path: root/src/tls/tls_client.c

/*++
/* NAME
/*	tls_client
/* SUMMARY
/*	client-side TLS engine
/* SYNOPSIS
/*	#include <tls.h>
/*
/*	TLS_APPL_STATE *tls_client_init(init_props)
/*	const TLS_CLIENT_INIT_PROPS *init_props;
/*
/*	TLS_SESS_STATE *tls_client_start(start_props)
/*	const TLS_CLIENT_START_PROPS *start_props;
/*
/*	TLS_SESS_STATE *tls_client_post_connect(TLScontext, start_props)
/*	TLS_SESS_STATE *TLScontext;
/*	const TLS_CLIENT_START_PROPS *start_props;
/*
/*	void	tls_client_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 clients,
/*	the OpenSSL library and the TLS entropy and cache manager.
/*
/*	The SMTP client will attempt to verify the server hostname
/*	against the names listed in the server certificate. When
/*	a hostname match is required, the verification fails
/*	on certificate verification or hostname mis-match errors.
/*	When no hostname match is required, hostname verification
/*	failures are logged but they do not affect the TLS handshake
/*	or the SMTP session.
/*
/*	The rules for peer name wild-card matching differ between
/*	RFC 2818 (HTTP over TLS) and RFC 2830 (LDAP over TLS), while
/*	RFC RFC3207 (SMTP over TLS) does not specify a rule at all.
/*	Postfix uses a restrictive match algorithm. One asterisk
/*	('*') is allowed as the left-most component of a wild-card
/*	certificate name; it matches the left-most component of
/*	the peer hostname.
/*
/*	Another area where RFCs aren't always explicit is the
/*	handling of dNSNames in peer certificates. RFC 3207 (SMTP
/*	over TLS) does not mention dNSNames. Postfix follows the
/*	strict rules in RFC 2818 (HTTP over TLS), section 3.1: The
/*	Subject Alternative Name/dNSName has precedence over
/*	CommonName.  If at least one dNSName is provided, Postfix
/*	verifies those against the peer hostname and ignores the
/*	CommonName, otherwise Postfix verifies the CommonName
/*	against the peer hostname.
/*
/*	tls_client_init() is called once when the SMTP client
/*	initializes.
/*	Certificate details are also decided during this phase,
/*	so peer-specific certificate selection is not possible.
/*
/*	tls_client_start() activates the TLS session over an established
/*	stream. We expect that network buffers are flushed and
/*	the TLS handshake can begin immediately.
/*
/*	tls_client_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. This consists of one or more of
/*	TLS_CERT_FLAG_PRESENT, TLS_CERT_FLAG_ALTNAME, TLS_CERT_FLAG_TRUSTED,
/*	TLS_CERT_FLAG_MATCHED and TLS_CERT_FLAG_SECURED.
/* .IP TLScontext->peer_CN
/*	Extracted CommonName of the peer, or zero-length string if the
/*	information could not be extracted.
/* .IP TLScontext->issuer_CN
/*	Extracted CommonName of the issuer, or zero-length string if the
/*	information could not be extracted.
/* .IP TLScontext->peer_cert_fprint
/*	At the fingerprint security level, if the peer presented a certificate
/*	the fingerprint of the certificate.
/* .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 TLS library historically provides,
/*	including tls_client_stop() and the underlying tls_stream(3)
/*	and tls_bio_ops(3) routines.
/*
/*	With the current TLS library implementation, this means
/*	that an event-driven application is responsible for calling
/*	and retrying SSL_connect(), SSL_read(), SSL_write() and
/*	SSL_shutdown().
/*
/*	To maintain control over TLS I/O, an event-driven client
/*	invokes tls_client_start() with a null VSTREAM argument and
/*	with an fd argument that specifies the I/O file descriptor.
/*	Then, tls_client_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_connect(), and if successful,
/*	for invoking tls_client_post_connect() to finish the work
/*	that was started by tls_client_start(). In case of unrecoverable
/*	failure, tls_client_post_connect() 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
/*
/*	Wietse Venema
/*	Google, Inc.
/*	111 8th Avenue
/*	New York, NY 10011, USA
/*
/*	Victor Duchovni
/*	Morgan Stanley
/*--*/

/* System library. */

#include <sys_defs.h>

#ifdef USE_TLS
#include <string.h>

#ifdef STRCASECMP_IN_STRINGS_H
#include <strings.h>
#endif

/* Utility library. */

#include <argv.h>
#include <mymalloc.h>
#include <vstring.h>
#include <vstream.h>
#include <stringops.h>
#include <msg.h>
#include <iostuff.h>			/* non-blocking */
#include <midna_domain.h>

