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
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Likely causes
What usually produces this signal
- Upstream stopped
- Wrong address or port
- DNS failure
- Timeout or protocol mismatch
Safe next checks
Collect evidence before changing the system
Fetch only the HTTP response headers
curl -I <url>The output matches the expected target and exits without an error.
List listening TCP ports and their processes
lsof -iTCP -sTCP:LISTEN -n -PThe output matches the expected target and exits without an error.
Check whether a TCP port accepts connections
nc -vz <host> <port>The output matches the expected target and exits without an error.
Resolve a hostname to its IP address
dig +short <domain>The output matches the expected target and exits without an error.