Fix NCP behaviour on TLS reconnect.

If a client reconnects on a soft-restart from the same port (due to --bind in use on the client), both sides will handle this as a "reconnect" and not a "full new connect" internally, re-using existing crypto context.

The client will still ask the server for pushed options, and the server code to handle this refuses to do NCP if a key has already been negotiated (because there is no way to *change* the cipher after that) - which ends up in "the client uses the non-negotiated cipher from the config file, while the server uses the previously-negotiated NCP cipher", and nothing works.

The easy workaround: if we find us in the situation that we think NCP has already been done, just re-push "cipher o->ciphername" with the current cipher for this client context.

All credits for this go to Stefan Behrens <[email protected]...> who found and diagnosed the issue in trac #887, came up with a first patch to solve the issue quite similar to this (simplified) one, and helped testing.

Trac: #887

---
 src/openvpn/push.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

src/openvpn/push.c +5 -3

index 8c3104e..bcef0ef 100644
@@ -372,15 +372,17 @@ prepare_push_reply(struct context *c, struct gc_arena *gc,
     /* Push cipher if client supports Negotiable Crypto Parameters */
     if (tls_peer_info_ncp_ver(peer_info) >= 2 && o->ncp_enabled)
     {
-        /* if we have already created our key, we cannot change our own
-         * cipher, so disable NCP and warn = explain why
+        /* if we have already created our key, we cannot *change* our own
+         * cipher -> so log the fact and push the "what we have now" cipher
+         * (so the client is always told what we expect it to use)
          */
         const struct tls_session *session = &tls_multi->session[TM_ACTIVE];
         if (session->key[KS_PRIMARY].crypto_options.key_ctx_bi.initialized)
         {
             msg( M_INFO, "PUSH: client wants to negotiate cipher (NCP), but "
                  "server has already generated data channel keys, "
-                 "ignoring client request" );
+                 "re-sending previously negotiated cipher '%s'",
+                 o->ciphername );
         }
         else
         {