4xx 客户端错误 (Client Error)

客户端一侧的问题，例如请求语法错误或权限不足。400、401、403、404 是典型代表。

400Bad Request 常用The server cannot process the request due to a client error such as malformed syntax.401Unauthorized 常用Authentication is required and has failed or not been provided (i.e. unauthenticated).402Payment RequiredReserved for future use; some APIs use it to signal a payment or quota requirement.403Forbidden 常用The server understood the request but refuses to authorize it. Re-authenticating will not help.404Not Found 常用The server cannot find the requested resource. The most well-known status code.405Method Not Allowed 常用The HTTP method is known but not supported for the target resource.406Not AcceptableThe server cannot produce a response matching the request’s Accept headers.407Proxy Authentication RequiredAuthentication with a proxy is required before the request can proceed.408Request TimeoutThe server timed out waiting for the complete request from the client.409Conflict 常用The request conflicts with the current state of the resource.410GoneThe resource is permanently gone with no forwarding address.411Length RequiredThe server refuses the request because it lacks a Content-Length header.412Precondition FailedA condition in the request headers evaluated to false on the server.413Content Too LargeThe request body is larger than the server will process (formerly Payload Too Large).414URI Too LongThe request URI is longer than the server will interpret.415Unsupported Media TypeThe request payload format is not supported by the server.416Range Not SatisfiableThe requested Range lies beyond the size of the resource.417Expectation FailedThe expectation in the request Expect header could not be met.418I'm a teapotAn April Fools joke: a teapot cannot brew coffee. Widely implemented as an easter egg.421Misdirected RequestThe request reached a server that cannot produce a response for that scheme/authority.422Unprocessable Content 常用The request is well-formed but has semantic errors (e.g. validation failed).423LockedThe resource being accessed is locked.424Failed DependencyThe request failed because a request it depended on also failed.425Too EarlyThe server is unwilling to risk processing a request that might be replayed.426Upgrade RequiredThe client should switch to a different protocol given in the Upgrade header.428Precondition RequiredThe origin server requires the request to be conditional.429Too Many Requests 常用The client has sent too many requests in a given time (rate limiting).431Request Header Fields Too LargeThe header fields are too large, individually or collectively.451Unavailable For Legal ReasonsThe resource is blocked due to legal demands such as censorship. Named after Fahrenheit 451.