tcptls/iostream: Add support for setting SNI on client TLS connections

If the hostname field of the ast_tcptls_session_args structure is
set (which it is for websocket client connections), that hostname
will now automatically be used in an SNI TLS extension in the client
hello.

Resolves: asterisk#713

UserNote: Secure websocket client connections now send SNI in
the TLS client hello.

include/asterisk/iostream.h

@@ -106,6 +106,17 @@ void ast_iostream_set_timeout_sequence(struct ast_iostream *stream, struct timev
  */
 void ast_iostream_set_exclusive_input(struct ast_iostream *stream, int exclusive_input);
 
+/*!
+ * \brief Set the iostream's SNI hostname for TLS client connections
+ *
+ * \param stream A pointer to an iostream
+ * \param sni_hostname The hostname to use for SNI when in client mode
+ *
+ * \retval 0 if the hostname was set successfully.
+ * \retval -1 if memory could not be allocated for the hostname.
+ */
+int ast_iostream_set_sni_hostname(struct ast_iostream *stream, const char *sni_hostname);
+
 /*!
  * \brief Get an iostream's file descriptor.
  *

main/iostream.c

@@ -46,6 +46,7 @@ struct ast_iostream {
 	int rbuflen;
 	char *rbufhead;
 	char rbuf[2048];
+	char *sni_hostname;
 };
 
 #if defined(DO_SSL)
@@ -152,6 +153,16 @@ void ast_iostream_set_exclusive_input(struct ast_iostream *stream, int exclusive
 	stream->exclusive_input = exclusive_input;
 }
 
+int ast_iostream_set_sni_hostname(struct ast_iostream *stream, const char *sni_hostname)
+{
+	ast_assert(stream != NULL);
+
+	ast_free(stream->sni_hostname);
+	stream->sni_hostname = ast_strdup(sni_hostname);
+
+	return stream->sni_hostname ? 0 : -1;
+}
+
 static ssize_t iostream_read(struct ast_iostream *stream, void *buf, size_t size)
 {
 	struct timeval start;
@@ -591,13 +602,9 @@ int ast_iostream_close(struct ast_iostream *stream)
 
 static void iostream_dtor(void *cookie)
 {
-#ifdef AST_DEVMODE
-	/* Since the ast_assert below is the only one using stream,
-	 * and ast_assert is only available with AST_DEVMODE, we
-	 * put this in a conditional to avoid compiler warnings. */
 	struct ast_iostream *stream = cookie;
-#endif
 
+	ast_free(stream->sni_hostname);
 	ast_assert(stream->fd == -1);
 }
 
@@ -639,6 +646,15 @@ int ast_iostream_start_tls(struct ast_iostream **pstream, SSL_CTX *ssl_ctx, int
 	 */
 	SSL_set_fd(stream->ssl, stream->fd);
 
+	if (client && !ast_strlen_zero(stream->sni_hostname)) {
+		if (!SSL_set_tlsext_host_name(stream->ssl, stream->sni_hostname)) {
+			ast_log(LOG_ERROR, "Unable to set SNI hostname '%s'\n",
+				stream->sni_hostname);
+			errno = EIO;
+			return -1;
+		}
+	}
+
 	res = ssl_setup(stream->ssl);
 	if (res <= 0) {
 		int sslerr = SSL_get_error(stream->ssl, res);

main/tcptls.c

@@ -741,6 +741,13 @@ struct ast_tcptls_session_instance *ast_tcptls_client_create(struct ast_tcptls_s
 	/* Set current info */
 	ast_sockaddr_copy(&desc->old_address, &desc->remote_address);
 
+	if (!ast_strlen_zero(desc->hostname)) {
+		if (ast_iostream_set_sni_hostname(tcptls_session->stream, desc->hostname) != 0) {
+			ast_log(LOG_WARNING, "Unable to set SNI hostname '%s' on connection '%s'\n",
+				desc->hostname, desc->name);
+		}
+	}
+
 	return tcptls_session;
 
 error: