Adding upstream version 3.7.10.upstream/3.7.10

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1250 insertions, 0 deletions

diff --git a/src/tls/tls_client.c b/src/tls/tls_client.c
new file mode 100644
index 0000000..68f9255
--- /dev/null
+++ b/src/tls/tls_client.c
@@ -0,0 +1,1250 @@
+/*++
+/* 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_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);
+}
+
+/* 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     verbose;
+
+    verbose = TLScontext->log_mask &
+	(TLS_LOG_CERTMATCH | TLS_LOG_VERBOSE | TLS_LOG_PEERCERT);
+
+    /*
+     * On exit both peer_CN and issuer_CN should be set.
+     */
+    TLScontext->issuer_CN = tls_issuer_CN(peercert, TLScontext);
+    TLScontext->peer_CN = tls_peer_CN(peercert, TLScontext);
+
+    /*
+     * Is the certificate trust chain trusted and matched?  Any required name
+     * checks are now performed internally in OpenSSL.
+     */
+    if (SSL_get_verify_result(TLScontext->con) == X509_V_OK) {
+	TLScontext->peer_status |= TLS_CERT_FLAG_TRUSTED;
+	if (TLScontext->must_fail) {
+	    msg_panic("%s: cert valid despite trust init failure",
+		      TLScontext->namaddr);
+	} else if (TLS_MUST_MATCH(TLScontext->level)) {
+
+	    /*
+	     * Fully secured only if not insecure like half-dane.  We use
+	     * TLS_CERT_FLAG_MATCHED to satisfy policy, but
+	     * TLS_CERT_FLAG_SECURED to log the effective security.
+	     * 
+	     * Would ideally also exclude "verify" (as opposed to "secure")
+	     * here, because that can be subject to insecure MX indirection,
+	     * but that's rather incompatible (and not even the case with
+	     * explicitly chosen non-default match patterns).  Users have
+	     * been warned.
+	     */
+	    if (!TLS_NEVER_SECURED(TLScontext->level))
+		TLScontext->peer_status |= TLS_CERT_FLAG_SECURED;
+	    TLScontext->peer_status |= TLS_CERT_FLAG_MATCHED;
+
+	    if (verbose) {
+		const char *peername = SSL_get0_peername(TLScontext->con);
+
+		if (peername)
+		    msg_info("%s: matched peername: %s",
+			     TLScontext->namaddr, peername);
+		tls_dane_log(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_MATCHED(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);
+    }
+}
+
+/* add_namechecks - tell OpenSSL what names to check */
+
+static void add_namechecks(TLS_SESS_STATE *TLScontext,
+			           const TLS_CLIENT_START_PROPS *props)
+{
+    SSL    *ssl = TLScontext->con;
+    int     namechecks_count = 0;
+    int     i;
+
+    /* RFC6125: No part-label 'foo*bar.example.com' wildcards for SMTP */
+    SSL_set_hostflags(ssl, X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS);
+
+    for (i = 0; i < props->matchargv->argc; ++i) {
+	const char *name = props->matchargv->argv[i];
+	const char *aname;
+	int     match_subdomain = 0;
+
+	if (strcasecmp(name, "nexthop") == 0) {
+	    name = props->nexthop;
+	} else if (strcasecmp(name, "dot-nexthop") == 0) {
+	    name = props->nexthop;
+	    match_subdomain = 1;
+	} else if (strcasecmp(name, "hostname") == 0) {
+	    name = props->host;
+	} else {
+	    if (*name == '.') {
+		if (*++name == 0) {
+		    msg_warn("%s: ignoring invalid match name: \".\"",
+			     TLScontext->namaddr);
+		    continue;
+		}
+		match_subdomain = 1;
+	    }
+#ifndef NO_EAI
+	    else {
+
+		/*
+		 * 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.
+		 */
+		unsigned char *cp = (unsigned char *) name;
+
+		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 (name[3]) {
+			name = name + 3;
+			match_subdomain = 1;
+		    }
+		}
+	    }
+#endif
+	}
+
+	/*
+	 * 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.