/* Global library. */

#include <mail_params.h>

/* TLS library. */

#include <tls_mgr.h>
#define TLS_INTERNAL
#include <tls.h>

/* Application-specific. */

#define STR	vstring_str
#define LEN	VSTRING_LEN

/* load_clnt_session - load session from client cache (non-callback) */

static SSL_SESSION *load_clnt_session(TLS_SESS_STATE *TLScontext)
{
    const char *myname = "load_clnt_session";
    SSL_SESSION *session = 0;
    VSTRING *session_data = vstring_alloc(2048);

    /*
     * Prepare the query.
     */
    if (TLScontext->log_mask & TLS_LOG_CACHE)
	/* serverid contains transport:addr:port information */
	msg_info("looking for session %s in %s cache",
		 TLScontext->serverid, TLScontext->cache_type);

    /*
     * We only get here if the cache_type is not empty. This code is not
     * called unless caching is enabled and the cache_type is stored in the
     * server SSL context.
     */
    if (TLScontext->cache_type == 0)
	msg_panic("%s: null client session cache type in session lookup",
		  myname);

    /*
     * Look up and activate the SSL_SESSION object. Errors are non-fatal,
     * since caching is only an optimization.
     */
    if (tls_mgr_lookup(TLScontext->cache_type, TLScontext->serverid,
		       session_data) == TLS_MGR_STAT_OK) {
	session = tls_session_activate(STR(session_data), LEN(session_data));
	if (session) {
	    if (TLScontext->log_mask & TLS_LOG_CACHE)
		/* serverid contains transport:addr:port information */
		msg_info("reloaded session %s from %s cache",
			 TLScontext->serverid, TLScontext->cache_type);
	}
    }

    /*
     * Clean up.
     */
    vstring_free(session_data);

    return (session);
}

/* new_client_session_cb - name new session and save it to client cache */

static int new_client_session_cb(SSL *ssl, SSL_SESSION *session)
{
    const char *myname = "new_client_session_cb";
    TLS_SESS_STATE *TLScontext;
    VSTRING *session_data;

    /*
     * The cache name (if caching is enabled in tlsmgr(8)) and the cache ID
     * string for this session are stored in the TLScontext. It cannot be
     * null at this point.
     */
    if ((TLScontext = SSL_get_ex_data(ssl, TLScontext_index)) == 0)
	msg_panic("%s: null TLScontext in new session callback", myname);

    /*
     * We only get here if the cache_type is not empty. This callback is not
     * set unless caching is enabled and the cache_type is stored in the
     * server SSL context.
     */
    if (TLScontext->cache_type == 0)
	msg_panic("%s: null session cache type in new session callback",
		  myname);

    if (TLScontext->log_mask & TLS_LOG_CACHE)
	/* serverid contains transport:addr:port information */
	msg_info("save session %s to %s cache",
		 TLScontext->serverid, TLScontext->cache_type);

    /*
     * Passivate and save the session object. Errors are non-fatal, since
     * caching is only an optimization.
     */
    if ((session_data = tls_session_passivate(session)) != 0) {
	tls_mgr_update(TLScontext->cache_type, TLScontext->serverid,
		       STR(session_data), LEN(session_data));
	vstring_free(session_data);
    }

    /*
     * Clean up.
     */
    SSL_SESSION_free(session);			/* 200502 */

    return (1);
}

/* uncache_session - remove session from the external cache */

static void uncache_session(SSL_CTX *ctx, TLS_SESS_STATE *TLScontext)
{
    SSL_SESSION *session = SSL_get_session(TLScontext->con);

    SSL_CTX_remove_session(ctx, session);
    if (TLScontext->cache_type == 0 || TLScontext->serverid == 0)
	return;

    if (TLScontext->log_mask & TLS_LOG_CACHE)
	/* serverid contains transport:addr:port information */
	msg_info("remove session %s from client cache", TLScontext->serverid);

    tls_mgr_delete(TLScontext->cache_type, TLScontext->serverid);
}

/* tls_client_init - initialize client-side TLS engine */

TLS_APPL_STATE *tls_client_init(const TLS_CLIENT_INIT_PROPS *props)
{
    long    off = 0;
    int     cachable;
    int     scache_timeout;
    SSL_CTX *client_ctx;
    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 client-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

