *TLScontext;
  /*
  /*	void	tls_server_stop(app_ctx, stream, failure, TLScontext)
  /*	TLS_APPL_STATE *app_ctx;
  /*	VSTREAM	*stream;
  /*	int	failure;
  /*	TLS_SESS_STATE *TLScontext;
  /* DESCRIPTION
  /*	This module is the interface between Postfix TLS servers,
  /*	the OpenSSL library, and the TLS entropy and cache manager.
  /*
  /*	See "EVENT_DRIVEN APPLICATIONS" below for using this code
  /*	in event-driven programs.
  /*
  /*	tls_server_init() is called once when the SMTP server
  /*	initializes.
  /*	Certificate details are also decided during this phase,
  /*	so that peer-specific behavior is not possible.
  /*
  /*	tls_server_start() activates the TLS feature for the VSTREAM
  /*	passed as argument. We assume that network buffers are flushed
  /*	and the TLS handshake can begin	immediately.
  /*
  /*	tls_server_stop() sends the "close notify" alert via
  /*	SSL_shutdown() to the peer and resets all connection specific
  /*	TLS data. As RFC2487 does not specify a separate shutdown, it
  /*	is assumed that the underlying TCP connection is shut down
  /*	immediately afterwards. Any further writes to the channel will
  /*	be discarded, and any further reads will report end-of-file.
  /*	If the failure flag is set, no SSL_shutdown() handshake is performed.
  /*
  /*	Once the TLS connection is initiated, information about the TLS
  /*	state is available via the TLScontext structure:
  /* .IP TLScontext->protocol
  /*	the protocol name (SSLv2, SSLv3, TLSv1),
  /* .IP TLScontext->cipher_name
  /*	the cipher name (e.g. RC4/MD5),
  /* .IP TLScontext->cipher_usebits
  /*	the number of bits actually used (e.g. 40),
  /* .IP TLScontext->cipher_algbits
  /*	the number of bits the algorithm is based on (e.g. 128).
  /* .PP
  /*	The last two values may differ from each other when export-strength
  /*	encryption is used.
  /*
  /*	If the peer offered a certificate, part of the certificate data are
  /*	available as:
  /* .IP TLScontext->peer_status
  /*	A bitmask field that records the status of the peer certificate
  /*	verification. One or more of TLS_CERT_FLAG_PRESENT and
  /*	TLS_CERT_FLAG_TRUSTED.
  /* .IP TLScontext->peer_CN
  /*	Extracted CommonName of the peer, or zero-length string
  /*	when information could not be extracted.
  /* .IP TLScontext->issuer_CN
  /*	Extracted CommonName of the issuer, or zero-length string
  /*	when information could not be extracted.
  /* .IP TLScontext->peer_cert_fprint
  /*	Fingerprint of the certificate, or zero-length string when no peer
  /*	certificate is available.
  /* .PP
  /*	If no peer certificate is presented the peer_status is set to 0.
  /* EVENT_DRIVEN APPLICATIONS
  /* .ad
  /* .fi
  /*	Event-driven programs manage multiple I/O channels.  Such
  /*	programs cannot use the synchronous VSTREAM-over-TLS
  /*	implementation that the current TLS library provides,
  /*	including tls_server_stop() and the underlying tls_stream(3)
  /*	and tls_bio_ops(3) routines.
  /*
  /*	With the current TLS library implementation, this means
  /*	that the application is responsible for calling and retrying
  /*	SSL_accept(), SSL_read(), SSL_write() and SSL_shutdown().
  /*
  /*	To maintain control over TLS I/O, an event-driven server
  /*	invokes tls_server_start() with a null VSTREAM argument and
  /*	with an fd argument that specifies the I/O file descriptor.
  /*	Then, tls_server_start() performs all the necessary
  /*	preparations before the TLS handshake and returns a partially
  /*	populated TLS context. The event-driven application is then
  /*	responsible for invoking SSL_accept(), and if successful,
  /*	for invoking tls_server_post_accept() to finish the work
  /*	that was started by tls_server_start(). In case of unrecoverable
  /*	failure, tls_server_post_accept() destroys the TLS context
  /*	and returns a null pointer value.
  /* LICENSE
  /* .ad
  /* .fi
  /*	This software is free. You can do with it whatever you want.
  /*	The original author kindly requests that you acknowledge
  /*	the use of his software.
  /* AUTHOR(S)
  /*	Originally written by:
  /*	Lutz Jaenicke
  /*	BTU Cottbus
  /*	Allgemeine Elektrotechnik
  /*	Universitaetsplatz 3-4
  /*	D-03044 Cottbus, Germany
  /*
  /*	Updated by:
  /*	Wietse Venema
  /*	IBM T.J. Watson Research
  /*	P.O. Box 704
  /*	Yorktown Heights, NY 10598, USA
  /*
  /*	Victor Duchovni
  /*	Morgan Stanley
  /*--*/
  
