How do I troubleshoot a ZCenter deployment or access failure caused by blocked ports?

Issue/Introduction

This article describes how to troubleshoot port and dependency connectivity issues when ZCenter is inaccessible after deployment, an API returns 502/503, a service fails to start, or services cannot discover one another. It distinguishes the external proxy port from the internal Gateway port to prevent confusion between ports 8100 and 8200.

Symptoms

• A client connection to http://<host>:8100 times out, is refused, or cannot be established.
• Port 8100 accepts a connection but returns 502/503, or the page opens but API requests fail.
• Service startup logs report that connections to 3306, 6379, 5672, or 8848 were refused, timed out, or failed authentication.
• Gateway is accessible, but /api/portal/..., /api/vm/..., or another API returns 503 or has no route.

Environment

• Product: ZCenter
• Version: 5.1.0
• Component: Caddy/reverse proxy, Gateway, Nacos, Base, and business services
• Deployment: Single-node or multi-node. Deployment parameters, container mappings, load balancers, or firewall policies might override the actual ports.

Possible causes