Many times I faced this issue “WARNING: inbound connection timed out (ORA-3136)” in alert log and if you are using data guard, then you can see that dgmgrl status will show failed because standby is not able to sync up with primary.
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.
From 10.2 onwards the default value of this parameter 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.
Root cause of this issue could be
1. malicious client is flooding database servers with connect requests that consumes resources
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
As a workaround to avoid only this warning messages, you can set the parameters SQLNET.INBOUND_CONNECT_TIMEOUT=0 in $ORACLE_HOME/network/admin/sqlnet.ora file
Once you set this parameter the warnings in alert log will go away. If you are using data guard, may be you want to enable the databases on which you were getting warnings.
Hope this helps !!