  /* System library. */
  
  #include <sys_defs.h>
  
  #ifdef USE_TLS
  #include <unistd.h>
  #include <string.h>
  
  /* Utility library. */
  
  #include <mymalloc.h>
  #include <vstring.h>
  #include <vstream.h>
  #include <dict.h>
  #include <stringops.h>
  #include <msg.h>
  #include <hex_code.h>
  #include <iostuff.h>			/* non-blocking */
  
  /* Global library. */
  
  #include <mail_params.h>
  
  /* TLS library. */
  
  #include <tls_mgr.h>
  #define TLS_INTERNAL
  #include <tls.h>
  
  #define STR(x)	vstring_str(x)
  #define LEN(x)	VSTRING_LEN(x)
  
  /* Application-specific. */
  
   /*
    * The session_id_context indentifies the service that created a session.
    * This information is used to distinguish between multiple TLS-based
    * servers running on the same server. We use the name of the mail system.
    */
  static const char server_session_id_context[] = "Postfix/TLS";
  
  #if OPENSSL_VERSION_NUMBER >= 0x1000000fL
  #define GET_SID(s, v, lptr)	((v) = SSL_SESSION_get_id((s), (lptr)))
  
  #else					/* Older OpenSSL releases */
  #define GET_SID(s, v, lptr) \
      do { (v) = (s)->session_id; *(lptr) = (s)->session_id_length; } while (0)
  
  #endif					/* OPENSSL_VERSION_NUMBER */
  
  /* get_server_session_cb - callback to retrieve session from server cache */
  
  static SSL_SESSION *get_server_session_cb(SSL *ssl, unsigned char *session_id,
  					          int session_id_length,
  					          int *unused_copy)
  {
      const char *myname = "get_server_session_cb";
      TLS_SESS_STATE *TLScontext;
      VSTRING *cache_id;
      VSTRING *session_data = vstring_alloc(2048);
      SSL_SESSION *session = 0;
  
      if ((TLScontext = SSL_get_ex_data(ssl, TLScontext_index)) == 0)
  	msg_panic("%s: null TLScontext in session lookup callback", myname);
  
  #define GEN_CACHE_ID(buf, id, len, service) \
      do { \
  	buf = vstring_alloc(2 * (len + strlen(service))); \
  	hex_encode(buf, (char *) (id), (len)); \
  	vstring_sprintf_append(buf, "&s=%s", (service)); \
  	vstring_sprintf_append(buf, "&l=%ld", (long) SSLeay()); \
      } while (0)
  
  
      GEN_CACHE_ID(cache_id, session_id, session_id_length, TLScontext->serverid);
  
      if (TLScontext->log_mask & TLS_LOG_CACHE)
  	msg_info("%s: looking up session %s in %s cache", TLScontext->namaddr,
  		 STR(cache_id), TLScontext->cache_type);
  
      /*
       * Load the session from cache and decode it.
       */
      if (tls_mgr_lookup(TLScontext->cache_type, STR(cache_id),
  		       session_data) == TLS_MGR_STAT_OK) {
  	session = tls_session_activate(STR(session_data), LEN(session_data));
  	if (session && (TLScontext->log_mask & TLS_LOG_CACHE))
  	    msg_info("%s: reloaded session %s from %s cache",
  		     TLScontext->namaddr, STR(cache_id),
  		     TLScontext->cache_type);
      }
  
