ECS Load balancer DNS unable to access privately hosted RDS MSSQL

There could be a few reasons why your application load balancer (ALB) is not able to connect to the RDS MSSQL instance when accessed using the ALB DNS domain name URL. Here are a few things to check:

Security group settings: Make sure that the security group associated with the RDS MSSQL instance allows incoming traffic from the security group associated with the ECS Fargate container or the security group associated with the ALB.

VPC settings: Ensure that the RDS MSSQL instance and the ECS Fargate container are in the same VPC and that they are able to communicate with each other. Also, check that the VPC has the correct routing table settings to allow communication between the RDS MSSQL instance and the ECS Fargate container.

Network ACLs: Check if you have any Network ACLs that are blocking traffic between the ECS Fargate container and the RDS MSSQL instance.

DNS resolution: Make sure that the DNS resolution is set up correctly for the ALB DNS domain name URL and it is pointing to the correct IP address.

Application settings: Ensure that the application settings within the ECS Fargate container are configured correctly to connect to the RDS MSSQL instance using the ALB DNS domain name URL.

By checking these items one by one, you should be able to identify the issue causing the connection failure and take the necessary steps to resolve it