HTTP服务器正常运行但部分服务判定其下线，寻求排查方向

Hey Dor, let’s work through this together—since your server responds to ping and you can access it directly, the problem is almost certainly tied to how services like isitup and IFTTT are checking your server, not basic network connectivity. Here’s where to start your troubleshooting:

• Grab a terminal and run curl -I your-server-url to pull the full HTTP response header. Tools like isitup rely heavily on getting a 200 OK response—if your server is returning a 4xx (like 403 Forbidden) or 5xx (like 500 Internal Server Error) code (even if the page loads fine in your browser), these automated checkers will flag it as down.
• Also, look for unusual headers (like an overly restrictive Cache-Control or non-standard Content-Type) that might throw off the tools’ parsing logic.

• Make sure isitup and IFTTT are checking the correct port: default is 80 for HTTP, 443 for HTTPS. If your LAMP server is running on a non-standard port (e.g., 8080), these tools won’t detect it unless you explicitly specify the port in their settings.
• Double-check HTTP vs HTTPS: if your server redirects HTTP to HTTPS, some monitoring tools don’t follow redirects automatically. Test the full chain with curl -L your-server-url to see if the final response is valid.

• Even if you can reach the server locally, there might be a routing quirk that only affects external services. Try accessing your server from a mobile hotspot or another network outside your local setup.
• Also, confirm your public IP hasn’t changed—if you’re using a dynamic IP, isitup/IFTTT might still be pointing to an outdated address.

• Raspbian’s Apache logs live at /var/log/apache2/access.log and /var/log/apache2/error.log. Run tail -f /var/log/apache2/access.log in one terminal, then trigger a check from isitup or IFTTT. Look for:
  • Requests coming from those services’ IP addresses that return non-200 codes.
  • Any error messages in error.log that coincide with the check times (like permission issues or misconfigured modules).

• Even without a firewall, Apache might have rules blocking automated requests. Check your Apache configs (/etc/apache2/apache2.conf, /etc/apache2/sites-available/your-site.conf) for LimitRequest directives or mod_security rules that could flag isitup/IFTTT as bots.
• Also, inspect your .htaccess file (if you have one) for user-agent blocking rules—some monitoring tools use specific user-agent strings that might be getting blocked accidentally.

• Run nslookup your-server-domain from a machine outside your local network to ensure it resolves to your current public IP. If you recently updated your domain’s DNS, propagation might not be complete, so isitup/IFTTT could be hitting an old IP.
• Check for DNS issues like misconfigured CNAME records or DNSSEC validation errors that might prevent external services from resolving your domain correctly.

• IFTTT’s "Website is up" trigger sometimes requires a specific text string to be present in the response body. If you recently updated your site’s content, run curl your-server-url | grep "your-target-string" to confirm the string IFTTT is looking for still exists.

Once you’ve gone through these steps, you’ll likely spot the root cause. If you find something specific in the logs or response codes, share those details and we can narrow it down further!

内容的提问来源于stack exchange，提问作者Dor Moshkovitz