      /*
       * Clean up.
       */
      vstring_free(cache_id);
      vstring_free(session_data);
  
      return (session);
  }
  
  /* uncache_session - remove session from internal & external cache */
  
  static void uncache_session(SSL_CTX *ctx, TLS_SESS_STATE *TLScontext)
  {
      VSTRING *cache_id;
      SSL_SESSION *session = SSL_get_session(TLScontext->con);
      const unsigned char *sid;
      unsigned int sid_length;
  
      SSL_CTX_remove_session(ctx, session);
  
      if (TLScontext->cache_type == 0)
  	return;
  
      GET_SID(session, sid, &sid_length);
      GEN_CACHE_ID(cache_id, sid, sid_length, TLScontext->serverid);
  
      if (TLScontext->log_mask & TLS_LOG_CACHE)
  	msg_info("%s: remove session %s from %s cache", TLScontext->namaddr,
  		 STR(cache_id), TLScontext->cache_type);
  
      tls_mgr_delete(TLScontext->cache_type, STR(cache_id));
      vstring_free(cache_id);
  }
  
  /* new_server_session_cb - callback to save session to server cache */
  
  static int new_server_session_cb(SSL *ssl, SSL_SESSION *session)
  {
      const char *myname = "new_server_session_cb";
      VSTRING *cache_id;
      TLS_SESS_STATE *TLScontext;
      VSTRING *session_data;
      const unsigned char *sid;
      unsigned int sid_length;
  
      if ((TLScontext = SSL_get_ex_data(ssl, TLScontext_index)) == 0)
  	msg_panic("%s: null TLScontext in new session callback", myname);
  
      GET_SID(session, sid, &sid_length);
      GEN_CACHE_ID(cache_id, sid, sid_length, TLScontext->serverid);
  
      if (TLScontext->log_mask & TLS_LOG_CACHE)
  	msg_info("%s: save session %s to %s cache", TLScontext->namaddr,
  		 STR(cache_id), TLScontext->cache_type);
  
      /*
       * Passivate and save the session state.
       */
      session_data = tls_session_passivate(session);
      if (session_data)
  	tls_mgr_update(TLScontext->cache_type, STR(cache_id),
  		       STR(session_data), LEN(session_data));
  
      /*
       * Clean up.
       */
      if (session_data)
  	vstring_free(session_data);
      vstring_free(cache_id);
      SSL_SESSION_free(session);			/* 200502 */
  
      return (1);
  }
  
  #define NOENGINE	((ENGINE *) 0)
  #define TLS_TKT_NOKEYS -1		/* No keys for encryption */
  #define TLS_TKT_STALE	0		/* No matching keys for decryption */
  #define TLS_TKT_ACCEPT	1		/* Ticket decryptable and re-usable */
  #define TLS_TKT_REISSUE	2		/* Ticket decryptable, not re-usable */
  
  /* ticket_cb - configure tls session ticket encrypt/decrypt context */
  
  #if defined(SSL_OP_NO_TICKET) \
      && !defined(OPENSSL_NO_TLSEXT) \
      && OPENSSL_VERSION_NUMBER >= 0x0090808fL
  
  static int ticket_cb(SSL *con, unsigned char name[], unsigned char iv[],
  		          EVP_CIPHER_CTX * ctx, HMAC_CTX * hctx, int create)
  {
      static const EVP_MD *sha256;
      static const EVP_CIPHER *ciph;
      TLS_TICKET_KEY *key;
      TLS_SESS_STATE *TLScontext = SSL_get_ex_data(con, TLScontext_index);
      int     timeout = ((int) SSL_CTX_get_timeout(SSL_get_SSL_CTX(con))) / 2;
  
      if ((!sha256 && (sha256 = EVP_sha256()) == 0)
  	|| (!ciph && (ciph = EVP_get_cipherbyname(var_tls_tkt_cipher)) == 0)
  	|| (key = tls_mgr_key(create ? 0 : name, timeout)) == 0
  	|| (create && RAND_bytes(iv, TLS_TICKET_IVLEN) <= 0))
  	return (create ? TLS_TKT_NOKEYS : TLS_TKT_STALE);
  
