WARNING: inbound connection timed out (ORA-3136)

The ORA-3136 will be written into alert.log when users fails to provide credentials and cannot authenticate within the set timeout value.

The default value set for below parameters in sqlnet.ora is 60 seconds.

SQLNET.INBOUND_CONNECT_TIMEOUT
INBOUND_CONNECT_TIMEOUT_listener_name

You will see below error in the alert.log file

Fatal NI connect error 12170.
VERSION INFORMATION:
TNS for Linux: Version 19.0.0.0.0 - Production
Oracle Bequeath NT Protocol Adapter for Linux: Version 19.0.0.0.0 - Production
TCP/IP NT Protocol Adapter for Linux: Version 19.0.0.0.0 - Production
Version 19.5.0.0.0
Time: 14-OCT-2023 18:06:02
Tracing not turned on.
Tns error struct:
ns main err code: 12535

TNS-12535: TNS:operation timed out
ns secondary err code: 12606
nt main err code: 0
nt secondary err code: 0
nt OS err code: 0
Client address: (ADDRESS=(PROTOCOL=tcp)(HOST=xxx.xx.xxx.xx)(PORT=51290))
2023-10-12T16:06:02.222415-07:00
WARNING: inbound connection timed out (ORA-3136)
2023-10-12T16:15:18.866821-07:00

In the above log you can see the client IP and port, you can able to check corresponding entry in the listener log as well. Also, you can see the time stamp of user initiated the connection and time stamp when user got error, it will be more than 60 sec.

The main reason for the error is …

1. When user or application trying to connect using wrong credentials or no attend made under 1 min threshold default value of the instance.

You can able to reproduce the issue by entering wrong credentials

 
SQL> sqlplus HR/xxxx@orcl
SQL*Plus: Release 19.0.0.0.0 - Production on Fri Oct 13 19:31:30 2023
Version 19.18.0.0.0
Copyright (c) 1982, 2022, Oracle. All rights reserved.

ERROR:
ORA-01017: invalid username/password; logon denied

$ tail -f orcl_alert.log
Fri Oct 13 19:32:31 2023
WARNING: inbound connection timed out (ORA-3136)

2. If the database has some performance issues or any network delay can cause long time and cannot authenticate user within default time out i.e. 60 seconds

If the database has some load causing the slowness, then increasing SQLNET.INBOUND_CONNECT_TIMEOUT to higher value will help to reduce the errors.

No DB restart required when you make the change in sqlnet.ora but note that it will be applicable to next server process.

3. The server receives the request but cannot be able to authenticate with in default time out i.e. 60 seconds

Find out what causing the delay in authentication if required increase the time out value.

If you are receiving error frequently you can enable tracing using below command
SQL> alter system set events '3136 trace name errorstack level 3';

 
You can trun off tracing using
SQL> alter system set events '3136 trace name context off';

Note that tracing will be generated under USER_DUMP_DEST or BACKGROUND_DUMP_DEST

If you still seeing these warnings I would suggest to raise a ticket with oracle support

Thanks,
https://oracleracexpert.com