HTTP Status Codes - Part 3 of Chapter 3 from HTTP: The Definitive Guide

Status code	Reason phrase	Meaning
500	Internal Server Error	Used when the server encounters an error that prevents it from servicing the request.
501	Not Implemented	Used when a client makes a request that is beyond the server's capabilities (e.g., using a request method that the server does not support).
502	Bad Gateway	Used when a server acting as a proxy or gateway encounters a bogus response from the next link in the request response chain (e.g., if it is unable to connect to its parent gateway).
503	Service Unavailable	Used to indicate that the server currently cannot service the request but will be able to in the future. If the server knows when the resource will become available, it can include a Retry-After header in the response. See "Response Headers" for more on the Retry-After header.
504	Gateway Timeout	Similar to status code 408, except that the response is coming from a gateway or proxy that has timed out waiting for a response to its request from another server.
505	HTTP Version Not Supported	Used when a server receives a request in a version of the protocol that it can't or won't support. Some server applications elect not to support older versions of the protocol.

Status code

Reason phrase

Meaning

500

Internal Server Error

Used when the server encounters an error that prevents it from servicing the request.

501

Not Implemented

Used when a client makes a request that is beyond the server's capabilities (e.g., using a request method that the server does not support).

502

Bad Gateway

Used when a server acting as a proxy or gateway encounters a bogus response from the next link in the request response chain (e.g., if it is unable to connect to its parent gateway).

503

Service Unavailable

Used to indicate that the server currently cannot service the request but will be able to in the future. If the server knows when the resource will become available, it can include a Retry-After header in the response. See "Response Headers" for more on the Retry-After header.

504

Gateway Timeout

Similar to status code 408, except that the response is coming from a gateway or proxy that has timed out waiting for a response to its request from another server.

505

HTTP Version Not
Supported

Used when a server receives a request in a version of the protocol that it can't or won't support. Some server applications elect not to support older versions of the protocol.