      HMAC_Init_ex(hctx, key->hmac, TLS_TICKET_MACLEN, sha256, NOENGINE);
  
      if (create) {
  	EVP_EncryptInit_ex(ctx, ciph, NOENGINE, key->bits, iv);
  	memcpy((void *) name, (void *) key->name, TLS_TICKET_NAMELEN);
  	if (TLScontext->log_mask & TLS_LOG_CACHE)
  	    msg_info("%s: Issuing session ticket, key expiration: %ld",
  		     TLScontext->namaddr, (long) key->tout);
      } else {
  	EVP_DecryptInit_ex(ctx, ciph, NOENGINE, key->bits, iv);
  	if (TLScontext->log_mask & TLS_LOG_CACHE)
  	    msg_info("%s: Decrypting session ticket, key expiration: %ld",
  		     TLScontext->namaddr, (long) key->tout);
      }
      TLScontext->ticketed = 1;
      return (TLS_TKT_ACCEPT);
  }
  
  #endif
  
  /* tls_server_init - initialize the server-side TLS engine */
  
  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 = (*var_tls_tkt_cipher && scache_timeout > 0
  		  && !(off & SSL_OP_NO_TICKET));
      if (ticketable) {
  	const EVP_CIPHER *ciph;
  
  	if ((ciph = EVP_get_cipherbyname(var_tls_tkt_cipher)) == 0
  	    || EVP_CIPHER_mode(ciph) != EVP_CIPH_CBC_MODE
  	    || EVP_CIPHER_iv_length(ciph) != TLS_TICKET_IVLEN
  	    || EVP_CIPHER_key_length(ciph) < TLS_TICKET_IVLEN
  	    || EVP_CIPHER_key_length(ciph) > TLS_TICKET_KEYLEN) {
  	    msg_warn("%s: invalid value: %s; session tickets disabled",
  		     VAR_TLS_TKT_CIPHER, var_tls_tkt_cipher);
  	    ticketable = 0;
  	}
      }
      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, TLS_SSL_OP_PROTOMASK(protomask));
  
      /*
       * 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);
  }
  
   /*
    * This is the actual startup routine for a new connection. We expect that
    * the SMTP buffers are flushed and the "220 Ready to start TLS" was sent to
    * the client, so that we can immediately start the TLS handshake process.
    */
  TLS_SESS_STATE *tls_server_start(const TLS_SERVER_START_PROPS *props)
  {
      int     sts;
      TLS_SESS_STATE *TLScontext;
      const char *cipher_list;
      TLS_APPL_STATE *app_ctx = props->ctx;
      int     log_mask = app_ctx->log_mask;
  
      /*
       * Implicitly enable logging of trust chain errors when verified certs
       * are required.
       */
      if (props->requirecert)
  	log_mask |= TLS_LOG_UNTRUSTED;
  
      if (log_mask & TLS_LOG_VERBOSE)
  	msg_info("setting up TLS connection from %s", props->namaddr);
  
      cipher_list = tls_set_ciphers(app_ctx, "TLS", props->cipher_grade,
  				  props->cipher_exclusions);
      if (cipher_list == 0) {
  	msg_warn("%s: %s: aborting TLS session", props->namaddr,
  		 vstring_str(app_ctx->why));
  	return (0);
      }
      if (log_mask & TLS_LOG_VERBOSE)
  	msg_info("%s: TLS cipher list \"%s\"", props->namaddr, cipher_list);
  
      /*
       * Allocate a new TLScontext for the new connection and get an SSL
       * structure. Add the location of TLScontext to the SSL to later retrieve
       * the information inside the tls_verify_certificate_callback().
       */
      TLScontext = tls_alloc_sess_context(log_mask, props->namaddr);
      TLScontext->cache_type = app_ctx->cache_type;
  
