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Add message explaining early TLS client hello failure

In my tests an OpenSSL 1.1.1 server does not accept TLS 1.0 only clients
anymore. Unfortunately, Debian 8 still has OpenVPN 2.3.4, which is
TLS 1.0 only without setting tls-version-min.

We currently log only
OpenSSL: error:14209102:SSL
routines:tls_early_post_process_client_hello:unsupported protocol
which indicates the right technical error but is not very helpful to a
person without deep knowledge in SSL/TLS and OpenVPN's TLS version
history.

This commit adds a hopefully helpful message and also tells users how
to fix the old Debian 8 clients. The error message will be displayed on
the server side only.

Note that connecting with an OpenSSL 1.1.1 client to a TLS 1.0 only
server works fine.

This behaviour is also not specific to OpenVPN. Using an openssl s_client
with the -tls1 option against an openssl s_server exhibits the same
behaviour.

Patch V2: fixed message grammar, use tls-version-min 1.0 and clarify
2.3.6 and older to be actually between 2.3.2 and 2.3.6

Signed-off-by: Arne Schwabe <arne@rfc2549.org>

Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20180926120159.19874-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17491.html
Signed-off-by: David Sommerseth <davids@openvpn.net>

Arne Schwabe authored on 2018/09/26 21:01:59
Showing 1 changed files

src/openvpn/crypto_openssl.c

History View file @ d319468

@@ -199,7 +199,16 @@ crypto_print_openssl_errors(const unsigned int flags)
                                      "in common with the client. Your --tls-cipher setting might be "
                                      "too restrictive.");
+                             }
+                    -
                     +        else if (ERR_GET_REASON(err) == SSL_R_UNSUPPORTED_PROTOCOL)
                     +        {
                     +            msg(D_CRYPT_ERRORS, "TLS error: Unsupported protocol. This typically "
                     +                 "indicates that client and server have no common TLS version enabled. "
                     +                 "This can be caused by mismatched tls-version-min and tls-version-max "
                     +                 "options on client and server. "
                     +                 "If your OpenVPN client is between v2.3.6 and v2.3.2 try adding "
                     +                 "tls-version-min 1.0 to the client configuration to use TLS 1.0+ "
                     +                 "instead of TLS 1.0 only");
                     +        }
                              msg(flags, "OpenSSL: %s", ERR_error_string(err, NULL));
+                         }
+                     }