upstream prematurely closed connection while reading response header Nginx / Apache Origin: Edge & web server OSI layer 7 · Application Last updated 5 Nov 2026

Nginx: [error] upstream prematurely closed connection while reading response header from upstream

The backend ended the connection before sending a complete response header.

Exact message you may see upstream prematurely closed connection while reading response header from upstream[error] upstream prematurely closed connection while reading response header502 Bad Gateway upstream prematurely closed connectionupstream prematurely closed FastCGI stdout while reading response header from upstreamupstream prematurely closed connection while reading upstream
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Seen on: Nginx Linux

Meaning

Usually the app/PHP-FPM worker crashed (fatal error, segfault, memory limit) or exited, or a proxy timeout/keepalive race closed the socket. Check PHP-FPM/app logs for fatal errors, request_terminate_timeout, max_execution_time and OOM kills.

Common causes

  • PHP fatal/segfault or exit() mid-request
  • Worker killed by OOM or timeout (request_terminate_timeout)
  • Backend keepalive closed the idle connection (race)
  • Wrong protocol (HTTP to a TLS/FastCGI port)
  • App crashed on large payloads

⚡ Quick fix

  1. Read the app/PHP-FPM error log for the crash
  2. Align timeouts; disable/adjust backend keepalive
  3. Check dmesg for OOM kills

Detailed fix by platform

Linux

  1. sudo journalctl -u php8.3-fpm --since "10 min ago" | tail -30

How to diagnose

  1. Backend log — Fatal or crash?
  2. Timeouts — request_terminate_timeout?
  3. Protocol — Correct?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Edge & web server. Errors that are often confused with or caused by this one (click to open):

502 — HTTP 502 Bad Gateway (HTTP)502AH00898: Error reading from remote server (502 Proxy Error) — Apache: [proxy_http:error] AH01102: error reading status line from remote server 127.0.0.1:3000 / AH00898: Error reading from remote server returned by /api (Nginx / Apache)AH00898: Error readin…connect() failed (111: Connection refused) while connecting to upstream — Nginx: [error] connect() failed (111: Connection refused) while connecting to upstream, upstream: "http://127.0.0.1:3000/" (Nginx / Apache)connect() failed (111…nginx: host not found in upstream — Nginx: nginx: [emerg] host not found in upstream "app" in /etc/nginx/conf.d/default.conf:5 (Nginx / Apache)nginx: host not found…no live upstreams while connecting to upstream — Nginx: [error] no live upstreams while connecting to upstream (503/502) (Nginx / Apache)no live upstreams whi…recv() failed (104: Connection reset by peer) while reading response header — Nginx: [error] recv() failed (104: Connection reset by peer) while reading response header from upstream (Nginx / Apache)recv() failed (104: C…upstream timed out — Nginx: upstream timed out (110: Connection timed out) while reading response header from upstream (Nginx / Apache)upstream timed outupstream …

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