TLS version negotiation

Updated the TLS negotiation logic to adaptively try to connect using
the highest TLS version supported by both client and server.
Previously, OpenVPN (when linked with OpenSSL) would always connect
using TLS 1.0.

Also added tls-version-min directive to force a higher TLS version
than 1.0:

tls-version-min <version> ['or-highest'] -- sets the minimum
TLS version we will accept from the peer.  Examples for version
include "1.0" (default), "1.1", or "1.2".  If 'or-highest' is
specified and version is not recognized, we will only accept
the highest TLS version supported by the local SSL implementation.

Examples:

tls-version-min 1.1 -- fail the connection unless peer can
  connect at TLS 1.1 or higher.

tls-version-min 1.2 or-highest -- require that the peer
  connect at TLS 1.2 or higher, however if the local SSL
  implementation doesn't support TLS 1.2 (as it wouldn't
  if linked with an older version of OpenSSL), reduce the
  minimum required version to the highest version supported
  by the local SSL implementation (such as TLS 1.0).  This
  is intended to allow client configurations to target higher
  TLS versions that are supported on the server, even if some
  older clients don't support these versions yet.

 [
    This is a merged patch from on the following commits
    on git://github.com/jamesyonan/openvpn.git

    03a5599202bdc3ba07983dc4efdae387fb8fb436
    d23005413b0e0f28a3c48a6342f494763d5c9b40
 ]

Signed-off-by: James Yonan <james@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
URL: http://thread.gmane.org/gmane.network.openvpn.devel/7743
URL: http://thread.gmane.org/gmane.network.openvpn.devel/7744
Message-Id: 51C77F12.1090802@openvpn.net
Signed-off-by: David Sommerseth <davids@redhat.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/src/openvpn/ssl_backend.h b/src/openvpn/ssl_backend.h
index b1dce22..4d2958c 100644
--- a/src/openvpn/ssl_backend.h
+++ b/src/openvpn/ssl_backend.h
@@ -101,6 +101,29 @@ void tls_free_lib();
 void tls_clear_error();
 
 /**
+ * Parse a TLS version specifier
+ *
+ * @param vstr		The TLS version string
+ * @param extra	        An optional extra parameter, may be NULL
+ *
+ * @return 		One of the TLS_VER_x constants or TLS_VER_BAD
+ *                      if a parse error should be flagged.
+ */
+#define TLS_VER_BAD   -1
+#define TLS_VER_1_0    0 /* default */
+#define TLS_VER_1_1    1
+#define TLS_VER_1_2    2
+int tls_version_min_parse(const char *vstr, const char *extra);
+
+/**
+ * Return the maximum TLS version (as a TLS_VER_x constant)
+ * supported by current SSL implementation
+ *
+ * @return 		One of the TLS_VER_x constants (but not TLS_VER_BAD).
+ */
+int tls_version_max(void);
+
+/**
  * Initialise a library-specific TLS context for a server.
  *
  * @param ctx		TLS context to initialise