Oracle Database / Connections & Oracle Net
ORA-12537: investigate a connection closed during negotiation
A closed connection during establishment can involve server process startup, authentication or network interception. Collect both ends of the failure before changing Oracle Net settings.
Environment and inputs: Oracle Database 19c baseline · SQL*Plus-compatible SQL unless RMAN, DGMGRL, ADRCI or a host shell is specified. Check release, patch level, edition and container first.
Start with this diagnostic
Use the stated SQL, RMAN, DGMGRL or host shell and replace sample names with approved targets. DBA and V$ views need authorized catalog access in the correct container. Do not grant broad privileges for an example.
lsnrctl status LISTENERRead the result
On the server host, this identifies the listener endpoints and diagnostic log location. A healthy status after the event does not explain why an earlier connection closed.
Resolve and verify
Match the failing client's timestamp with listener and server diagnostics, then compare one working client using the same service. Investigate executable permissions or network controls only when evidence points there.
Operational precautions
Avoid broad permission changes on Oracle binaries or disabling authentication protections based on this error alone.