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599 Network Connect Timeout Error

Non-standard proxy code indicating the proxy could not establish a connection to the upstream in time.

Status

HTTP/1.1 599 Network Connect Timeout Error

Details

  • Category: 5xx
  • Status: Unofficial (Legacy HTTP proxies). Not registered with IANA. A server, framework, or CDN convention.
  • Specification: none, not defined by any RFC
  • Cacheable: Not cacheable
  • Response body: Allowed
  • Retry: Retry after a delay

599 is the connect-phase counterpart to 598: the proxy could not open a connection to the upstream before its timeout expired. Some HTTP client libraries also use 599 internally as a synthetic status for connection failures that never produced a real response.

The standard equivalents are 504 for a gateway timeout and 502 when the connection fails outright.

Common causes

  • The upstream host was unreachable or dropped connection attempts.
  • DNS resolution failure inside the proxy.
  • A synthetic status generated by a client library for a transport-level failure.

How to fix it

As the client

  • Retry with backoff. Confirm the destination resolves and is reachable from your network.
  • Check whether the code came from your HTTP library rather than from a server, since several libraries synthesize it.

As the server

  • Return 502 or 504 so the failure mode is expressed in a registered code.

Examples

A proxy connect timeout

HTTP response

HTTP/1.1 599 Network connect timeout error

Notes for proxy users

A 599 from a client library is usually a transport failure rather than a response from any server. Check DNS, the proxy endpoint and port, and whether the destination accepts connections at all before treating it as a target-side error.

See also