Nginx Fixes

12 troubleshooting guides — each with the error, why it happens, and copy-paste commands to fix it.

Nginx 400: "The Plain HTTP Request Was Sent to HTTPS Port" — The port speaks TLS; the client spoke HTTP.
Nginx 504: "Upstream Timed Out (110: Connection Timed Out)" — The app behind nginx didn't answer in time.
nginx: "Host Not Found in Upstream" — DNS at Startup vs Runtime — nginx resolves proxy_pass hostnames at CONFIG LOAD, not per request.
Nginx 400: "Request Header or Cookie Too Large" — Buffer Tuning Done Right — A request header (usually a bloated cookie or a huge token) exceeded large_client_header_buffers (or the 4k cookie line).
ERR_TOO_MANY_REDIRECTS: The Redirect Loop You Created — A redirect loop means every response says 'go somewhere else' until the browser gives up.
Nginx 413 Request Entity Too Large (client_max_body_size) — Nginx refuses request bodies over 1MB by default.
Nginx 502 "Upstream Sent Too Big Header" — Buffers, Not the App — When a proxied app returns very large headers (big cookies, long auth tokens), Nginx's default response buffers overflow and it returns 502.
Nginx 503: "worker_connections Are Not Enough" — Real Limits vs Bandwidth — Nginx returns 503 when worker_connections (default 512/1024) can't cover concurrent sockets.
Nginx 400 "Request Header or Cookie Too Large" — Nginx rejects requests whose headers overflow its default buffers — usually a fat cookie.
Nginx 502: "Upstream Closed Prematurely" — Crashed or Timing Out Backend — The backend accepted the connection then died mid-response.
Nginx 403 Forbidden — Permissions, Index, or Deliberate Deny — Nginx refused to serve — and it's telling you why in error.
nginx: "Unknown Directive" — Module Not Loaded (or Wrong Version) — nginx parsed your config and found a directive no loaded module provides.

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Nginx Fixes

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Vultr — our lab-environment pick for this stack