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HTTP response status codes indicate whether a specific HTTP request has been successfully completed or not.

The 4xx class of status code is intended for cases in which the client seems to have erred. Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. 1

The Netdata Agent calculates the ratio of client error HTTP requests over the last minute. This metric does not include the 401 errors.

Client error responses (4XX)

The client error codes below contain the descriptions as provided by Mozilla.

  • 400 Bad Request: The server could not understand the request due to invalid syntax.

  • 401 Unauthorized: Although the HTTP standard specifies “unauthorized”, semantically this
    response means “unauthenticated”. That is, the client must authenticate itself to get the
    requested response.

  • 402 Payment Required: This response code is reserved for future use. The initial aim for
    creating this code was using it for digital payment systems, however this status code is used very
    rarely and no standard convention exists.

  • 403 Forbidden: The client does not have access rights to the content; that is, it is
    unauthorized, so the server is refusing to give the requested resource. Unlike 401 Unauthorized,
    the client’s identity is known to the server.

  • 404 Not Found: The server can not find the requested resource. In the browser, this means the
    URL is not recognized. In an API, this can also mean that the endpoint is valid but the resource
    itself does not exist. Servers may also send this response instead of 403 Forbidden to hide the
    existence of a resource from an unauthorized client. This response code is probably the most well
    known due to its frequent occurrence on the web.

  • 405 Method Not Allowed: The request method is known by the server but is not supported by the
    target resource. For example, an API may not allow calling DELETE to remove a resource.

  • 406 Not Acceptable: This response is sent when the web server, after performing server-driven
    content negotiation, doesn’t find any content that conforms to the criteria given by the user

  • 407 Proxy Authentication Required: This is similar to 401 Unauthorized but authentication is
    needed to be done by a proxy.

  • 408 Request Timeout: This response is sent on an idle connection by some servers, even without
    any previous request by the client. It means that the server would like to shut down this unused
    connection. This response is used much more since some browsers, like Chrome, Firefox 27+, or IE9,
    use HTTP pre-connection mechanisms to speed up surfing. Also note that some servers merely shut
    down the connection without sending this message.

  • 409 Conflict: This response is sent when a request conflicts with the current state of the

  • 410 Gone: This response is sent when the requested content has been permanently deleted from
    server, with no forwarding address. Clients are expected to remove their caches and links to the
    resource. The HTTP specification intends this status code to be used for “limited-time,
    promotional services”. APIs should not feel compelled to indicate resources that have been deleted
    with this status code.

  • 411 Length Required: Server rejected the request because the Content-Length header field is not
    defined and the server requires it.

  • 412 Precondition Failed: The client has indicated preconditions in its headers which the server
    does not meet.

  • 413 Payload Too Large: Request entity is larger than limits defined by server. The server might
    close the connection or return an Retry-After header field.

  • 414 URI Too Long: The URI requested by the client is longer than the server is willing to

  • 415 Unsupported Media Type: The media format of the requested data is not supported by the
    server, so the server is rejecting the request.

  • 416 Range Not Satisfiable: The range specified by the Range header field in the request cannot
    be fulfilled. It’s possible that the range is outside the size of the target URI’s data.

  • 417 Expectation Failed: This response code means the expectation indicated by the Expect request
    header field cannot be met by the server.

  • 418 I’m a teapot: The server refuses the attempt to brew coffee with a teapot.

  • 421 Misdirected Request: The request was directed at a server that is not able to produce a
    response. This can be sent by a server that is not configured to produce responses for the
    combination of scheme and authority that are included in the request URI.

  • 422 Unprocessable Entity (WebDAV):
    The request was well-formed but was unable to be followed due to semantic errors.

  • 423 Locked (WebDAV):
    The resource that is being accessed is locked.

  • 424 Failed Dependency (WebDAV):
    The request failed due to failure of a previous request.

  • 425 Too Early: Indicates that the server is unwilling to risk processing a request that might be

  • 426 Upgrade Required: The server refuses to perform the request using the current protocol but
    might be willing to do so after the client upgrades to a different protocol. The server sends an
    Upgrade header in a 426 response to indicate the required protocol(s).

  • 428 Precondition Required: The origin server requires the request to be conditional. This
    response is intended to prevent the ‘lost update’ problem, where a client GETs a resource’s state,
    modifies it and PUTs it back to the server, when meanwhile a third party has modified the state on
    the server, leading to a conflict.

  • 429 Too Many Requests: The user has sent too many requests in a given amount of time (“rate

  • 431 Request Header Fields Too Large: The server is unwilling to process the request because its
    header fields are too large. The request may be resubmitted after reducing the size of the request
    header fields.

  • 451 Unavailable For Legal Reasons: The user agent requested a resource that cannot legally be
    provided, such as a web page censored by a government.

References and sources
  1. https://datatracker.ietf.org/doc/html/rfc2616#section-10.4
  2. https://developer.mozilla.org/en-US/docs/Web/HTTP/Status#client_error_responses

Troubleshooting section:

General approach

To identify the HTTP response code your web server sends back:

  1. Open the Netdata dashboard.
  2. Inspect the detailed_response_codes chart for your web server. This chart keeps
    track of exactly what error codes your web server sends out.

You should also check server logs for more details about how the server is handling the requests. For example, web servers such as Apache or Nginx produce two files called access.log and error.log (by default under /var/log/{nginx, apache2}/{access.log, error.log})

Troubleshoot 404 codes in the server side

The 404 requests indicate outdated links on your website or in other websites that redirect to your website. To check for dead links on your on website, use a broken link checker software periodically.