Fix client's poor man NCP fallback

This commit fixes two separate issues which are closely linked.

First, a 2.5 client cannot connect to a server which does not support NCP
and is not using one of the default --data-ciphers (AES-*-GCM).

This is because the 2.5 client does not use its configured --data-ciphers
cipher in the "fall back to OCC based cipher negotiation" case.  Fix this.

Second, do not allow the 2.5 client to use --data-ciphers-fallback in the
above situation because that is not it's intended use (only to be used if
there is no pushed cipher [NCP] and no OCC provided cipher).

To reproduce the error use a client with only --data-ciphers set against
a server without NCP.

        OPTIONS ERROR: failed to negotiate cipher with server.
        Add the server's cipher  ('AES-256-CBC') to --data-ciphers
        (currently 'AES-256-CBC') if you want to connect to this server.

Reported by: Richard Bonhomme <tincanteksup@gmail.com>

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Steffan Karger <steffan@karger.me>
Message-Id: <20200814080619.2108-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20734.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

diff --git a/src/openvpn/ssl_ncp.c b/src/openvpn/ssl_ncp.c
index f522b8f0cec0cdb028d172e672ec72fe4d5b3845..c9ab85ce2793070ae46f2010e0df45533478a543 100644 (file)
--- a/src/openvpn/ssl_ncp.c
+++ b/src/openvpn/ssl_ncp.c
@@ -296,13 +296,14 @@ check_pull_client_ncp(struct context *c, const int found)
     }
     /* If the server did not push a --cipher, we will switch to the
      * remote cipher if it is in our ncp-ciphers list */
-    bool useremotecipher = tls_poor_mans_ncp(&c->options,
-                                             c->c2.tls_multi->remote_ciphername);
-
+    if(tls_poor_mans_ncp(&c->options, c->c2.tls_multi->remote_ciphername))
+    {
+        return true;
+    }
 
     /* We could not figure out the peer's cipher but we have fallback
      * enabled */
-    if (!useremotecipher && c->options.enable_ncp_fallback)
+    if (!c->c2.tls_multi->remote_ciphername && c->options.enable_ncp_fallback)
     {
         return true;
     }