ORA-12537: TNS: connection closed error

The ORA-12537 is an information message and means that the connection has been closed. This can be caused by a number of reasons:
1. Database is shut down (maybe for nightly backup), but connection to database was kept by client.
2. A time-out occurred on the client-connection.
3. When firewall closes idle connections.
3. A configuration problem in sqlnet.ora or listener.ora.
4. You have a path name that is too long for the Oracle TNS client on windows.



This issue is due to the connection string being large enough that the connection packet is being split into two packets and then sent to the listener. There are several workarounds for the issue.

a. Install the exe in a shorter named directory.
b. Modify the TNS Names entry so that it is shorter.
c. Replace domain names with IP Addresses if that shortens and vice versa.
d. Remove any parameters from the entry that are not needed.
e. Patch the Database Listener to the most recent version.
5. Oracle security is enables (for example in Oracle Apps). Solution either:
a) Edit sqlnet.ora file and add your client-ip to tcp.invited_nodes.
b) Remove secutity by editing sqlnet.ora and comment out parameter tcp.validnode_checking = yes by adding # at the beginning of the line.