+	 */
+#ifndef NO_EAI
+	if (!allascii(name) && (aname = midna_domain_to_ascii(name)) != 0) {
+	    if (msg_verbose)
+		msg_info("%s asciified to %s", name, aname);
+	    name = aname;
+	}
+#endif
+
+	if (!match_subdomain) {
+	    if (SSL_add1_host(ssl, name))
+		++namechecks_count;
+	    else
+		msg_warn("%s: error loading match name: \"%s\"",
+			 TLScontext->namaddr, name);
+	} else {
+	    char   *dot_name = concatenate(".", name, (char *) 0);
+
+	    if (SSL_add1_host(ssl, dot_name))
+		++namechecks_count;
+	    else
+		msg_warn("%s: error loading match name: \"%s\"",
+			 TLScontext->namaddr, dot_name);
+	    myfree(dot_name);
+	}
+    }
+
+    /*
+     * If we failed to add any names, OpenSSL will perform no namechecks, so
+     * we set the "must_fail" bit to avoid verification false-positives.
+     */
+    if (namechecks_count == 0) {
+	msg_warn("%s: could not configure peer name checks",
+		 TLScontext->namaddr);
+	TLScontext->must_fail = 1;
+    }
+}
+
+/* tls_auth_enable - set up TLS authentication */
+
+static int tls_auth_enable(TLS_SESS_STATE *TLScontext,
+			           const TLS_CLIENT_START_PROPS *props)
+{
+    const char *sni = 0;
+
+    if (props->sni && *props->sni) {
+#ifndef NO_EAI
+	const char *aname;
+
+#endif
+
+	/*
+	 * MTA-STS policy plugin compatibility: with servername=hostname,
+	 * Postfix must send the MX hostname (not CNAME expanded).
+	 */
+	if (strcmp(props->sni, "hostname") == 0)
+	    sni = props->host;
+	else if (strcmp(props->sni, "nexthop") == 0)
+	    sni = props->nexthop;
+	else
+	    sni = props->sni;
+
+	/*
+	 * The SSL_set_tlsext_host_name() documentation does not promise that
+	 * every implementation will convert U-label form to A-label form.
+	 */
+#ifndef NO_EAI
+	if (!allascii(sni) && (aname = midna_domain_to_ascii(sni)) != 0) {
+	    if (msg_verbose)
+		msg_info("%s asciified to %s", sni, aname);
+	    sni = aname;
+	}
+#endif
+    }
+    switch (TLScontext->level) {
+    case TLS_LEV_HALF_DANE:
+    case TLS_LEV_DANE:
+    case TLS_LEV_DANE_ONLY:
+
+	/*
+	 * 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 make use of SNI the best
+	 * opportunity to find the certificate they promised via the
+	 * associated TLSA RRs.
+	 * 
+	 * Since the hostname is DNSSEC-validated, it must be a DNS FQDN and
+	 * therefore valid for use with SNI.
