523 Origin Is Unreachable
Meaning
A Cloudflare-specific code indicating that the origin server was unreachable. This could be due to DNS issues, network routing problems, or the server not responding at the network level.
When to Use
When the proxy (Cloudflare) cannot even reach the origin network or get any response due to network errors. It’s as if the server doesn’t exist at the given address or is unreachable due to network partitions.
Example
Cloudflare returns 523 Origin Is Unreachable if, for example, the DNS for the origin is resolvable but no IP route is available, or the server is completely unreachable (no ping, no TCP handshake). It tells the user the origin can't be contacted at all.
When Not to Use
Do not use on typical servers. This is specifically a proxy-to-origin network issue. Standard clients or servers won't use 523; they'd either time out or fail connection without an HTTP status. This code is part of Cloudflare’s specific error reporting.
Source
Unofficial (Cloudflare)