To migrate a legacy application to GCP without knowing what ports it uses and ensuring the environment is secure, the best approach is to use a "Lift & Shift" method in an isolated project and analyze the traffic using VPC Flow logs. Here’s a step-by-step explanation:

Isolated Project:

Create a new, isolated project within your GCP environment to host the legacy application. This isolation ensures that any potential misconfigurations do not affect other projects.

Lift & Shift:

Migrate the application as-is (lift and shift) to the new isolated project. This involves moving the application without altering its architecture.

Enable Internal TCP Traffic:

Configure VPC Firewall rules to allow all internal TCP traffic within the VPC network. This step ensures that the application components can communicate internally without interruption.

Use VPC Flow Logs:

Enable VPC Flow logs to capture information about the traffic to and from your application. VPC Flow logs provide details about the source, destination, port, and protocol of the traffic.

Analyze Traffic:

Analyze the VPC Flow logs to identify the necessary ports and protocols used by the application.

Based on this analysis, create specific firewall rules to allow only the required traffic, thereby tightening security.

Implementation Steps:

Navigate to the VPC network section in the GCP Console.

Create a new VPC or use an existing one, and configure firewall rules to allow internal TCP traffic.

Enable VPC Flow logs from the VPC network settings.

Migrate your application to the new project.

Monitor and analyze the VPC Flow logs to refine your firewall rules.

By following these steps, you can safely migrate the application, understand its network requirements, and secure it appropriately in the new GCP environment.

Google Cloud VPC Documentation

VPC Flow Logs Documentation