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
---
src/openvpn/crypto_openssl.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/src/openvpn/crypto_openssl.c b/src/openvpn/crypto_openssl.c
index 9ec2048d..43d75b89 100644
--- a/src/openvpn/crypto_openssl.c
+++ b/src/openvpn/crypto_openssl.c
@@ -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));
}
}
--
2.19.0
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