    /*
     * Initialize the OpenSSL library, possibly loading its configuration
     * file.
     */
    if (tls_library_init() == 0)
	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. RFC2487 is only specified for TLSv1, but
     * we want to be as compatible as possible, so we will start off with a
     * SSLv2 greeting allowing the best we can offer: TLSv1. We can restrict
     * this with the options setting later, anyhow.
     */
    ERR_clear_error();
    client_ctx = SSL_CTX_new(TLS_client_method());
    if (client_ctx == 0) {
	msg_warn("cannot allocate client 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(client_ctx, 0);
#endif

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

    /*
     * Presently we use TLS only with SMTP where truncation attacks are not
     * possible as a result of application framing.  If we ever use TLS in
     * some other application protocol where truncation could be relevant,
     * we'd need to disable truncation detection conditionally, or explicitly
     * clear the option in that code path.
     */
    off |= SSL_OP_IGNORE_UNEXPECTED_EOF;

    /*
     * Protocol selection is destination dependent, so we delay the protocol
     * selection options to the per-session SSL object.
     */
    off |= tls_bug_bits();
    SSL_CTX_set_options(client_ctx, off);

    /* Enable all supported protocols */
#if OPENSSL_VERSION_NUMBER >= 0x1010000fUL
    SSL_CTX_set_min_proto_version(client_ctx, 0);
#endif

    /*
     * Set the call-back routine for verbose logging.
     */
    if (log_mask & TLS_LOG_DEBUG)
	SSL_CTX_set_info_callback(client_ctx, tls_info_callback);

    /*
     * Load the CA public key certificates for both the client cert and for
     * the verification of server 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(client_ctx,
				    props->CAfile, props->CApath) < 0) {
	/* tls_set_ca_certificate_info() already logs a warning. */
	SSL_CTX_free(client_ctx);		/* 200411 */
	return (0);
    }

    /*
     * We do not need a client certificate, so the certificates are only
     * loaded (and checked) if supplied. A clever client would handle
     * multiple client certificates and decide based on the list of
     * acceptable CAs, sent by the server, which certificate to submit.
     * OpenSSL does however not do this and also has no call-back hooks to
     * easily implement it.
     * 
     * Load the client 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). The client
     * certificate is presented after the server chooses the session cipher,
     * so we will just present the right cert for the chosen cipher (if it
     * uses certificates).
     */
    if (tls_set_my_certificate_key_info(client_ctx,
					props->chain_files,
					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(client_ctx);		/* 200411 */
	return (0);
    }

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

    /*
     * According to the OpenSSL documentation, temporary RSA key is needed
     * export ciphers are in use. We have to provide one, so well, we just do
     * it.
     */
    SSL_CTX_set_tmp_rsa_callback(client_ctx, tls_tmp_rsa_cb);
#endif

    /*
     * With OpenSSL 1.0.2 and later the client EECDH curve list becomes
     * configurable with the preferred curve negotiated via the supported
     * curves extension.
     */
    tls_auto_eecdh_curves(client_ctx, var_tls_eecdh_auto);

    /*
     * Finally, the setup for the server certificate checking, done "by the
     * book".
     */
    SSL_CTX_set_verify(client_ctx, SSL_VERIFY_NONE,
		       tls_verify_certificate_callback);

    /*
     * Initialize the session cache.
     * 
     * Since the client does not search an internal cache, we simply disable it.
     * It is only useful for expiring old sessions, but we do that in the
     * tlsmgr(8).
     * 
     * 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).
     */
    if (tls_mgr_policy(props->cache_type, &cachable,
		       &scache_timeout) != TLS_MGR_STAT_OK)
	scache_timeout = 0;
    if (scache_timeout <= 0)
	cachable = 0;

    /*
     * Allocate an application context, and populate with mandatory protocol
     * and cipher data.
     */
    app_ctx = tls_alloc_app_context(client_ctx, 0, log_mask);

    /*
     * The external session cache is implemented by the tlsmgr(8) process.
     */
    if (cachable) {

	app_ctx->cache_type = mystrdup(props->cache_type);

	/*
	 * OpenSSL does not use callbacks to load sessions from a client
	 * cache, so we must invoke that function directly. Apparently,
	 * OpenSSL does not provide a way to pass session names from here to
	 * call-back routines that do session lookup.
	 * 
	 * OpenSSL can, however, automatically save newly created sessions for
	 * us by callback (we create the session name in the call-back
	 * function).
	 * 
	 * XXX gcc 2.95 can't compile #ifdef .. #endif in the expansion of
	 * SSL_SESS_CACHE_CLIENT | SSL_SESS_CACHE_NO_INTERNAL_STORE |
	 * SSL_SESS_CACHE_NO_AUTO_CLEAR.
	 */
#ifndef SSL_SESS_CACHE_NO_INTERNAL_STORE
#define SSL_SESS_CACHE_NO_INTERNAL_STORE 0
#endif

