Error map

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

Nginx / Apache · layer 3, Edge & web server

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Centre: this error. First ring: errors it is linked with (7). Outer ring: what those are linked with. Colours show the layer of a request each error belongs to. Click any dot to re-centre the map on it; hover to follow its connections. The centre dot opens the error page.

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upstream prematurely closed connection while reading response header — Nginx: [error] upstream prematurely closed connection while reading response header from upstream (Nginx / Apache)upstream pr…upstream timed out — Nginx: upstream timed out (110: Connection timed out) while reading response header from upstream (Nginx / Apache)upstream timed out502 — HTTP 502 Bad Gateway (HTTP)502nginx: 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 fou…AH00898: 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 read…no live upstreams while connecting to upstream — Nginx: [error] no live upstreams while connecting to upstream (503/502) (Nginx / Apache)no live upstreams w…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:…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 (1…2006 — MySQL ERROR 2006 (HY000): MySQL server has gone away (MySQL)2006408 — HTTP 408 Request Timeout (HTTP)408504 — HTTP 504 Gateway Timeout (HTTP)504ECONNREFUSED — Error: connect ECONNREFUSED 127.0.0.1:5432 (Node.js)ECONNREFUSED407 — HTTP 407 Proxy Authentication Required (HTTP)407500 — HTTP 500 Internal Server Error (HTTP)500cURL error 6 — cURL error 6: Could not resolve host: api.example.com (PHP)cURL error 6sudo: unable to resolve host — sudo: unable to resolve host <hostname>: Name or service not known (Linux)sudo: unable to res…Temporary failure in name resolution — ping: google.com: Temporary failure in name resolution (Linux)Temporary failure i…417 — HTTP 417 Expectation Failed (HTTP)417511 — HTTP 511 Network Authentication Required (HTTP)511AH00558 — Apache: AH00558: apache2: Could not reliably determine the server's fully qualified domain name (Nginx / Apache)AH00558nginx: client intended to send too large body — Nginx: [error] client intended to send too large body: 5242880 bytes (413 Request Entity Too Large) (Nginx / Apache)nginx: client inten…ModSecurity: Access denied with code 403 — Nginx/Apache: ModSecurity: Access denied with code 403 (phase 2). Matched "Operator GE matched 5 at TX:anomaly_score" (Nginx / Apache)ModSecurity: Access…upstream sent too big header while reading response header — Nginx: [error] upstream sent too big header while reading response header from upstream (502 Bad Gateway) (Nginx / Apache)upstream sent too b…499 Client Closed Request / client prematurely closed connection — Nginx: [info] client prematurely closed connection while reading response header from upstream / access log status 499 (Nginx / Apache)499 Client Closed R…connect() to unix:/run/php/php-fpm.sock failed (13: Permission denied) — Nginx: [crit] connect() to unix:/run/php/php8.3-fpm.sock failed (13: Permission denied) while connecting to upstream (Nginx / Apache)connect() to unix:/…WebSocket connection failed through Nginx (missing Upgrade headers) — Browser: WebSocket connection to 'wss://example.com/socket' failed: Error during WebSocket handshake: Unexpected response code: 400 (nginx proxy) (Nginx / Apache)WebSocket connectio…proxy_pass with a URI changes the path (404 from upstream) — Nginx: reverse proxy returns 404 — location /api/ { proxy_pass http://app:3000/; } strips the prefix (or keeps it) (Nginx / Apache)proxy_pass with a U…

Directly related (7)

upstream timed outNginx / Apache

Nginx: upstream timed out (110: Connection timed out) while reading response header from upstream

Nginx waited longer than proxy_read_timeout (60 s default) for the backend (app server, PHP-FPM) to respond, then returned 504.

502HTTP

HTTP 502 Bad Gateway

A proxy or gateway (Nginx, load balancer, CDN) got an invalid response — or no response — from the upstream server behind it.

nginx: host not found in upstreamNginx / Apache

Nginx: nginx: [emerg] host not found in upstream "app" in /etc/nginx/conf.d/default.conf:5

Nginx couldn't resolve the hostname in an `upstream`/`proxy_pass` at startup.

