301 vs 302 Redirect Guide
301 vs 302: which redirect to use, what each does to caching and SEO, and how to test the response code. Free instant header check, no sign-up.
DiagnoseHTTP Errors & Site Reachability Guides
Guides for HTTP status errors (403, 404, 5xx), redirect loops, connection failures, security headers, and open ports. Each guide includes a free check for your URL.
HTTP — 29 guides
400 Bad Request Checklist
400 Bad Request? Separate malformed syntax from oversized headers or cookies, then see which layer rejected it. Free instant check, no sign-up.
Diagnose403 Forbidden Checklist
403 Forbidden? Find the cause in 4 checks: origin permissions, WAF rules, CDN access controls, routing. Free instant check, no sign-up.
Diagnose404 Not Found Root Cause Guide
404 Not Found on a page that should exist? Trace it in 3 steps: deployment artifacts, rewrite rules, final route. Free instant check, no sign-up.
Diagnose405 Method Not Allowed Checklist
405 Method Not Allowed? Read the Allow header to see valid methods, then tell a disabled route from a proxy or CORS block. Free instant check, no sign-up.
Diagnose413 Payload Too Large Checklist
413 Payload Too Large means your upload crossed a size cap. Pin which layer set it — CDN, reverse proxy, or app — in 3 checks. Free instant check, no sign-up.
Diagnose429 Too Many Requests Checklist
429 Too Many Requests? Tell an origin rate limit from a CDN or WAF block in 3 checks: final status code, Retry-After, which layer responded. Free instant check, no sign-up.
Diagnose500 Internal Server Error Checklist
500 Internal Server Error? Separate an application crash from a gateway fault in 3 checks: final status code, origin reachability, response path. Free instant check, no sign-up.
Diagnose502 Bad Gateway Checklist
502 Bad Gateway? Isolate the cause in 3 checks: proxy-to-upstream connectivity, service health, target routing. Free instant check, no sign-up.
Diagnose503 Service Unavailable Checklist
503 Service Unavailable? Pin down the cause in 4 checks: maintenance mode, overload, rate limiting, backend readiness. Free instant check, no sign-up.
Diagnose504 Gateway Timeout Checklist
504 Gateway Timeout means the upstream was too slow, not broken. Fix it in 3 checks: upstream latency, port reachability, DNS target. Free instant check, no sign-up.
Diagnose521 Web Server Is Down (Cloudflare) Fix
Cloudflare Error 521 means your origin refused the connection. Fix it in 3 checks: server up, Cloudflare IPs allowed, port open. Free instant check, no sign-up.
Diagnose522 Connection Timed Out (Cloudflare) Fix
Cloudflare Error 522 means the connection to your origin timed out. Fix it in 3 checks: silent firewall drops, origin overload, routing. Free instant check, no sign-up.
DiagnoseERR_CONNECTION_CLOSED in Chrome: Fix Checklist
ERR_CONNECTION_CLOSED means a TCP FIN closed the connection mid-request. Check TLS, keep-alive, antivirus, and reachability. Free instant check, no sign-up.
DiagnoseERR_CONNECTION_REFUSED Fix Guide
ERR_CONNECTION_REFUSED is an instant TCP RST, not a timeout. Fix it in 3 checks: listening port, bind address, firewall reject rules. Free instant check, no sign-up.
DiagnoseERR_CONNECTION_RESET Fix Guide
ERR_CONNECTION_RESET is a TCP RST that drops an open connection. Fix it in 3 checks: server abort, middlebox, port mismatch. Free instant check, no sign-up.
DiagnoseERR_CONNECTION_TIMED_OUT Fix Guide
ERR_CONNECTION_TIMED_OUT means packets are silently dropped. Fix it in 3 checks: port reachability, network route, origin health. Free instant check, no sign-up.
DiagnoseERR_CONTENT_LENGTH_MISMATCH Fix
ERR_CONTENT_LENGTH_MISMATCH means the server sent fewer bytes than promised. Spot the proxy, disk, or worker fault in 3 checks. Free instant check, no sign-up.
DiagnoseERR_EMPTY_RESPONSE Fix
ERR_EMPTY_RESPONSE means the server closed the connection with no reply. Tell a crash from an HTTPS-port mix-up in 3 checks. Free instant check, no sign-up.
DiagnoseERR_TOO_MANY_REDIRECTS: Find and Break the Redirect Loop
ERR_TOO_MANY_REDIRECTS? Locate the loop in 4 checks: origin rules, CDN settings, cookies, HTTPS enforcement. Free instant redirect trace, no sign-up.
DiagnoseFiltered vs Closed Port in Nmap: What's the Difference
Nmap port "filtered" vs "closed"? What each state means — RST, dropped packets, or ICMP unreachable — checked in 4 steps. Free instant port check, no sign-up.
DiagnoseHSTS Preload Checklist
HSTS preload is hard to undo. Verify 3 things first: HTTPS consistency, subdomain readiness, certificate coverage. Free instant check, no sign-up.
DiagnoseHTTP Security Headers Guide
HTTP security headers explained: HSTS, CSP, X-Frame-Options, and more, with copy-paste configs for each. Free instant header check, no sign-up.
DiagnoseMixed Content Blocked: Fix Insecure Resources on HTTPS Pages
Your HTTPS page loads but scripts or images are blocked as mixed content. Locate the http:// references, tell active from passive, and upgrade them safely.
DiagnoseNo 'Access-Control-Allow-Origin' Header (CORS)
No 'Access-Control-Allow-Origin' header? Fix it in 3 checks: failed preflight, exact origin echoed back, and credentials mode. Free instant check, no sign-up.
DiagnosePort 443 Closed Checklist
Port 443 closed? Trace it in 4 checks: external reachability, listener status, firewall rules, proxy forwarding. Free instant port check, no sign-up.
DiagnoseRisky Open Ports Security Checklist
Open ports you didn't expect? Review each in 4 steps: identify the service, judge exposure, close or restrict, re-scan. Free instant port check, no sign-up.
DiagnoseTTFB / Compression / Cache-Control Checklist
Slow TTFB, missing compression, weak caching? Check all 3 in one pass: response time, Content-Encoding, Cache-Control. Free instant check, no sign-up.
DiagnoseX-Robots-Tag noindex Fix Guide
X-Robots-Tag: noindex blocking your pages? Find the injection point in 3 checks: server config, CDN rules, app middleware. Free instant check, no sign-up.
Diagnose[Ad] Guide Cluster Inline