Fix NCP behaviour on TLS reconnect.

If a client reconnects on a hard-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 <sbehrens@giantdisaster.de>
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

Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20170518102246.5496-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14666.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>

diff --git a/src/openvpn/push.c b/src/openvpn/push.c
index 8c3104ed7690cdf24463c1d610cecf955d744cf4..bcef0ef49c45811eefbb2aeec165a2268012343e 100644 (file)
--- a/src/openvpn/push.c
+++ b/src/openvpn/push.c
@@ -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
         {
@@ -388,8 +390,8 @@ prepare_push_reply(struct context *c, struct gc_arena *gc,
              * TODO: actual negotiation, instead of server dictatorship. */
             char *push_cipher = string_alloc(o->ncp_ciphers, &o->gc);
             o->ciphername = strtok(push_cipher, ":");
-            push_option_fmt(gc, push_list, M_USAGE, "cipher %s", o->ciphername);
         }
+        push_option_fmt(gc, push_list, M_USAGE, "cipher %s", o->ciphername);
     }
     else if (o->ncp_enabled)
     {