39 browser, certificate, DNS and edge errors — each with the most likely cause first, the exact fix, and a free check that tells you whether it is happening on your own domain.
Browser & network errors
Chrome, Edge and Firefox connection failures — the request never completed.
ERR_CONNECTION_TIMED_OUT — DNS resolved and the browser tried to open a TCP connection, but nothing answered before the timeout. The server is unreachable,…
ERR_CONNECTION_REFUSED — Something answered the connection attempt and rejected it outright. That means the host is reachable but nothing is listening on that port…
ERR_CONNECTION_RESET — The connection was established and then abruptly torn down before the response finished. Usually a crashing worker process, a proxy timing…
ERR_CONNECTION_CLOSED — The server closed the connection cleanly but before sending a complete response. Close cousin of a reset, and the same short list of…
ERR_EMPTY_RESPONSE — The connection succeeded and then the server returned zero bytes. Typically a fatal application error that killed the process before any…
ERR_TOO_MANY_REDIRECTS — Two rules are bouncing the request back and forth — most often an HTTPS redirect at the application level fighting a proxy that is…
ERR_ADDRESS_UNREACHABLE — DNS returned an address and the network has no route to it. Common when a DNS record points at a private, retired, or IPv6 address the…
ERR_NETWORK_CHANGED — The device switched networks mid-request — Wi-Fi to mobile, a VPN connecting, or an IP address changing. Almost always a client-side…
ERR_INTERNET_DISCONNECTED — Chrome has no usable network route at all. This is a client-side state; no server configuration produces it.
ERR_BLOCKED_BY_CLIENT — The browser itself refused to send the request because an extension — usually an ad or tracker blocker — matched it against a block list.…
ERR_HTTP2_PROTOCOL_ERROR — The HTTP/2 connection broke a protocol rule and the browser aborted the stream. Usually caused by a server or proxy with a buggy HTTP/2…
ERR_QUIC_PROTOCOL_ERROR — The browser tried HTTP/3 over QUIC and the connection failed. Often a network path that mishandles UDP, or a CDN advertising HTTP/3 support…
TLS & certificate errors
The connection was refused because the certificate or TLS handshake failed.
ERR_SSL_PROTOCOL_ERROR — Chrome tried to start an encrypted connection and the server answered with something that is not valid TLS. In almost every case the server…
ERR_CERT_DATE_INVALID — The certificate the server presented is outside its validity window — almost always expired. Either the certificate genuinely lapsed, or…
ERR_CERT_AUTHORITY_INVALID — The browser could not build a trust path from the certificate the server sent to a root it trusts. Usually the server is serving only the…
ERR_CERT_COMMON_NAME_INVALID — The certificate is valid and trusted, but it was issued for a different hostname than the one in the address bar. The classic case is a…
ERR_SSL_VERSION_OR_CIPHER_MISMATCH — The browser and server have no protocol version or cipher suite in common, so the handshake ends before it starts. Almost always an origin…
ERR_SSL_OBSOLETE_VERSION — A warning rather than a hard failure: the connection succeeded but negotiated TLS 1.0 or 1.1, which browsers flag as obsolete and will…
SSL handshake failed — A generic handshake failure reported by servers, CDNs and API clients rather than browsers. The connection reached the server but the TLS…
Your connection is not private — Chrome's full-page warning shown whenever certificate validation fails. It is not a single error — the real cause is the NET::ERR_CERT_*…
ERR_CERT_REVOKED — The CA has explicitly revoked this certificate — usually because the private key was compromised, the certificate was reissued, or the…
ERR_BAD_SSL_CLIENT_AUTH_CERT — The server asked for a client certificate during the handshake and rejected the one presented — or none was available. This is mutual TLS,…
DNS errors
The hostname could not be resolved to an IP address.
DNS_PROBE_FINISHED_NXDOMAIN — The DNS lookup completed and the authoritative answer was 'this name does not exist'. Either the record is genuinely missing, the domain…
ERR_NAME_NOT_RESOLVED — Chrome could not turn the hostname into an IP address. Closely related to NXDOMAIN but broader: it also covers resolver failures and…
DNS_PROBE_STARTED — A transient state Chrome shows while it retries a failed lookup with its own probe. If it sticks, the resolver is not answering at all —…
DNS_PROBE_FINISHED_BAD_CONFIG — The probe finished and concluded the device's DNS configuration itself is broken — a bad resolver address, a corrupted network profile, or…
DNS_PROBE_FINISHED_NO_INTERNET — The DNS probe could not reach any resolver because there is no working route to the internet. The domain is irrelevant here — connectivity…
Server DNS address could not be found — The human-readable wrapper Chrome shows around a DNS failure. The technical code underneath — usually DNS_PROBE_FINISHED_NXDOMAIN or…
Cloudflare & edge errors
The CDN reached your origin and got nothing usable back.
Cloudflare Error 520 — Cloudflare reached your origin and got back a response it could not interpret — an empty reply, a malformed header, or a connection dropped…
Cloudflare Error 521 — Cloudflare's connection to the origin was refused. The origin either has nothing listening on the expected port, or its firewall is…
Cloudflare Error 522 — Cloudflare tried to open a connection to the origin and nothing answered before the timeout. Usually a firewall silently dropping…
Cloudflare Error 523 — Cloudflare could not route to the origin at all. Typically the DNS record points at an invalid or unroutable IP, or the origin's network…
Cloudflare Error 524 — Cloudflare connected to the origin successfully, but the origin took longer than 100 seconds to send a complete response. The request is…
Cloudflare Error 525 — Cloudflare reached the origin on port 443 but the TLS handshake failed. Nearly always the origin has no valid certificate, no certificate…
Cloudflare Error 526 — Cloudflare completed the handshake with the origin but refused its certificate — expired, self-signed, or not matching the hostname —…
Cloudflare Error 1015 — A Cloudflare rate-limiting rule matched the request. Either the client genuinely exceeded a limit, or a rule is scoped too broadly and is…
Cloudflare Error 1020 — A Cloudflare WAF or firewall rule blocked the request. The origin never received it. The Security Events log names the exact rule that…
Application errors
The server answered, but the application behind it failed.
Error establishing a database connection — WordPress loaded but could not connect to MySQL. Either the credentials in wp-config.php are wrong, the database server is down, or…
White screen of death — PHP hit a fatal error with display_errors off, so the page rendered as blank white with an HTTP 200 or 500. Nothing is visibly broken to a…