HTTP 599 – Network Connect Timeout Error
(Informal convention) Used by some HTTP clients and proxies to indicate the connection to the upstream server timed out before it could even be established.
Common use case
Common in HTTP client libraries and proxies to distinguish "couldn't even connect" failures from "connected but got no response" ones.
History
Another informal, unofficial convention, distinct from - but often confused with - 598.
Did you know?
It is the highest-numbered status code in common informal use, occupying essentially the last free slot below the 600 boundary that formal HTTP status codes are expected to stay under.