+	 */
+	if (SSL_dane_enable(TLScontext->con, 0) <= 0) {
+	    msg_warn("%s: error enabling DANE-based certificate validation",
+		     TLScontext->namaddr);
+	    tls_print_errors();
+	    return (0);
+	}
+	/* RFC7672 Section 3.1.1 specifies no name checks for DANE-EE(3) */
+	SSL_dane_set_flags(TLScontext->con, DANE_FLAG_NO_DANE_EE_NAMECHECKS);
+
+	/* Per RFC7672 the SNI name is the TLSA base domain */
+	sni = props->dane->base_domain;
+	add_namechecks(TLScontext, props);
+	break;
+
+    case TLS_LEV_FPRINT:
+	/* Synthetic DANE for fingerprint security */
+	if (SSL_dane_enable(TLScontext->con, 0) <= 0) {
+	    msg_warn("%s: error enabling fingerprint certificate validation",
+		     props->namaddr);
+	    tls_print_errors();
+	    return (0);
+	}
+	SSL_dane_set_flags(TLScontext->con, DANE_FLAG_NO_DANE_EE_NAMECHECKS);
+	break;
+
+    case TLS_LEV_SECURE:
+    case TLS_LEV_VERIFY:
+	if (TLScontext->dane != 0 && TLScontext->dane->tlsa != 0) {
+	    /* Synthetic DANE for per-destination trust-anchors */
+	    if (SSL_dane_enable(TLScontext->con, NULL) <= 0) {
+		msg_warn("%s: error configuring local trust anchors",
+			 props->namaddr);
+		tls_print_errors();
+		return (0);
+	    }
+	}
+	add_namechecks(TLScontext, props);
+	break;
+    default:
+	break;
+    }
+
+    if (sni) {
+	if (strlen(sni) > TLSEXT_MAXLEN_host_name) {
+	    msg_warn("%s: ignoring too long SNI hostname: %.100s",
+		     props->namaddr, sni);
+	    return (0);
+	}
+
+	/*
+	 * 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);
+	    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 (TLScontext->log_mask & TLS_LOG_DEBUG)
+	    msg_info("%s: SNI hostname: %s", props->namaddr, sni);
+    }
+    return (1);
+}
+
+/* tls_client_init - initialize client-side TLS engine */
+
+TLS_APPL_STATE *tls_client_init(const TLS_CLIENT_INIT_PROPS *props)
+{
+    SSL_CTX *client_ctx;
+    TLS_APPL_STATE *app_ctx;
+    const EVP_MD *fpt_alg;
+    long    off = 0;
+    int     cachable;
+    int     scache_timeout;
+    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();
+
+    /*
+     * 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 ((fpt_alg = tls_validate_digest(props->mdalg)) == 0) {
+	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);
+
+    /*
+     * This is a prerequisite for enabling DANE support in OpenSSL, but not a
+     * commitment to use DANE, thus suitable for both DANE and non-DANE TLS
+     * connections.  Indeed we need this not just for DANE, but aslo for
+     * fingerprint and "tafile" support.  Since it just allocates memory, it
+     * should never fail except when we're likely to fail anyway.  Rather
+     * than try to run with crippled TLS support, just give up using TLS.
+     */
+    if (SSL_CTX_dane_enable(client_ctx) <= 0) {
+	msg_warn("OpenSSL DANE initialization failed: disabling TLS support");
+	tls_print_errors();
+	return (0);
+    }
+    tls_dane_digest_init(client_ctx, fpt_alg);
+
+    /*
+     * 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);
+
+    /*
+     * 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);
+    }
+
+    /*
+     * 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);
+}
+
+ /*
+  * 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;
+    int     min_proto;
+    int     max_proto;
+    const char *cipher_list;
+    SSL_SESSION *session = 0;
+    TLS_SESS_STATE *TLScontext;
+    TLS_APPL_STATE *app_ctx = props->ctx;
+    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_MATCH(props->tls_level))
+	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_proto_mask_lims(props->protocols, &min_proto, &max_proto);
+    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);
+    }
+
+    /*
+     * Though RFC7672 set the floor at SSLv3, we really can and should
+     * require TLS 1.0, since e.g. we send SNI, which is a TLS 1.0 extension.
+     * No DANE domains have been observed to support only SSLv3.
+     * 
+     * XXX: Would be nice to make that TLS 1.2 at some point.  Users can choose
+     * to exclude TLS 1.0 and TLS 1.1 if they find they don't run into any
+     * problems doing that.
+     */
+    if (TLS_DANE_BASED(props->tls_level))
+	protomask |= TLS_PROTOCOL_SSLv2 | TLS_PROTOCOL_SSLv3;
+
+    /*
+     * 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;
+    TLScontext->level = props->tls_level;
+
+    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);
+
+    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);
+    }
+#define CARP_VERSION(which) do { \
+        if (which##_proto != 0) \
+            msg_warn("%s: error setting %simum TLS version to: 0x%04x", \
+                     TLScontext->namaddr, #which, which##_proto); \
+        else \
+            msg_warn("%s: error clearing %simum TLS version", \
+                     TLScontext->namaddr, #which); \
+    } while (0)
+
+    /*
+     * Apply session protocol restrictions.
