For example:

tnsping dbrs

This message indicates that a connection could not be established with Recovery Service.

TNS Ping Utility for Linux: Version 19.0.0.0.0 - Production on 26-APR-2023 06:09:46
Used parameter files:

/u01/app/oracle/product/19.0.0.0/dbhome_1/network/admin/sqlnetdb.ora

Used TNSNAMES adapter to resolve the alias
Attempting to contact (DESCRIPTION = (FAILOVER = on) (CONNECT_TIMEOUT = 3) (RETRY_COUNT = 3) (TRANSPORT_CONNECT_TIMEOUT = 3) (ADDRESS_LIST = (LOAD_BALANCE = on) (ADDRESS = (PROTOCOL = TCPS)(HOST = sales-server)(PORT = 1421)) (ADDRESS = (PROTOCOL = TCPS)(HOST = sales-server)(PORT = 1421))) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = sales.example.com)))
TNS-12535: TNS:operation timed out

Probable Cause 1

Port 8005 is not open to allow HTTP traffic.

Solution

Add an ingress rule to allow HTTP traffic from Destination Port Range 8005. You must add this rule to the security list used by the VCN in which your database resides.

See, Review Subnet Size Requirements and Security Rules for Recovery Service Subnet

Probable Cause 2

Port 2484 is not open to allow SQL Net traffic

Solution

Add an ingress rule to allow SQL Net traffic from Destination Port Range 2484. You must add this ingress rule to the security list used by the VCN in which your database resides.

See, Review Subnet Size Requirements and Security Rules for Recovery Service Subnet

Probable Cause 3

An egress rule may be preventing network traffic on ports 8005 and 2484.

Solution

If your database VCN restricts network traffic between subnets, then ensure to add an egress rule for ports 2484 and 8005 from the database subnet to the Recovery Service subnet that you create.

See, Review Subnet Size Requirements and Security Rules for Recovery Service Subnet

Probable Cause 4

You could be using a custom DNS setup, which will lead to incorrect IP address resolution.

Solution

Perform a nslookup on the host names provided in the dbrsnames.ora file. You can also obtain the host names when you run the tnsping command. The IP address must match the IP addresses provided in the protected database hosts.txt file. You can download the hosts.txt file from the protected database details page in the OCI Console.

See, Downloading Protected Database Network Connection Details.

Probable Cause

There are insufficient unallocated IP addresses in the subnet used for Recovery Service operations in the database VCN.

Solution

To prevent a recurrence of this issue, ensure that sufficient unallocated IP addresses remain available in the subnet, or use a different Recovery Service subnet.

See, Register Recovery Service Subnet