I'm trying to determine if it is possible to connect an Oracle client that is not configured for encrypted connections to a TLS or mTLS connected Oracle Autonomous database using an stunnel client connection.

First I tested a TLS capable client to verify connectivity. Next I set up stunnel to make the same TLS connection. I have configured stunnel as follows:

foreground = yes debug = info

[oracle] accept = 127.0.0.1:1521 connect = adb.us-ashburn-1.oraclecloud.com:1521 client = yes

Next, I attempted a non-encrypted Oracle database connection to 127.0.0.1:1521, but got the following errors from stunnel:

2023.11.14 10:58:50 LOG6[0]: TLS connected: new session negotiated 2023.11.14 10:58:50 LOG6[0]: TLSv1.2 ciphersuite: AES256-GCM-SHA384 (256-bit encryption) 2023.11.14 10:58:50 LOG3[0]: SSL_read: ../ssl/record/ssl3_record.c:1065: error:14191044:SSL routines:tls1_enc:internal error 2023.11.14 10:58:50 LOG5[0]: Connection reset: 752 byte(s) sent to TLS, 8 byte(s) sent to socket ^C2023.11.14 10:58:57 LOG3[ui]: Received SIGINT; terminating

Could this be a configuration issue in stunnel or might there be something in the Oracle database protocol itself that is different for TLS connections that would prevent tunneling?