	SSL_CTX_set_session_cache_mode(client_ctx,
				       SSL_SESS_CACHE_CLIENT |
				       SSL_SESS_CACHE_NO_INTERNAL_STORE |
				       SSL_SESS_CACHE_NO_AUTO_CLEAR);
	SSL_CTX_sess_set_new_cb(client_ctx, new_client_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 to
	 * twice the cache lifetime.  This way a session always lasts longer
	 * than its lifetime in the cache.
	 */
	SSL_CTX_set_timeout(client_ctx, 2 * scache_timeout);
    }
    return (app_ctx);
}

/* match_servername -  match servername against pattern */

static int match_servername(const char *certid,
			            const TLS_CLIENT_START_PROPS *props)
{
    const ARGV *cmatch_argv;
    const char *nexthop = props->nexthop;
    const char *hname = props->host;
    const char *domain;
    const char *parent;
    const char *aname;
    int     match_subdomain;
    int     i;
    int     idlen;
    int     domlen;

    if ((cmatch_argv = props->matchargv) == 0)
	return 0;

#ifndef NO_EAI

    /*
     * DNS subjectAltNames are required to be ASCII.
     * 
     * Per RFC 6125 Section 6.4.4 Matching the CN-ID, follows the same rules
     * (6.4.1, 6.4.2 and 6.4.3) that apply to subjectAltNames.  In
     * particular, 6.4.2 says that the reference identifier is coerced to
     * ASCII, but no conversion is stated or implied for the CN-ID, so it
     * seems it only matches if it is all ASCII.  Otherwise, it is some other
     * sort of name.
     */
    if (!allascii(certid))
	return (0);
    if (!allascii(nexthop) && (aname = midna_domain_to_ascii(nexthop)) != 0) {
	if (msg_verbose)
	    msg_info("%s asciified to %s", nexthop, aname);
	nexthop = aname;
    }
#endif

    /*
     * Match the certid against each pattern until we find a match.
     */
    for (i = 0; i < cmatch_argv->argc; ++i) {
	match_subdomain = 0;
	if (!strcasecmp(cmatch_argv->argv[i], "nexthop"))
	    domain = nexthop;
	else if (!strcasecmp(cmatch_argv->argv[i], "hostname"))
	    domain = hname;
	else if (!strcasecmp(cmatch_argv->argv[i], "dot-nexthop")) {
	    domain = nexthop;
	    match_subdomain = 1;
	} else {
	    domain = cmatch_argv->argv[i];
	    if (*domain == '.') {
		if (domain[1]) {
		    ++domain;
		    match_subdomain = 1;
		}
	    }
#ifndef NO_EAI

	    /*
	     * Besides U+002E (full stop) IDNA2003 allows labels to be
	     * separated by any of the Unicode variants U+3002 (ideographic
	     * full stop), U+FF0E (fullwidth full stop), and U+FF61
	     * (halfwidth ideographic full stop). Their respective UTF-8
	     * encodings are: E38082, EFBC8E and EFBDA1.
	     * 
	     * IDNA2008 does not permit (upper) case and other variant
	     * differences in U-labels. The midna_domain_to_ascii() function,
	     * based on UTS46, normalizes such differences away.
	     * 
	     * The IDNA to_ASCII conversion does not allow empty leading labels,
	     * so we handle these explicitly here.
	     */
	    else {
		unsigned char *cp = (unsigned char *) domain;

		if ((cp[0] == 0xe3 && cp[1] == 0x80 && cp[2] == 0x82)
		    || (cp[0] == 0xef && cp[1] == 0xbc && cp[2] == 0x8e)
		    || (cp[0] == 0xef && cp[1] == 0xbd && cp[2] == 0xa1)) {
		    if (domain[3]) {
			domain = domain + 3;
			match_subdomain = 1;
		    }
		}
	    }
	    if (!allascii(domain)
		&& (aname = midna_domain_to_ascii(domain)) != 0) {
		if (msg_verbose)
		    msg_info("%s asciified to %s", domain, aname);
		domain = aname;
	    }
#endif
	}

	/*
	 * Sub-domain match: certid is any sub-domain of hostname.
	 */
	if (match_subdomain) {
	    if ((idlen = strlen(certid)) > (domlen = strlen(domain)) + 1
		&& certid[idlen - domlen - 1] == '.'
		&& !strcasecmp(certid + (idlen - domlen), domain))
		return (1);
	    else
		continue;
	}

