tls_server.c

[詳解]

/*++
/* 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 */

 /* OpenSSL 1.1.0 bitrot */
#if OPENSSL_VERSION_NUMBER >= 0x10100000L
typedef const unsigned char *session_id_t;

#else
typedef unsigned char *session_id_t;

#endif

/* get_server_session_cb - callback to retrieve session from server cache */

static SSL_SESSION *get_server_session_cb(SSL *ssl, session_id_t 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) OpenSSL_version_num()); \
    } 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();

#if OPENSSL_VERSION_NUMBER < 0x10100000L

    /*
     * 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();
#endif

    /*
     * 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();
    server_ctx = SSL_CTX_new(TLS_server_method());
    if (server_ctx == 0) {
    msg_warn("cannot allocate server SSL_CTX: disabling TLS support");
    tls_print_errors();
    return (0);
    }
#ifdef SSL_SECOP_PEER
    /* Backwards compatible security as a base for opportunistic TLS. */
    SSL_CTX_set_security_level(server_ctx, 0);
#endif

    /*
     * 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);
    }

    /*
     * 2015-12-05: Ephemeral RSA removed from OpenSSL 1.1.0-dev
     */
#if OPENSSL_VERSION_NUMBER < 0x10100000L

    /*
     * 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);
#endif

    /*
     * 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.
     */
    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);
    }
#ifdef SSL_SECOP_PEER
    /* When authenticating the peer, use 80-bit plus OpenSSL security level */
    if (props->requirecert)
    SSL_set_security_level(TLScontext->con, 1);
#endif

    /*
     * 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);

    /*
     * Give them a clue. Problems with trust chain verification are
     * logged when the session is first negotiated, before the session is
     * stored into the cache. We don't want mystery failures, so log the
     * fact the real problem is to be found in the past.
     */
    if (!TLS_CERT_IS_TRUSTED(TLScontext)
        && (TLScontext->log_mask & TLS_LOG_UNTRUSTED)) {
        if (TLScontext->session_reused == 0)
        tls_log_verify_error(TLScontext);
        else
        msg_info("%s: re-using session with untrusted certificate, "
             "look for details earlier in the log",
             TLScontext->namaddr);
    }
    } 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 */