AH00898: Error reading from remote server (502 Proxy Error)Nginx / Apache

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

mod_proxy couldn't read a valid response from the backend, so Apache returned 502/503.

no live upstreams while connecting to upstreamNginx / Apache

Nginx: [error] no live upstreams while connecting to upstream (503/502)

All servers in an `upstream` group are marked unavailable, so Nginx has nothing to proxy to.

recv() failed (104: Connection reset by peer) while reading response headerNginx / Apache

Nginx: [error] recv() failed (104: Connection reset by peer) while reading response header from upstream

The upstream (PHP-FPM, Node, Gunicorn) closed the connection abruptly while Nginx waited for the response.

connect() failed (111: Connection refused) while connecting to upstreamNginx / Apache

Nginx: [error] connect() failed (111: Connection refused) while connecting to upstream, upstream: "http://127.0.0.1:3000/"

Nginx tried to proxy a request but nothing listens on the upstream address, so the client gets 502.

Two steps away (19)

2006MySQL

MySQL ERROR 2006 (HY000): MySQL server has gone away

The connection to MySQL was closed — usually by a timeout or a packet larger than `max_allowed_packet`.

408HTTP

HTTP 408 Request Timeout

The server closed the connection because the client took too long to send the complete request.

504HTTP

HTTP 504 Gateway Timeout

A proxy or gateway waited too long for the upstream server to respond and gave up.

ECONNREFUSEDNode.js

Error: connect ECONNREFUSED 127.0.0.1:5432

Nothing is listening at that host:port — the target service is down or you’re connecting to the wrong address.

407HTTP

HTTP 407 Proxy Authentication Required

A proxy between you and the server requires credentials before it will forward your request.

500HTTP

HTTP 500 Internal Server Error

The server hit an unexpected condition — usually an unhandled exception or misconfiguration in the application.

cURL error 6PHP

cURL error 6: Could not resolve host: api.example.com

PHP cURL couldn’t resolve the hostname via DNS.

sudo: unable to resolve hostLinux

sudo: unable to resolve host <hostname>: Name or service not known

The machine’s hostname isn’t in /etc/hosts (or DNS), so sudo prints this warning — usually after changing the hostname.

Temporary failure in name resolutionLinux

ping: google.com: Temporary failure in name resolution

The Linux host can’t resolve domain names — no working DNS server is configured or reachable.

417HTTP

HTTP 417 Expectation Failed

The server or a proxy can’t meet the Expect header in the request — almost always Expect: 100-continue.

511HTTP

HTTP 511 Network Authentication Required

A captive portal (hotel, airport, office Wi-Fi) is intercepting your traffic until you log in or accept its terms.

AH00558Nginx / Apache

Apache: AH00558: apache2: Could not reliably determine the server's fully qualified domain name

A harmless startup warning: Apache has no global ServerName, so it guessed one from the host’s IP.

nginx: client intended to send too large bodyNginx / Apache

Nginx: [error] client intended to send too large body: 5242880 bytes (413 Request Entity Too Large)

A request body exceeded `client_max_body_size` (default 1 MB), so Nginx returned 413 before reaching your app.

ModSecurity: Access denied with code 403Nginx / Apache

Nginx/Apache: ModSecurity: Access denied with code 403 (phase 2). Matched "Operator GE matched 5 at TX:anomaly_score"

The ModSecurity WAF (OWASP Core Rule Set) blocked the request as suspicious.

upstream sent too big header while reading response headerNginx / Apache

Nginx: [error] upstream sent too big header while reading response header from upstream (502 Bad Gateway)

The backend's response headers (cookies, Set-Cookie, long headers) exceeded Nginx's proxy/fastcgi buffer size.

499 Client Closed Request / client prematurely closed connectionNginx / Apache

Nginx: [info] client prematurely closed connection while reading response header from upstream / access log status 499

The client (browser, mobile app, load balancer) disconnected before Nginx finished the response; Nginx logs status 499.

connect() to unix:/run/php/php-fpm.sock failed (13: Permission denied)Nginx / Apache

Nginx: [crit] connect() to unix:/run/php/php8.3-fpm.sock failed (13: Permission denied) while connecting to upstream

Nginx can't open the PHP-FPM (or app) Unix socket because of socket ownership/permissions or SELinux.

WebSocket connection failed through Nginx (missing Upgrade headers)Nginx / Apache

Browser: WebSocket connection to 'wss://example.com/socket' failed: Error during WebSocket handshake: Unexpected response code: 400 (nginx proxy)

Nginx isn't forwarding the `Upgrade`/`Connection` headers needed for WebSockets.

proxy_pass with a URI changes the path (404 from upstream)Nginx / Apache

Nginx: reverse proxy returns 404 — location /api/ { proxy_pass http://app:3000/; } strips the prefix (or keeps it)

Whether `proxy_pass` has a URI part decides if the matched prefix is stripped, so the backend receives an unexpected path.

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