ORA-12537: TNS:connection closed

What does it mean?

The TNS connection between client and server ended because the communication channel was closed unexpectedly. The session drops while the connection is being established or immediately afterwards.

Why does it happen?

The listener handed the client off to a server process, but the process could not be created or terminated immediately: instance crash, PROCESSES limit reached, operating system resource shortage, a firewall/middle tier cutting the connection, or a TCP.VALIDNODE_CHECKING/TCP.INVITED_NODES rule in sqlnet.ora rejecting the client. Sometimes an idle firewall between client and server drops the connection.

How to fix it

Examine listener.log and sqlnet.log. Check the PROCESSES/SESSIONS limits: SELECT * FROM v$resource_limit WHERE resource_name IN ('processes','sessions'); Verify that the TCP.INVITED_NODES/TCP.EXCLUDED_NODES lists in the server-side sqlnet.ora do not reject the client IP (a logon version mismatch raises ORA-28040, not this error). Increase the idle timeout on the firewall or send keepalives with SQLNET.EXPIRE_TIME. Check the alert.log for instance crash records.