      TLScontext->serverid = mystrdup(props->serverid);
      TLScontext->am_server = 1;
      TLScontext->stream = props->stream;
      TLScontext->mdalg = props->mdalg;
  
      ERR_clear_error();
      if ((TLScontext->con = (SSL *) SSL_new(app_ctx->ssl_ctx)) == 0) {
  	msg_warn("Could not allocate 'TLScontext->con' with SSL_new()");
  	tls_print_errors();
  	tls_free_context(TLScontext);
  	return (0);
      }
      if (!SSL_set_ex_data(TLScontext->con, TLScontext_index, TLScontext)) {
  	msg_warn("Could not set application data for 'TLScontext->con'");
  	tls_print_errors();
  	tls_free_context(TLScontext);
  	return (0);
      }
  
      /*
       * Before really starting anything, try to seed the PRNG a little bit
       * more.
       */
      tls_int_seed();
      (void) tls_ext_seed(var_tls_daemon_rand_bytes);
  
      /*
       * Initialize the SSL connection to accept state. This should not be
       * necessary anymore since 0.9.3, but the call is still in the library
       * and maintaining compatibility never hurts.
       */
      SSL_set_accept_state(TLScontext->con);
  
      /*
       * Connect the SSL connection with the network socket.
       */
      if (SSL_set_fd(TLScontext->con, props->stream == 0 ? props->fd :
  		   vstream_fileno(props->stream)) != 1) {
  	msg_info("SSL_set_fd error to %s", props->namaddr);
  	tls_print_errors();
  	uncache_session(app_ctx->ssl_ctx, TLScontext);
  	tls_free_context(TLScontext);
  	return (0);
      }
  
      /*
       * If the debug level selected is high enough, all of the data is dumped:
       * TLS_LOG_TLSPKTS will dump the SSL negotiation, TLS_LOG_ALLPKTS will
       * dump everything.
       * 
       * We do have an SSL_set_fd() and now suddenly a BIO_ routine is called?
       * Well there is a BIO below the SSL routines that is automatically
       * created for us, so we can use it for debugging purposes.
       */
      if (log_mask & TLS_LOG_TLSPKTS)
  	BIO_set_callback(SSL_get_rbio(TLScontext->con), tls_bio_dump_cb);
  
      /*
       * If we don't trigger the handshake in the library, leave control over
       * SSL_accept/read/write/etc with the application.
       */
      if (props->stream == 0)
  	return (TLScontext);
  
      /*
       * Turn on non-blocking I/O so that we can enforce timeouts on network
       * I/O.
       */
      non_blocking(vstream_fileno(props->stream), NON_BLOCKING);
  
      /*
       * Start TLS negotiations. This process is a black box that invokes our
       * call-backs for session caching and certificate verification.
       * 
       * Error handling: If the SSL handhake fails, we print out an error message
       * and remove all TLS state concerning this session.
       */
      sts = tls_bio_accept(vstream_fileno(props->stream), props->timeout,
  			 TLScontext);
      if (sts <= 0) {
  	if (ERR_peek_error() != 0) {
  	    msg_info("SSL_accept error from %s: %d", props->namaddr, sts);
  	    tls_print_errors();
  	} else if (errno != 0) {
  	    msg_info("SSL_accept error from %s: %m", props->namaddr);
  	} else {
  	    msg_info("SSL_accept error from %s: lost connection",
  		     props->namaddr);
  	}
  	tls_free_context(TLScontext);
  	return (0);
      }
      return (tls_server_post_accept(TLScontext));
  }
  
  /* tls_server_post_accept - post-handshake processing */
  
  TLS_SESS_STATE *tls_server_post_accept(TLS_SESS_STATE *TLScontext)
  {
      SSL_CIPHER_const SSL_CIPHER *cipher;
      X509   *peer;
      char    buf[CCERT_BUFSIZ];
  
      /* Turn off packet dump if only dumping the handshake */
      if ((TLScontext->log_mask & TLS_LOG_ALLPKTS) == 0)
  	BIO_set_callback(SSL_get_rbio(TLScontext->con), 0);
  
