WARNING: inbound connection timed out (ORA-3136)

The "WARNING: inbound connection timed out (ORA-3136)" in the alert log indicates that the client was not able to complete it's authentication within the period of time specified by parameter SQLNET.INBOUND_CONNECT_TIMEOUT.

You may also witness ORA-12170 without timeout error on the database server sqlnet.log file.This entry would also have the clinet address which failed to get authenticated. Some applications or JDBC thin driver applications may not have these details.

From 10.2 onwards the default value of parameter SQLNET.INBOUND_CONNECT_TIMEOUT is 60 seconds, hence if the client is not able authenticate within 60 secs , the warning would appear in the alert log and the client connection will be terminated.

reasons for this error:
1. Server gets a connection request from a malicious client which is not supposed to connect to the database , in which case the error thrown is the correct behavior. You can get the client address for which the error was thrown via sqlnet log file.
2. The server receives a valid client connection request but the client takes a long time to authenticate more than the default 60 seconds.
3. The DB server is heavily loaded due to which it cannot finish the client logon within the timeout specified.

following checks can be done :
1. Check whether local connection on the database server is sucessful & quick.
2. If local connections are quick ,then check for underlying network delay with the help of your network administrator.
3. Check whether your Database performance has degraded by anyway.
4. Check alert log for any critical errors for eg, ORA-600 or ORA-7445 and get them resolved first.
These critical errors might have triggered the slowness of the database server.

As a workaround to avoid only this warning messages, you can set the parameters SQLNET.INBOUND_CONNECT_TIMEOUT
and INBOUND_CONNECT_TIMEOUT_listenername to the value more than 60.
To set these parameter -
1. In server side sqlnet.ora file add
SQLNET.INBOUND_CONNECT_TIMEOUT
For e.g
SQLNET.INBOUND_CONNECT_TIMEOUT = 120

2. In listener.ora file -
INBOUND_CONNECT_TIMEOUT_listenername = 110
For e.g if the listener name is LISTENER then -
INBOUND_CONNECT_TIMEOUT_LISTENER = 110