gnutls backend doesn't reuse sessions

[kroeckx]

Hi,

It seems that curl has support for session IDs but not for session tickets. When linked with gnutls client hello indicates support for TLS tickets so the server sends a TLS ticket back and does not send a session id back. But curl doesn't seem to handle the TLS tickets.

In case of OpenSSL it doesn't send a TLS ticket extension in the client hello and so gets a session id back, and then it properly reuses the session.

[bagder]

We'll welcome help and patches to fix this. This seems to be the GnuTLS docs for how it works: https://www.gnutls.org/manual/html_node/Session-resumption.html

[mkauf]

Probably TLS session tickets don't work because curl does not use the same SSL context for multiple requests. See TODO: Cache/share OpenSSL contexts

I have tested this: After disabling TLS session tickets, reusing the SSL sessions works properly. TLS session tickets are already disabled for OpenSSL.

For GnuTLS 3.5.6 and newer versions, this can be fixed with this patch:

diff --git a/lib/vtls/gtls.c b/lib/vtls/gtls.c
index a992f99..faa70ac 100644
--- a/lib/vtls/gtls.c
+++ b/lib/vtls/gtls.c
@@ -380,6 +380,7 @@ gtls_connect_step1(struct connectdata *conn,
                    int sockindex)
 {
   struct Curl_easy *data = conn->data;
+  unsigned int init_flags;
   gnutls_session_t session;
   int rc;
   bool sni = TRUE; /* default is SNI enabled */
@@ -526,7 +527,14 @@ gtls_connect_step1(struct connectdata *conn,
   }

   /* Initialize TLS session as a client */
-  rc = gnutls_init(&conn->ssl[sockindex].session, GNUTLS_CLIENT);
+  init_flags = GNUTLS_CLIENT;
+
+#if defined(GNUTLS_NO_TICKETS)
+  /* Disable TLS session tickets */
+  init_flags |= GNUTLS_NO_TICKETS;
+#endif
+
+  rc = gnutls_init(&conn->ssl[sockindex].session, init_flags);
   if(rc != GNUTLS_E_SUCCESS) {
     failf(data, "gnutls_init() failed: %d", rc);
     return CURLE_SSL_CONNECT_ERROR;

The same problem also exists for mbed TLS. Patch:

diff --git a/lib/vtls/mbedtls.c b/lib/vtls/mbedtls.c
index 8bcaddd..213a58f 100644
--- a/lib/vtls/mbedtls.c
+++ b/lib/vtls/mbedtls.c
@@ -373,6 +373,11 @@ mbed_connect_step1(struct connectdata *conn,
   mbedtls_ssl_conf_ciphersuites(&connssl->config,
                                 mbedtls_ssl_list_ciphersuites());

+#if defined(MBEDTLS_SSL_SESSION_TICKETS)
+  mbedtls_ssl_conf_session_tickets(&connssl->config,
+                                   MBEDTLS_SSL_SESSION_TICKETS_DISABLED);
+#endif
+
   /* Check if there's a cached ID we can/should use here! */
   if(data->set.general_ssl.sessionid) {
     void *old_session = NULL;

What do you think?