Oracle Autonomous Transaction Processing Database is Unreachable During a Connection Test

If you receive the following error during a connection test and are using the connectivity agent to connect to an Oracle Autonomous Transaction Processing database running on a private subnet, you must verify that the Oracle Autonomous Transaction Processing database IP address can be resolved from the agent host.

Network Adapter could not establish the connection. Please check the wallet 
credentials and ensure database is reachable

1. From the Oracle Autonomous Transaction Processing wallet file, extract the tnsnames.ora file.
2. Find the service name being used (for example, test_atp_low) and retrieve the host name (for example, test_atp.adb.us-phoenix-1.oraclecloud.com).
3. From the agent host, run the following command:

   nslookup ATP_hostname

4. If the hostname does not resolve to the Oracle Autonomous Transaction Processing database IP address, create an entry in the /etc/hosts file on the agent host:
   ATP_IP_Address ATP_Service_Hostname_from_tnsnames.ora Hostname

   For example:

   10.0.0.64 test_atp.adb.us-phoenix-1.oraclecloud.com test_atp

5. Test the connection again.