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🔥 Complete HTTP Status Code Guide (DevOps Version)

🟢 1xx – Informational (Rarely Used in Debugging)

Code Meaning DevOps Note
100 Continue Client can send body
101 Switching Protocols WebSocket upgrade

Usually not part of troubleshooting.

🟢 2xx – Success Codes

These mean the server handled the request correctly.

Code Meaning When You See It
200 OK Normal website/API response
201 Created Resource created (POST)
202 Accepted Async job started
204 No Content Health checks / DELETE

🔎 DevOps Tip:

If load balancer health check expects 200 but app returns 204 → can cause unhealthy status.

🟡 3xx – Redirection (Infrastructure-Level Important)

Code Meaning DevOps Usage
301 Permanent Redirect Domain migration
302 Temporary Redirect Short testing
307 Temporary (strict method) API-safe redirect
308 Permanent (strict method) Modern 301

🔥 301 vs 302 Difference (Very Important)

301 302
Browser caches permanently Browser may not cache
SEO transfers authority No SEO transfer
Used in domain migration Used in temporary switch

🔴 4xx – Client Errors (VERY COMMON)

These happen because of request issues.

🔹 400 – Bad Request

Malformed request.

Causes:

  • Invalid JSON
  • Wrong headers
  • Corrupt cookies

🔹 401 – Unauthorized

Authentication required.

Cause:

  • Missing token
  • Expired JWT
  • Wrong API key

🔹 403 – Forbidden

Server understood request but refuses.

Common DevOps Causes:

  • Cloudflare WAF block
  • Security group restriction
  • Nginx deny rule
  • IP blocked

🔹 404 – Not Found

Resource doesn’t exist.

Causes:

  • Wrong route
  • Deployment missing files
  • Wrong ingress path (Kubernetes)

🔹 405 – Method Not Allowed

Wrong HTTP method used.

Example: POST on GET-only route.

🔹 408 – Request Timeout

Client too slow.

🔹 409 – Conflict

Data conflict (DB level).

🔹 413 – Payload Too Large

Upload exceeded limit.

Often caused by:

  • Nginx client_max_body_size
  • Cloudflare upload limit

🔹 429 – Too Many Requests

Rate limiting.

Caused by:

  • API gateway limit
  • Cloudflare rate limiting
  • WAF rule

🔥 5xx – Server Errors (Critical)

These are backend/server/infrastructure problems.

🔹 500 – Internal Server Error

Application crashed.

Check:

  • App logs
  • Stack trace
  • Runtime error

🔹 501 – Not Implemented

Feature not supported.

🔹 502 – Bad Gateway (VERY COMMON)

Gateway received invalid response from backend.

Common Causes:

  • Backend down
  • Wrong upstream IP
  • Docker container crashed
  • Wrong port mapping
  • Load balancer target unhealthy

DevOps golden rule:

502 = Upstream problem

🔹 503 – Service Unavailable

Server overloaded or down.

Causes:

  • Auto-scaling delay
  • Too much traffic
  • Maintenance mode

🔹 504 – Gateway Timeout (VERY IMPORTANT)

Gateway waited too long.

Causes:

  • Slow DB
  • Slow backend API
  • Long-running query
  • Timeout mismatch (Nginx vs app)

DevOps rule:

504 = Performance problem

🔥 Cloudflare-Specific Errors (Important for You)

Code Meaning
520 Unknown error
521 Web server down
522 Connection timed out
523 Origin unreachable
525 SSL handshake failed
526 Invalid SSL certificate

Example: 522 → Your server not responding 525 → SSL mode mismatch

🧠 DevOps Debug Flow Based on Status Code

If 4xx:

  • Check request
  • Check auth
  • Check WAF

If 5xx:

  • Check backend
  • Check logs
  • Check container status
  • Check health checks
  • Check timeouts

🔥 Real Interview Question

If interviewer asks:

What is difference between 502 and 504?

Correct Answer:

  • 502 → Bad response from upstream
  • 504 → No response within timeout

⚡ Quick Cheat Sheet

Code Check This
401 Auth
403 Firewall/WAF
404 Route/Ingress
429 Rate limit
500 App logs
502 Backend down
503 Overload
504 Slow backend