	/*
	 * Exact match and initial "*" match. The initial "*" in a certid
	 * matches one (if var_tls_multi_label is false) or more hostname
	 * components under the condition that the certid contains multiple
	 * hostname components.
	 */
	if (!strcasecmp(certid, domain)
	    || (certid[0] == '*' && certid[1] == '.' && certid[2] != 0
		&& (parent = strchr(domain, '.')) != 0
		&& (idlen = strlen(certid + 1)) <= (domlen = strlen(parent))
		&& strcasecmp(var_tls_multi_wildcard == 0 ? parent :
			      parent + domlen - idlen,
			      certid + 1) == 0))
	    return (1);
    }
    return (0);
}

/* verify_extract_name - verify peer name and extract peer information */

static void verify_extract_name(TLS_SESS_STATE *TLScontext, X509 *peercert,
				        const TLS_CLIENT_START_PROPS *props)
{
    int     i;
    int     r;
    int     matched = 0;
    int     dnsname_match;
    int     verify_peername = 0;
    int     log_certmatch;
    int     verbose;
    const char *dnsname;
    const GENERAL_NAME *gn;
    general_name_stack_t *gens;

    /*
     * On exit both peer_CN and issuer_CN should be set.
     */
    TLScontext->issuer_CN = tls_issuer_CN(peercert, TLScontext);

    /*
     * Is the certificate trust chain valid and trusted?
     */
    if (SSL_get_verify_result(TLScontext->con) == X509_V_OK)
	TLScontext->peer_status |= TLS_CERT_FLAG_TRUSTED;

    /*
     * With fingerprint or dane we may already be done. Otherwise, verify the
     * peername if using traditional PKI or DANE with trust-anchors.
     */
    if (!TLS_CERT_IS_MATCHED(TLScontext)
	&& TLS_CERT_IS_TRUSTED(TLScontext)
	&& TLS_MUST_TRUST(props->tls_level))
	verify_peername = 1;

    /* Force cert processing so we can log the data? */
    log_certmatch = TLScontext->log_mask & TLS_LOG_CERTMATCH;

    /* Log cert details when processing? */
    verbose = log_certmatch || (TLScontext->log_mask & TLS_LOG_VERBOSE);

    if (verify_peername || log_certmatch) {

	/*
	 * Verify the dNSName(s) in the peer certificate against the nexthop
	 * and hostname.
	 * 
	 * If DNS names are present, we use the first matching (or else simply
	 * the first) DNS name as the subject CN. The CommonName in the
	 * issuer DN is obsolete when SubjectAltName is available. This
	 * yields much less surprising logs, because we log the name we
	 * verified or a name we checked and failed to match.
	 * 
	 * XXX: The nexthop and host name may both be the same network address
	 * rather than a DNS name. In this case we really should be looking
	 * for GEN_IPADD entries, not GEN_DNS entries.
	 * 
	 * XXX: In ideal world the caller who used the address to build the
	 * connection would tell us that the nexthop is the connection
	 * address, but if that is not practical, we can parse the nexthop
	 * again here.
	 */
	gens = X509_get_ext_d2i(peercert, NID_subject_alt_name, 0, 0);
	if (gens) {
	    r = sk_GENERAL_NAME_num(gens);
	    for (i = 0; i < r; ++i) {
		gn = sk_GENERAL_NAME_value(gens, i);
		if (gn->type != GEN_DNS)
		    continue;

		/*
		 * Even if we have an invalid DNS name, we still ultimately
		 * ignore the CommonName, because subjectAltName:DNS is
		 * present (though malformed). Replace any previous peer_CN
		 * if empty or we get a match.
		 * 
		 * We always set at least an empty peer_CN if the ALTNAME cert
		 * flag is set. If not, we set peer_CN from the cert
		 * CommonName below, so peer_CN is always non-null on return.
		 */
		TLScontext->peer_status |= TLS_CERT_FLAG_ALTNAME;
		dnsname = tls_dns_name(gn, TLScontext);
		if (dnsname && *dnsname) {
		    if ((dnsname_match = match_servername(dnsname, props)) != 0)
			matched++;
		    /* Keep the first matched name. */
		    if (TLScontext->peer_CN
			&& ((dnsname_match && matched == 1)
			    || *TLScontext->peer_CN == 0)) {
			myfree(TLScontext->peer_CN);
			TLScontext->peer_CN = 0;
		    }
		    if (verbose)
			msg_info("%s: %ssubjectAltName: %s", props->namaddr,
				 dnsname_match ? "Matched " : "", dnsname);
		}
		if (TLScontext->peer_CN == 0)
		    TLScontext->peer_CN = mystrdup(dnsname ? dnsname : "");
		if (matched && !log_certmatch)
		    break;
	    }
	    if (verify_peername && matched)
		TLScontext->peer_status |= TLS_CERT_FLAG_MATCHED;

