Error map

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

Nginx / Apache · layer 3, Edge & web server

Open the full error page →

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.

2 · Network 3 · Edge & web server 4 · Application & API 6 · Platform & cloud
no live upstreams while connecting to upstream — Nginx: [error] no live upstreams while connecting to upstream (503/502) (Nginx / Apache)no live ups…502 — HTTP 502 Bad Gateway (HTTP)502upstream prematurely closed connection while reading response header — Nginx: [error] upstream prematurely closed connection while reading response header from upstream (Nginx / Apache)upstream prematurel…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 fou…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…nginx: 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…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…ECONNREFUSED — Error: connect ECONNREFUSED 127.0.0.1:5432 (Node.js)ECONNREFUSED407 — HTTP 407 Proxy Authentication Required (HTTP)407500 — HTTP 500 Internal Server Error (HTTP)500upstream timed out — Nginx: upstream timed out (110: Connection timed out) while reading response header from upstream (Nginx / Apache)upstream timed outAH00898: 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…cURL 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…403 — HTTP 403 Forbidden (HTTP)4031020 — Cloudflare Error 1020: Access Denied (Cloudflare)1020bind() failed (98: Address already in use) — nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use) (Linux)bind() failed (98: …413 — HTTP 413 Content Too Large (Request Entity Too Large) (HTTP)413EMFILE — Too many open files (EMFILE) (Linux)EMFILELink expired — WordPress: The link you followed has expired (WordPress)Link expiredupstream 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)

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.

upstream prematurely closed connection while reading response headerNginx / Apache

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

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

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.

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.

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.

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)

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.

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.

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.

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.

403HTTP

HTTP 403 Forbidden

The server understood the request but refuses to authorize it — you (or the server process) are not allowed to access this resour…

1020Cloudflare

Cloudflare Error 1020: Access Denied

A Cloudflare firewall/WAF custom rule blocked the request — the visitor’s IP, country, user agent or request pattern matched a bl…

bind() failed (98: Address already in use)Linux

nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)

Nginx (or another server) can’t listen on the port because another process already holds it — often Apache, another Nginx, or a s…

413HTTP

HTTP 413 Content Too Large (Request Entity Too Large)

The request body is bigger than the server is willing to accept — usually a file upload over the configured limit.

EMFILELinux

Too many open files (EMFILE)

The process hit its file-descriptor limit — sockets, files and pipes all count.

Link expiredWordPress

WordPress: The link you followed has expired

Usually an upload (theme, plugin, media) larger than PHP’s upload/post limits — WordPress receives an empty request and shows thi…

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.

Map another error

Popular: 500403400ORA-009331064Critical errorFileNotFoundException assemblyTypeError