From: Viktor Dukhovni <postfix-users@dukhovni.org>
Date: Thu, 25 Jun 2020 22:20:58 +0000 (-0200)
Subject: Send correct SNI domain when the TLS base domain is CNAME expanded
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=refs%2Fheads%2Fdane-sni-3.2;p=thirdparty%2Fpostfix.git

Send correct SNI domain when the TLS base domain is CNAME expanded
---

diff --git a/postfix/HISTORY b/postfix/HISTORY
index 707bad4b7..0f0fc60fc 100644
--- a/postfix/HISTORY
+++ b/postfix/HISTORY
@@ -23326,3 +23326,18 @@ Apologies for any names omitted.
 	Bugfix (introduced: Postfix 3.1): "postfix tls deploy-server-cert"
 	did not handle a missing optional argument. File:
 	conf/postfix-tls-script.
+
+20200626
+
+	Bugfix (introduced: Postfix 2.11): The Postfix smtp(8) client did not
+	send the right SNI name when the TLSA base domain was a secure CNAME
+	expansion of the MX hostname (or non-MX nexthop domain).  Domains with
+	CNAME expanded MX hosts are not conformant with RFC5321, and so are
+	rare.  Even more rare are MX hosts with TLSA records for their CNAME
+	expansion.  For this to matter, the remote SMTP server would also have
+	to select its certificate based on the SNI name in such a way that the
+	original MX host would yield a different certificate.  Among the ~2
+	million hosts in the DANE survey, none meet the conditions for
+	returning a different certificate for the expanded CNAME.  Therefore,
+	sending the correct SNI name should not break existing mail flows.
+	Fixed by Viktor Dukhovni.  File: src/tls/tls_client.c.
diff --git a/postfix/src/tls/tls_client.c b/postfix/src/tls/tls_client.c
index 6bc1c3acd..72f90090c 100644
--- a/postfix/src/tls/tls_client.c
+++ b/postfix/src/tls/tls_client.c
@@ -973,6 +973,7 @@ TLS_SESS_STATE *tls_client_start(const TLS_CLIENT_START_PROPS *props)
 #ifdef TLSEXT_MAXLEN_host_name
     if (TLS_DANE_BASED(props->tls_level)
 	&& strlen(props->host) <= TLSEXT_MAXLEN_host_name) {
+	const char *sni;
 
 	/*
 	 * With DANE sessions, send an SNI hint.  We don't care whether the
@@ -984,15 +985,19 @@ TLS_SESS_STATE *tls_client_start(const TLS_CLIENT_START_PROPS *props)
 	 * 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.  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, props->host)) {
+	sni = props->dane->base_domain;
+	if (!SSL_set_tlsext_host_name(TLScontext->con, sni)) {
 	    msg_warn("%s: error setting SNI hostname to: %s", props->namaddr,
-		     props->host);
+		     sni);
 	    tls_free_context(TLScontext);
 	    return (0);
 	}