	    /*
	     * (Sam Rushing, Ironport) Free stack *and* member GENERAL_NAME
	     * objects
	     */
	    sk_GENERAL_NAME_pop_free(gens, GENERAL_NAME_free);
	}

	/*
	 * No subjectAltNames, peer_CN is taken from CommonName.
	 */
	if (TLScontext->peer_CN == 0) {
	    TLScontext->peer_CN = tls_peer_CN(peercert, TLScontext);
	    if (*TLScontext->peer_CN)
		matched = match_servername(TLScontext->peer_CN, props);
	    if (verify_peername && matched)
		TLScontext->peer_status |= TLS_CERT_FLAG_MATCHED;
	    if (verbose)
		msg_info("%s %sCommonName %s", props->namaddr,
			 matched ? "Matched " : "", TLScontext->peer_CN);
	} else if (verbose) {
	    char   *tmpcn = tls_peer_CN(peercert, TLScontext);

	    /*
	     * Though the CommonName was superceded by a subjectAltName, log
	     * it when certificate match debugging was requested.
	     */
	    msg_info("%s CommonName %s", TLScontext->namaddr, tmpcn);
	    myfree(tmpcn);
	}
    } else
	TLScontext->peer_CN = tls_peer_CN(peercert, TLScontext);

    /*
     * 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", props->namaddr);
    }
}

/* verify_extract_print - extract and verify peer fingerprint */

static void verify_extract_print(TLS_SESS_STATE *TLScontext, X509 *peercert,
				         const TLS_CLIENT_START_PROPS *props)
{
    TLScontext->peer_cert_fprint = tls_cert_fprint(peercert, props->mdalg);
    TLScontext->peer_pkey_fprint = tls_pkey_fprint(peercert, props->mdalg);

    /*
     * Whether the level is "dane" or "fingerprint" when the peer certificate
     * is matched without resorting to a separate CA, we set both the trusted
     * and matched bits.  This simplifies logic in smtp_proto.c where "dane"
     * must be trusted and matched, since some "dane" TLSA RRsets do use CAs.
     * 
     * This also suppresses spurious logging of the peer certificate as
     * untrusted in verify_extract_name().
     */
    if (TLS_DANE_HASEE(props->dane)
	&& tls_dane_match(TLScontext, TLS_DANE_EE, peercert, 0))
	TLScontext->peer_status |=
	    TLS_CERT_FLAG_TRUSTED | TLS_CERT_FLAG_MATCHED;
}

 /*
  * This is the actual startup routine for the connection. We expect that the
  * buffers are flushed and the "220 Ready to start TLS" was received by us,
  * so that we can immediately start the TLS handshake process.
  */
TLS_SESS_STATE *tls_client_start(const TLS_CLIENT_START_PROPS *props)
{
    int     sts;
    int     protomask;
    const char *cipher_list;
    SSL_SESSION *session = 0;
    TLS_SESS_STATE *TLScontext;
    TLS_APPL_STATE *app_ctx = props->ctx;
    const char *sni = 0;
    char   *myserverid;
    int     log_mask = app_ctx->log_mask;

    /*
     * When certificate verification is required, log trust chain validation
     * errors even when disabled by default for opportunistic sessions. For
     * DANE this only applies when using trust-anchor associations.
     */
    if (TLS_MUST_TRUST(props->tls_level)
      && (!TLS_DANE_BASED(props->tls_level) || TLS_DANE_HASTA(props->dane)))
	log_mask |= TLS_LOG_UNTRUSTED;

    if (log_mask & TLS_LOG_VERBOSE)
	msg_info("setting up TLS connection to %s", props->namaddr);

    /*
     * First make sure we have valid protocol and cipher parameters
     * 
     * Per-session protocol restrictions must be applied to the SSL connection,
     * as restrictions in the global context cannot be cleared.
     */
    protomask = tls_protocol_mask(props->protocols);
    if (protomask == TLS_PROTOCOL_INVALID) {
	/* tls_protocol_mask() logs no warning. */
	msg_warn("%s: Invalid TLS protocol list \"%s\": aborting TLS session",
		 props->namaddr, props->protocols);
	return (0);
    }
    /* DANE requires SSLv3 or later, not SSLv2. */
    if (TLS_DANE_BASED(props->tls_level))
	protomask |= TLS_PROTOCOL_SSLv2;