      /*
       * The caller may want to know if this session was reused or if a new
       * session was negotiated.
       */
      TLScontext->session_reused = SSL_session_reused(TLScontext->con);
      if ((TLScontext->log_mask & TLS_LOG_CACHE) && TLScontext->session_reused)
  	msg_info("%s: Reusing old session%s", TLScontext->namaddr,
  		 TLScontext->ticketed ? " (RFC 5077 session ticket)" : "");
  
      /*
       * Let's see whether a peer certificate is available and what is the
       * actual information. We want to save it for later use.
       */
      peer = SSL_get_peer_certificate(TLScontext->con);
      if (peer != NULL) {
  	TLScontext->peer_status |= TLS_CERT_FLAG_PRESENT;
  	if (SSL_get_verify_result(TLScontext->con) == X509_V_OK)
  	    TLScontext->peer_status |= TLS_CERT_FLAG_TRUSTED;
  
  	if (TLScontext->log_mask & TLS_LOG_VERBOSE) {
  	    X509_NAME_oneline(X509_get_subject_name(peer),
  			      buf, sizeof(buf));
  	    msg_info("subject=%s", printable(buf, '?'));
  	    X509_NAME_oneline(X509_get_issuer_name(peer),
  			      buf, sizeof(buf));
  	    msg_info("issuer=%s", printable(buf, '?'));
  	}
  	TLScontext->peer_CN = tls_peer_CN(peer, TLScontext);
  	TLScontext->issuer_CN = tls_issuer_CN(peer, TLScontext);
  	TLScontext->peer_cert_fprint = tls_cert_fprint(peer, TLScontext->mdalg);
  	TLScontext->peer_pkey_fprint = tls_pkey_fprint(peer, TLScontext->mdalg);
  
  	if (TLScontext->log_mask & (TLS_LOG_VERBOSE | TLS_LOG_PEERCERT)) {
  	    msg_info("%s: subject_CN=%s, issuer=%s, fingerprint=%s"
  		     ", pkey_fingerprint=%s",
  		     TLScontext->namaddr,
  		     TLScontext->peer_CN, TLScontext->issuer_CN,
  		     TLScontext->peer_cert_fprint,
  		     TLScontext->peer_pkey_fprint);
  	}
  	X509_free(peer);
      } else {
  	TLScontext->peer_CN = mystrdup("");
  	TLScontext->issuer_CN = mystrdup("");
  	TLScontext->peer_cert_fprint = mystrdup("");
  	TLScontext->peer_pkey_fprint = mystrdup("");
      }
  
      /*
       * Finally, collect information about protocol and cipher for logging
       */
      TLScontext->protocol = SSL_get_version(TLScontext->con);
      cipher = SSL_get_current_cipher(TLScontext->con);
      TLScontext->cipher_name = SSL_CIPHER_get_name(cipher);
      TLScontext->cipher_usebits = SSL_CIPHER_get_bits(cipher,
  					     &(TLScontext->cipher_algbits));
  
      /*
       * If the library triggered the SSL handshake, switch to the
       * tls_timed_read/write() functions and make the TLScontext available to
       * those functions. Otherwise, leave control over SSL_read/write/etc.
       * with the application.
       */
      if (TLScontext->stream != 0)
  	tls_stream_start(TLScontext->stream, TLScontext);
  
      /*
       * All the key facts in a single log entry.
       */
      if (TLScontext->log_mask & TLS_LOG_SUMMARY)
  	msg_info("%s TLS connection established from %s: %s with cipher %s "
  	      "(%d/%d bits)", !TLS_CERT_IS_PRESENT(TLScontext) ? "Anonymous"
  		 : TLS_CERT_IS_TRUSTED(TLScontext) ? "Trusted" : "Untrusted",
  	 TLScontext->namaddr, TLScontext->protocol, TLScontext->cipher_name,
  		 TLScontext->cipher_usebits, TLScontext->cipher_algbits);
  
      tls_int_seed();
  
      return (TLScontext);
  }
  
  #endif					/* USE_TLS */
  