Networking Fixes

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

• nginx 502 Bad Gateway: Finding the Real Culprit — 502 means nginx reached for your upstream and got nothing usable back.
• certbot Renewals Failing Silently Until Your Cert Expires — Let's Encrypt certs last 90 days and certbot renews via a cron/systemd timer — until something small breaks it.
• nginx 504 Gateway Timeout: Slow Upstream or Wrong Timeout — 504 means nginx waited for your app and gave up.
• curl: (6) Could Not Resolve Host — The Three DNS Layers — Name resolution failed before any connection attempt.
• "ERR_CONNECTION_REFUSED" — Nothing Is Listening on That Port — Connection refused means the host was reachable but nothing accepted the connection on that port.
• HTTP 429 Too Many Requests: Read the Headers, Then Back Off — A 429 is an application-level 'slow down', not a network failure.
• "Connection Reset by Peer" — Which End Reset You, and Why — A reset (RST) means someone actively tore the connection down: the peer app, a middlebox, or the OS (port closed / queue full).
• Weird Network Failures: TLS Hangs, Large Transfers Stall — Check MTU — Small packets pass, large ones vanish: the classic MTU black hole.
• CORS Error: "Blocked by CORS Policy" — The Server's Job, Not the Browser's — CORS is the SERVER telling the browser who may call it.