HTTP Error Analyzer

Likely causes:

• Malformed request syntax (bad JSON, invalid headers)
• Request body too large for a proxy/server limit
• A client-side bug sending an invalid parameter

What to check:

• Inspect the actual request body/headers in browser DevTools → Network tab
• Check the app or API server's error log for the specific validation failure

Likely causes:

• Missing or expired auth token/session
• Wrong credentials sent
• Clock skew breaking time-based tokens (JWT, HMAC signatures)

What to check:

• Confirm the Authorization header is actually being sent
• Check server time is correct — `timedatectl status`

Likely causes:

• Valid auth, but insufficient permissions for this resource
• IP allowlist/firewall blocking the request
• File permissions denying the web server read access

What to check:

• Check web server error log for the specific denial reason
• Verify file/directory permissions with the chmod calculator if it's a static file

Likely causes:

• URL typo or the resource genuinely doesn't exist
• Missing rewrite/routing rule for a single-page app
• DNS pointing to the wrong server entirely

What to check:

• Confirm DNS resolves where you expect it to
• Check Nginx/Apache rewrite rules for SPA fallback routing

Likely causes:

• Client sent POST to a GET-only endpoint (or vice versa)
• A reverse proxy stripping or rewriting the HTTP method
• CORS preflight (OPTIONS) not handled by the server

What to check:

• Confirm which methods the endpoint actually accepts
• Check for an OPTIONS handler if this is a cross-origin API call

Likely causes:

• Client connection too slow to finish sending the request
• Server-side timeout set too aggressively for large uploads
• Network issue between client and server

What to check:

• Check client_body_timeout (Nginx) or similar directive
• Test from a different network to rule out client-side connectivity

Likely causes:

• Concurrent edits to the same resource (optimistic locking failure)
• Trying to create a resource that already exists (duplicate key)
• State machine violation (e.g., cancelling an already-completed order)

What to check:

• Check application logs for the specific constraint that failed
• Look for a version/ETag mismatch in the request

Likely causes:

• Rate limit genuinely exceeded
• A retry loop in client code hammering the endpoint
• Rate limit configured too strictly for legitimate traffic

What to check:

• Check the Retry-After response header for the cooldown period
• Review rate-limit config — Nginx limit_req zone, API gateway settings

Likely causes:

• Unhandled exception in application code
• Database connection failure
• Misconfiguration (missing environment variable, bad config file)

What to check:

• Check application error logs — this is almost always logged server-side
• Check PHP-FPM error log if running PHP: /var/log/php-fpm/www-error.log

Likely causes:

• Server doesn't support the requested HTTP method at all
• A proxy forwarding an unsupported method to the backend

What to check:

• Confirm the web server/framework actually implements this method

Likely causes:

• Upstream application server (PHP-FPM, Node, Gunicorn) crashed or isn't running
• Reverse proxy pointing to the wrong upstream address/port
• Upstream returned a malformed response

What to check:

• Check if PHP-FPM/your app process is actually running: `systemctl status php-fpm`
• Check the Nginx error log for the specific "connect() failed" message
• Verify the upstream host:port in your Nginx config matches where the app is actually listening

Likely causes:

• Server overloaded — all worker processes busy
• App deliberately in maintenance mode
• Upstream health check failing in a load balancer

What to check:

• Check worker/connection limits with the PHP-FPM or Nginx worker calculator
• Check if a maintenance-mode flag or file is set

Likely causes:

• Upstream application taking longer than the proxy's timeout to respond
• A slow database query blocking the response
• Network issue between the proxy and the upstream server

What to check:

• Check proxy_read_timeout (Nginx) against how long the backend actually takes
• Profile the slow endpoint — is it a specific slow query?