Comprehensive investigation from DNS to firewall to determine why websites cannot be accessed on Hong Kong servers

Brief Introduction: When a website cannot be accessed due to issues with the Hong Kong server, the troubleshooting should cover multiple aspects such as DNS resolution, network connectivity, service ports, firewalls, and CDN. This guide lists key checkpoints and common symptoms step by step to help quickly identify the root cause and restore access. It is suitable for operations and technical support teams as a reference for troubleshooting.

Initial Check of Domain Names and DNS Resolution

First, confirm whether the domain name is correctly resolved to the Hong Kong server IP. Use dig or nslookup to check the A/AAAA/CNAME records and TTL values, and verify the WHOIS information and domain name renewal status. Be aware of DNS caching and propagation delays, and check for misconfigured CNAMEs or corrupted resolutions. If an external DNS service is used, verify whether it responds properly at the Asia-Pacific nodes, and whether DNSSEC is enabled, causing resolution failures.

Network Connectivity and Route Tracing

Use ping, traceroute, or mtr to test the latency and packet loss paths from different regions to the Hong Kong server, and analyze for packet loss, abnormal routing hops, or loops. Check whether BGP routes are declared correctly, whether ISPs are blocking specific prefixes, and whether there is congestion on cross-border links. If necessary, perform external probes from multiple Internet service providers or cloud nodes to rule out connectivity issues at the operator level.

Server-side services and port listening

On the server side, use netstat, ss, or lsof to confirm whether the Web service is listening on ports 80/443. Check the status of the nginx or apache processes as well as the syntax of their configuration files, to see if any configuration changes have caused the service not to start or to bind to the wrong port. Verify whether the backend application pool or database connections are available, and troubleshoot service unavailability issues caused by high memory or load, as well as abnormal restart logs.

Firewall and Security Group Policy Troubleshooting

Check whether the server’s local firewall (iptables, nftables, ufw) and the cloud provider’s security groups have accidentally blocked the HTTP/HTTPS ports, and verify if any IP allowlists or rate limits are in place. Hong Kong data centers may sometimes be affected by facility-level ACLs or ISP firewalls. Check whether access is blocked due to GeoIP blocking, DDoS protection activation, or changes in firewall policies.

Reverse proxy, load balancing, and CDN configuration

If a reverse proxy, load balancing, or CDN is used, check the upstream host configuration, health check status, and caching policies. Common issues include mismatched Host headers or SNI, incorrect backend addresses, 504/502 gateway timeouts, or old configurations taking effect due to edge node caching. Compare the edge and origin server responses; if necessary, temporarily bypass the CDN for a direct connection test to identify the issue.

Issues related to certificates and HTTPS

HTTPS errors often cause the “cannot be opened” issue. Check whether the certificate has expired, whether the chain is complete, whether the domain name matches the certificate’s SANs, and the OCSP/CRL status. Verify TLS version and cipher suite compatibility, and confirm that the server has the correct certificate files and private key permissions enabled. Browser error messages and openssl s_Client output helps quickly identify issues with the certificate chain or handshake failures.

Log and packet capture analysis

Use access/error logs, application logs, and system logs to analyze at the user request time point, and check return codes, error stacks, and exception events. Use tcpdump or Wireshark to capture network packets simultaneously, and analyze the actual traffic of the three-way handshake, TLS handshake, or HTTP request responses to determine whether there are any intermediate packet losses, RST packets, or connection resets. Combine monitoring alerts to quickly identify problem trends.

Summary and Recommendations

It is recommended to troubleshoot step by step, starting from DNS, routing, port services, firewall, proxy/CDN, certificates, and logs, while recording the test results and changes for each step. If it cannot be located in a short time, please contact the server room or ISP to verify the status of the link and the blacklist. Use a rollback strategy to restore the available configuration, and establish regular health checks and alert mechanisms to ensure Hong Kong server Stable external access and rapid fault response.