Error map
SSL_do_handshake() failed while SSL handshaking to upstream Nginx: [error] SSL_do_handshake() failed (SSL: error:0A000410:SSL routines::sslv3 alert handshake failure) while SSL handshaking to upstream
Nginx / Apache · layer 3, Edge & web server
Centre: this error. First ring: errors it is linked with (8). 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.
Directly related (8)
Cloudflare Error 526: Invalid SSL Certificate
With SSL mode Full (strict), the origin’s certificate is expired, self-signed, or doesn’t match the hostname.
Cloudflare Error 525: SSL Handshake Failed
Cloudflare couldn’t complete a TLS handshake with your origin (SSL mode Full / Full strict).
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.
OpenSSL: error:14094410:SSL routines:ssl3_read_bytes:sslv3 alert handshake failure (SSL alert number 40)
The server aborted the handshake with alert 40 — no acceptable parameters (cipher, signature algorithm, client certificate or SNI…
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.
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.
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.
Nginx: [error] no resolver defined to resolve api.example.com, client: 203.0.113.5 (502 when proxy_pass uses a variable)
`proxy_pass` with a variable (or `set $upstream`) needs a `resolver`, which isn't configured.
Two steps away (24)
Cloudflare Error 520: Web Server Returned an Unknown Error
Cloudflare connected to your origin, but the origin returned an empty, unknown or invalid response.
NET::ERR_CERT_DATE_INVALID (certificate expired)
The site’s certificate has expired (or isn’t valid yet) — or the device clock is wrong.
NET::ERR_CERT_AUTHORITY_INVALID with a Cloudflare Origin Certificate (record not proxied)
Cloudflare Origin CA certificates are only trusted by Cloudflare — visitors reaching the origin directly (grey-cloud record, dire…
Cloudflare Error 521: Web Server Is Down
Cloudflare couldn’t connect to your origin — the connection was refused.
ERR_SSL_PROTOCOL_ERROR (This site can't provide a secure connection)
The browser couldn’t negotiate TLS — the server sent an invalid response, often plain HTTP on the HTTPS port.
HTTP 421 Misdirected Request
The request reached a server that isn’t configured for that host name — usually HTTP/2 connection reuse across domains sharing a…
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.
HTTP 407 Proxy Authentication Required
A proxy between you and the server requires credentials before it will forward your request.
HTTP 500 Internal Server Error
The server hit an unexpected condition — usually an unhandled exception or misconfiguration in the application.
cURL error 35: OpenSSL SSL_connect: SSL_ERROR_SYSCALL in connection to host:443
The TLS handshake failed — protocol/cipher mismatch, a proxy interfering, or the server closed the connection.
Firefox: Secure Connection Failed — SSL_ERROR_NO_CYPHER_OVERLAP (no common cipher)
Firefox and the server share no TLS cipher suite, so the secure handshake cannot even start.
OpenSSL: error:num=20:unable to get local issuer certificate / SSL: CERTIFICATE_VERIFY_FAILED
The client could not build a chain from the server's certificate to a CA it trusts — the issuer is missing locally or the server…
HTTP 400 Bad Request
The server could not understand the request because its syntax, headers or body are malformed or invalid.
Cloudflare-branded 502 Bad Gateway / 504 Gateway Timeout ("cloudflare" footer)
A plain-looking 502/504 page with “cloudflare” at the bottom comes from Cloudflare’s network itself (or an origin response it cou…
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.
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.
Nginx: [error] FastCGI sent in stderr: "PHP message: PHP Fatal error: Uncaught Error: Call to undefined function mysqli_connect()" while reading response header from upstream
PHP wrote an error to stderr while handling the request, so Nginx logs it (often a 500/502 follows).
Nginx: 400 Bad Request — Request Header Or Cookie Too Large / client sent too long header line
The request line or headers (often cookies) exceed `large_client_header_buffers`.
HTTP 404 Not Found
The server can’t find anything at the requested URL.
HTTP 405 Method Not Allowed
The URL exists, but it doesn’t accept the HTTP method you used (e.g. POST to a GET-only route).
TypeError: Missing parameter name at 1 (path-to-regexp)
An Express 5 (path-to-regexp v8) route uses old wildcard syntax like '*' or '/:id?'.
cURL error 6: Could not resolve host: api.example.com
PHP cURL couldn’t resolve the hostname via DNS.
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.
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.
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