Increase logging/tracing on Oracle 12c database I need to connect TLS-secured-ly to an Oracle 12c (from some random Java program, using JDBC connection)

I set up my listener to use TCPS, I made my wallet and everything and still my program is failing with a handshake failure.

Tracing it on client side is not helfpul.

How can I increase the logging on Oracle side (and where to find extra logs) to debug this?
You need to enable listener logging.

In your listener.ora, set:

LOG_DIRECTORY_listener=/path/to/your/oracle/home/network/admin/log
LOG_FILE_listener=listener.log
TRACE_DIRECTORY_listener=/path/to/your/oracle/home/network/admin/trace
TRACE_FILE_listener=listener.trc
TRACE_LEVEL_listener=10


... make sure you substitute your listener name for listener above.

You can also use ADR parameters instead - see the documentation.