OneLinersCommand workbench
Verified diagnostic signature

NGINX could not resolve, connect to, or receive a valid response from its upstream.

Match output from nginx, understand the likely cause, then continue with sourced, read-only checks.

Confidence
high
Platforms
linux
Verified
2026-07-24
Recognized output

Signature matched locally

host not found in upstream

OneLiners matches this pattern in your browser. Your pasted output is not stored or indexed.

Likely causes

What usually produces this signal

  1. Upstream stopped
  2. Wrong address or port
  3. DNS failure
  4. Timeout or protocol mismatch
Safe next checks

Collect evidence before changing the system

01

Fetch only the HTTP response headers

curl -I <url>

The output matches the expected target and exits without an error.

02

List listening TCP ports and their processes

lsof -iTCP -sTCP:LISTEN -n -P

The output matches the expected target and exits without an error.

03

Check whether a TCP port accepts connections

nc -vz <host> <port>

The output matches the expected target and exits without an error.

04

Resolve a hostname to its IP address

dig +short <domain>

The output matches the expected target and exits without an error.