+     */
+    if (protomask != 0)
+	SSL_set_options(TLScontext->con, TLS_SSL_OP_PROTOMASK(protomask));
+    if (!SSL_set_min_proto_version(TLScontext->con, min_proto))
+	CARP_VERSION(min);
+    if (!SSL_set_max_proto_version(TLScontext->con, max_proto))
+	CARP_VERSION(max);
+
+    /*
+     * When applicable, configure DNS-based or synthetic (fingerprint or
+     * local trust anchor) DANE authentication, enable an appropriate SNI
+     * name and peer name matching.
+     * 
+     * NOTE, this can change the effective security level, and needs to happen
+     * early.
+     */
+    if (!tls_auth_enable(TLScontext, props)) {
+	tls_free_context(TLScontext);
+	return (0);
+    }
+
+    /*
+     * Try to convey the configured TLSA records for this connection to the
+     * OpenSSL library.  If none are "usable", we'll fall back to "encrypt"
+     * when authentication is not mandatory, otherwise we must arrange to
+     * ensure authentication failure.
+     */
+    if (TLScontext->dane && TLScontext->dane->tlsa) {
+	int     usable = tls_dane_enable(TLScontext);
+	int     must_fail = usable <= 0;
+
+	if (usable == 0) {
+	    switch (TLScontext->level) {
+	    case TLS_LEV_HALF_DANE:
+	    case TLS_LEV_DANE:
+		msg_warn("%s: all TLSA records unusable, fallback to "
+			 "unauthenticated TLS", TLScontext->namaddr);
+		must_fail = 0;
+		TLScontext->level = TLS_LEV_ENCRYPT;
+		break;
+
+	    case TLS_LEV_FPRINT:
+		msg_warn("%s: all fingerprints unusable", TLScontext->namaddr);
+		break;
+	    case TLS_LEV_DANE_ONLY:
+		msg_warn("%s: all TLSA records unusable", TLScontext->namaddr);
+		break;
+	    case TLS_LEV_SECURE:
+	    case TLS_LEV_VERIFY:
+		msg_warn("%s: all trust anchors unusable", TLScontext->namaddr);
+		break;
+	    }
+	}
+	TLScontext->must_fail |= must_fail;
+    }
+
+    /*
+     * We compute the policy digest after we compute the SNI name in
+     * tls_auth_enable() and possibly update the TLScontext security level.
+     * 
+     * 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.
+     */
+    TLScontext->serverid =
+	tls_serverid_digest(TLScontext, props, cipher_list);
+
+    /*
+     * When authenticating the peer, use 80-bit plus OpenSSL security level
+     * 
+     * XXX: We should perhaps use security level 1 also for mandatory
+     * encryption, with only "may" tolerating weaker algorithms.  But that
+     * could mean no TLS 1.0 with OpenSSL >= 3.0 and encrypt, unless I get my
+     * patch in on time to conditionally re-enable SHA1 at security level 1,
+     * and we add code to make it so.
+     * 
+     * That said, with "encrypt", we could reasonably require TLS 1.2?
+     */
+    if (TLS_MUST_MATCH(TLScontext->level))
+	SSL_set_security_level(TLScontext->con, 1);
+
+    /*
+     * 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 */
+	}
+    }
+
+    /*
+     * 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)
+	tls_set_bio_callback(SSL_get_rbio(TLScontext->con), tls_bio_dump_cb);
+
+    /*
+     * 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)
+	tls_set_bio_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 = TLS_PEEK_PEER_CERT(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().
+	 */
+	TLScontext->peer_cert_fprint = tls_cert_fprint(peercert, props->mdalg);
+	TLScontext->peer_pkey_fprint = tls_pkey_fprint(peercert, props->mdalg);
+	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);
+    } 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);
+
+    /*
+     * 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 */