599 Network Connect Timeout Error

Informal: a connection timed out at the network level, used by some proxies and HTTP client libraries.

599 at a glance

Code: 599
Name: Network Connect Timeout Error
Class: 5xx Server error
Status: Non-standard used by Some HTTP proxies and clients (informal)
Documented by: Some HTTP proxies and clients (informal)
Cacheable by default: No by default. Caches only store responses with unrecognised status codes if explicit freshness (Cache-Control max-age) is given.

What 599 means

599 is informally used by some proxies to signal that connecting to the upstream timed out. Several HTTP client libraries, notably Python's Tornado HTTPClient, also report 599 for timeouts and connection errors where no response was received at all, so a "599" in your logs may never have crossed the network.

What typically causes a 599

Client-side timeouts or DNS/connection failures in Tornado-based and similar clients.
Proxies reporting connect timeouts to the upstream.

Fix it on the server

If you see 599 in a client's logs, check whether your server was reachable at all at that time.

Fix it as a client or visitor

Check DNS, network reachability and client timeout settings; retry with backoff.

Example response

HTTP 599: Timeout while connecting