    /*
     * 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().
     * 
     * If session caching was enabled when TLS was initialized, the cache type
     * is stored in the client SSL context.
     */
    TLScontext = tls_alloc_sess_context(log_mask, props->namaddr);
    TLScontext->cache_type = app_ctx->cache_type;

    if ((TLScontext->con = SSL_new(app_ctx->ssl_ctx)) == NULL) {
	msg_warn("Could not allocate 'TLScontext->con' with SSL_new()");
	tls_print_errors();
	tls_free_context(TLScontext);
	return (0);
    }

    /*
     * Per session cipher selection for sessions with mandatory encryption
     * 
     * The cipherlist is applied to the global SSL context, since it is likely
     * to stay the same between connections, so we make use of a 1-element
     * cache to return the same result for identical inputs.
     */
    cipher_list = tls_set_ciphers(TLScontext, props->cipher_grade,
				  props->cipher_exclusions);
    if (cipher_list == 0) {
	/* already warned */
	tls_free_context(TLScontext);
	return (0);
    }
    if (log_mask & TLS_LOG_VERBOSE)
	msg_info("%s: TLS cipher list \"%s\"", props->namaddr, cipher_list);

    /*
     * OpenSSL will ignore cached sessions that use the wrong protocol. So we
     * do not need to filter out cached sessions with the "wrong" protocol,
     * rather OpenSSL will simply negotiate a new session.
     * 
     * We salt the session lookup key with the protocol list, so that sessions
     * found in the cache are plausibly acceptable.
     * 
     * By the time a TLS client is negotiating ciphers it has already offered to
     * re-use a session, it is too late to renege on the offer. So we must
     * not attempt to re-use sessions whose ciphers are too weak. We salt the
     * session lookup key with the cipher list, so that sessions found in the
     * cache are always acceptable.
     * 
     * With DANE, (more generally any TLScontext where we specified explicit
     * trust-anchor or end-entity certificates) the verification status of
     * the SSL session depends on the specified list.  Since we verify the
     * certificate only during the initial handshake, we must segregate
     * sessions with different TA lists.  Note, that TA re-verification is
     * not possible with cached sessions, since these don't hold the complete
     * peer trust chain.  Therefore, we compute a digest of the sorted TA
     * parameters and append it to the serverid.
     */
    myserverid = tls_serverid_digest(props, protomask, cipher_list);

    TLScontext->serverid = myserverid;
    TLScontext->stream = props->stream;
    TLScontext->mdalg = props->mdalg;

    /* Alias DANE digest info from props */
    TLScontext->dane = props->dane;

    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);
    }

    /*
     * Apply session protocol restrictions.
     */
    if (protomask != 0)
	SSL_set_options(TLScontext->con, TLS_SSL_OP_PROTOMASK(protomask));

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

    /*
     * XXX To avoid memory leaks we must always call SSL_SESSION_free() after
     * calling SSL_set_session(), regardless of whether or not the session
     * will be reused.
     */
    if (TLScontext->cache_type) {
	session = load_clnt_session(TLScontext);
	if (session) {
	    SSL_set_session(TLScontext->con, session);
	    SSL_SESSION_free(session);		/* 200411 */
	}
    }
#ifdef TLSEXT_MAXLEN_host_name
    if (TLS_DANE_BASED(props->tls_level)) {

	/*
	 * With DANE sessions, send an SNI hint.  We don't care whether the
	 * server reports finding a matching certificate or not, so no
	 * callback is required to process the server response.  Our use of
	 * SNI is limited to giving servers that are (mis)configured to use
	 * SNI the best opportunity to find the certificate they promised via
	 * the associated TLSA RRs.  (Generally, server administrators should
	 * avoid SNI, and there are no plans to support SNI in the Postfix
	 * SMTP server).
	 * 
	 * Per RFC7672, the required SNI name is the TLSA "base domain" (the one
	 * used to construct the "_25._tcp.<fqdn>" TLSA record DNS query).
	 * 
	 * Since the hostname is DNSSEC-validated, it must be a DNS FQDN and
	 * thererefore valid for use with SNI.
	 */
	sni = props->dane->base_domain;
    } else if (props->sni && *props->sni) {
	if (strcmp(props->sni, "hostname") == 0)
	    sni = props->host;
	else if (strcmp(props->sni, "nexthop") == 0)
	    sni = props->nexthop;
	else
	    sni = props->sni;
    }
    if (sni && strlen(sni) <= TLSEXT_MAXLEN_host_name) {

