AWS DMS - Replication instance cannot connect to the rds

To troubleshoot the connection issue between your DMS replication instance and the MySQL RDS instance, you can check the following:

• Ensure the security group attached to your RDS instance allows inbound connections from the DMS replication instance on the MySQL port (default is 3306). Double-check the source IP or security group specified in the inbound rule.
• If your DMS and RDS instances are in different VPCs, ensure there's a VPC peering connection that allows traffic between these VPCs. Also, check the route tables to ensure they have routes that direct traffic appropriately between the VPCs.
• Check if the MySQL user has the necessary permissions and if there are any host restrictions on the user account that might prevent connections from the DMS instance.
• In DMS, ensure the endpoint configuration for the RDS instance (especially the server name and port) matches the actual RDS instance details.
• If there's a firewall appliance in your VPC, verify that they're not blocking the traffic between DMS and RDS instances.

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