libpq: Set Server Name Indication (SNI) for SSL connections

By default, have libpq set the TLS extension "Server Name Indication" (SNI).

This allows an SNI-aware SSL proxy to route connections.  (This
requires a proxy that is aware of the PostgreSQL protocol, not just
any SSL proxy.)

In the future, this could also allow the server to use different SSL
certificates for different host specifications.  (That would require
new server functionality.  This would be the client-side functionality
for that.)

Since SNI makes the host name appear in cleartext in the network
traffic, this might be undesirable in some cases.  Therefore, also add
a libpq connection option "sslsni" to turn it off.

Discussion: https://www.postgresql.org/message-id/flat/7289d5eb-62a5-a732-c3b9-438cee2cb709%40enterprisedb.com

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -8917,7 +8917,7 @@ DO $d$
     END;
 $d$;
 ERROR:  invalid option "password"
-HINT:  Valid options in this context are: service, passfile, channel_binding, connect_timeout, dbname, host, hostaddr, port, options, application_name, keepalives, keepalives_idle, keepalives_interval, keepalives_count, tcp_user_timeout, sslmode, sslcompression, sslcert, sslkey, sslrootcert, sslcrl, sslcrldir, requirepeer, ssl_min_protocol_version, ssl_max_protocol_version, gssencmode, krbsrvname, gsslib, target_session_attrs, use_remote_estimate, fdw_startup_cost, fdw_tuple_cost, extensions, updatable, fetch_size, batch_size, async_capable, keep_connections
+HINT:  Valid options in this context are: service, passfile, channel_binding, connect_timeout, dbname, host, hostaddr, port, options, application_name, keepalives, keepalives_idle, keepalives_interval, keepalives_count, tcp_user_timeout, sslmode, sslcompression, sslcert, sslkey, sslrootcert, sslcrl, sslcrldir, sslsni, requirepeer, ssl_min_protocol_version, ssl_max_protocol_version, gssencmode, krbsrvname, gsslib, target_session_attrs, use_remote_estimate, fdw_startup_cost, fdw_tuple_cost, extensions, updatable, fetch_size, batch_size, async_capable, keep_connections
 CONTEXT:  SQL statement "ALTER SERVER loopback_nopw OPTIONS (ADD password 'dummypw')"
 PL/pgSQL function inline_code_block line 3 at EXECUTE
 -- If we add a password for our user mapping instead, we should get a different

doc/src/sgml/libpq.sgml

@@ -1777,6 +1777,27 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
       </listitem>
      </varlistentry>
 
+     <varlistentry id="libpq-connect-sslsni" xreflabel="sslsni">
+      <term><literal>sslsni</literal><indexterm><primary>Server Name Indication</primary></indexterm></term>
+      <listitem>
+       <para>
+        By default, libpq sets the TLS extension <quote>Server Name
+        Indication</quote> (SNI) on SSL-enabled connections.  See <ulink
+        url="https://tools.ietf.org/html/rfc6066#section-3">RFC 6066</ulink>
+        for details.  By setting this parameter to 0, this is turned off.
+       </para>
+
+       <para>
+        The Server Name Indication can be used by SSL-aware proxies to route
+        connections without having to decrypt the SSL stream.  (Note that this
+        requires a proxy that is aware of the PostgreSQL protocol handshake,
+        not just any SSL proxy.)  However, SNI makes the destination host name
+        appear in cleartext in the network traffic, so it might be undesirable
+        in some cases.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="libpq-connect-requirepeer" xreflabel="requirepeer">
       <term><literal>requirepeer</literal></term>
       <listitem>
@@ -7797,6 +7818,16 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
      </para>
     </listitem>
 
+    <listitem>
+     <para>
+      <indexterm>
+       <primary><envar>PGSSLSNI</envar></primary>
+      </indexterm>
+      <envar>PGSSLSNI</envar>  behaves the same as the <xref
+      linkend="libpq-connect-sslsni"/> connection parameter.
+     </para>
+    </listitem>
+
     <listitem>
      <para>
       <indexterm>

src/interfaces/libpq/fe-connect.c

@@ -303,6 +303,10 @@ static const internalPQconninfoOption PQconninfoOptions[] = {
 		"SSL-Revocation-List-Dir", "", 64,
 	offsetof(struct pg_conn, sslcrldir)},
 
+	{"sslsni", "PGSSLSNI", "1", NULL,
+		"SSL-SNI", "", 1,
+	offsetof(struct pg_conn, sslsni)},
+
 	{"requirepeer", "PGREQUIREPEER", NULL, NULL,
 		"Require-Peer", "", 10,
 	offsetof(struct pg_conn, requirepeer)},
@@ -4095,6 +4099,8 @@ freePGconn(PGconn *conn)
 		free(conn->sslcrldir);
 	if (conn->sslcompression)
 		free(conn->sslcompression);
+	if (conn->sslsni)
+		free(conn->sslsni);
 	if (conn->requirepeer)
 		free(conn->requirepeer);
 	if (conn->ssl_min_protocol_version)

src/interfaces/libpq/fe-secure-openssl.c

@@ -1082,6 +1082,28 @@ initialize_SSL(PGconn *conn)
 	SSL_CTX_free(SSL_context);
 	SSL_context = NULL;
 
+	/*
+	 * Set Server Name Indication (SNI), if enabled by connection parameters.
+	 * Per RFC 6066, do not set it if the host is a literal IP address (IPv4
+	 * or IPv6).
+	 */
+	if (conn->sslsni && conn->sslsni[0] &&
+		!(strspn(conn->pghost, "0123456789.") == strlen(conn->pghost) ||
+		  strchr(conn->pghost, ':')))
+	{
+		if (SSL_set_tlsext_host_name(conn->ssl, conn->pghost) != 1)
+		{
+			char	   *err = SSLerrmessage(ERR_get_error());
+
+			appendPQExpBuffer(&conn->errorMessage,
+							  libpq_gettext("could not set SSL Server Name Indication (SNI): %s\n"),
+							  err);
+			SSLerrfree(err);
+			SSL_CTX_free(SSL_context);
+			return -1;
+		}
+	}
+
 	/*
 	 * Read the SSL key. If a key is specified, treat it as an engine:key
 	 * combination if there is colon present - we don't support files with

src/interfaces/libpq/libpq-int.h

@@ -383,6 +383,7 @@ struct pg_conn
 	char	   *sslrootcert;	/* root certificate filename */
 	char	   *sslcrl;			/* certificate revocation list filename */
 	char	   *sslcrldir;		/* certificate revocation list directory name */
+	char	   *sslsni;			/* use SSL SNI extension (0 or 1) */
 	char	   *requirepeer;	/* required peer credentials for local sockets */
 	char	   *gssencmode;		/* GSS mode (require,prefer,disable) */
 	char	   *krbsrvname;		/* Kerberos service name */