	/*
	 * Failure to set a valid SNI hostname is a memory allocation error,
	 * and thus transient.  Since we must not cache the session if we
	 * failed to send the SNI name, we have little choice but to abort.
	 */
	if (!SSL_set_tlsext_host_name(TLScontext->con, sni)) {
	    msg_warn("%s: error setting SNI hostname to: %s", props->namaddr,
		     sni);
	    tls_free_context(TLScontext);
	    return (0);
	}

	/*
	 * The saved value is not presently used client-side, but could later
	 * be logged if acked by the server (requires new client-side
	 * callback to detect the ack).  For now this just maintains symmetry
	 * with the server code, where do record the received SNI for
	 * logging.
	 */
	TLScontext->peer_sni = mystrdup(sni);
	if (log_mask & TLS_LOG_DEBUG)
	    msg_info("%s: SNI hostname: %s", props->namaddr, sni);
    }
#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);

    /*
     * 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);

    tls_dane_set_callback(app_ctx->ssl_ctx, TLScontext);

    /*
     * If we don't trigger the handshake in the library, leave control over
     * SSL_connect/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 certificate verification.
     * 
     * Error handling: If the SSL handshake fails, we print out an error message
     * and remove all TLS state concerning this session.
     */
    sts = tls_bio_connect(vstream_fileno(props->stream), props->timeout,
			  TLScontext);
    if (sts <= 0) {
	if (ERR_peek_error() != 0) {
	    msg_info("SSL_connect error to %s: %d", props->namaddr, sts);
	    tls_print_errors();
	} else if (errno != 0) {
	    msg_info("SSL_connect error to %s: %m", props->namaddr);
	} else {
	    msg_info("SSL_connect error to %s: lost connection",
		     props->namaddr);
	}
	uncache_session(app_ctx->ssl_ctx, TLScontext);
	tls_free_context(TLScontext);
	return (0);
    }
    return (tls_client_post_connect(TLScontext, props));
}

/* tls_client_post_connect - post-handshake processing */

TLS_SESS_STATE *tls_client_post_connect(TLS_SESS_STATE *TLScontext,
				        const TLS_CLIENT_START_PROPS *props)
{
    const SSL_CIPHER *cipher;
    X509   *peercert;

    /* 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", TLScontext->namaddr);

    /*
     * Do peername verification if requested and extract useful information
     * from the certificate for later use.
     */
    if ((peercert = SSL_get_peer_certificate(TLScontext->con)) != 0) {
	TLScontext->peer_status |= TLS_CERT_FLAG_PRESENT;

	/*
	 * Peer name or fingerprint verification as requested.
	 * Unconditionally set peer_CN, issuer_CN and peer_cert_fprint. Check
	 * fingerprint first, and avoid logging verified as untrusted in the
	 * call to verify_extract_name().
	 */
	verify_extract_print(TLScontext, peercert, props);
	verify_extract_name(TLScontext, peercert, props);

	if (TLScontext->log_mask &
	    (TLS_LOG_CERTMATCH | TLS_LOG_VERBOSE | TLS_LOG_PEERCERT))
	    msg_info("%s: subject_CN=%s, issuer_CN=%s, "
		     "fingerprint=%s, pkey_fingerprint=%s", props->namaddr,
		     TLScontext->peer_CN, TLScontext->issuer_CN,
		     TLScontext->peer_cert_fprint,
		     TLScontext->peer_pkey_fprint);
	X509_free(peercert);
    } else {
	TLScontext->issuer_CN = mystrdup("");
	TLScontext->peer_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));

    /*
     * The TLS engine is active. Switch to the tls_timed_read/write()
     * functions and make the TLScontext available to those functions.
     */
    if (TLScontext->stream != 0)
	tls_stream_start(props->stream, TLScontext);

    /*
     * Fully secured only if trusted, matched and not insecure like halfdane.
     * Should perhaps also exclude "verify" (as opposed to "secure") here,
     * because that can be subject to insecure MX indirection, but that's
     * rather incompatible.  Users have been warned.
     */
    if (TLS_CERT_IS_PRESENT(TLScontext)
	&& TLS_CERT_IS_TRUSTED(TLScontext)
	&& TLS_CERT_IS_MATCHED(TLScontext)
	&& !TLS_NEVER_SECURED(props->tls_level))
	TLScontext->peer_status |= TLS_CERT_FLAG_SECURED;

    /*
     * With the handshake done, extract TLS 1.3 signature metadata.
     */
    tls_get_signature_params(TLScontext);

    if (TLScontext->log_mask & TLS_LOG_SUMMARY)
	tls_log_summary(TLS_ROLE_CLIENT, TLS_USAGE_NEW, TLScontext);

    tls_int_seed();

    return (TLScontext);
}

#endif					/* USE_TLS */