Response codes 5xx

The final part of our series is devoted to responses of category 500-599. This category of status codes describes the problem that has arisen on the server side. If the user sees an error that is in the 5xx range, this means that he will not be able to influence it in any way from his side, and only the intervention of specialists who are responsible for the server’s operability is required.

The server must notify the user of the specific reason for the failure in the body of the message.

Response codes 5xx

The most well-known status codes in this category are:

• 500
• 501
• 502
• 503
• 504
• 505
• 506
• 507
• 508
• 509
• 510
• 511
• 520
• 521
• 526
• 529

More details about them below.

500 Internal Server Error

Generic response for code error. This is a catch-all internal server error message that is used when a more specific response is not needed.

Many web resources automatically respond with this status code when an error occurs while executing a request handler.

Error 500 is not dependent on the client, there may be software failures, database problems, resource limitations and other factors, so it is reasonable for the client to repeat the same request and hope that the server will be able to process it without errors next time.

501 Not Implemented

Error 501 indicates that the server either does not understand the request method or does not have the necessary instructions to process it. Code 501 may occur if the server does not support standard request protocols such as GET, OPTIONS, HEAD, POST, and others.

Sometimes the error message may read: “Please try again later. The feature you are requesting may become available.”

502 Bad Gateway

Probably, every Internet user has encountered this error at least once. The reason is the server’s request to an “intermediate” server (for example, VPN). The latter, when requesting the main server, received an incorrect answer, which the user will see on the screen as an error.

503 Service Unavailable

Also, a very popular status code. The server is temporarily unavailable. This is due to the fact that:

• the server is overloaded due to a large number of users and insufficient resources to process all requests;
• server maintenance that temporarily stops the server from running.

Customers are advised to try their request again later.

504 Gateway Timeout

A gateway timeout is a situation where an intermediary or proxy server has not received a response from an upstream server to complete the current request. This status code applies when a proxy or gateway is used to transfer data and two servers are waiting for a response.

505 HTTP Version Not Supported

One of the few errors that refers to interference from the client side. The current version of HTTP used by the client is not supported by the server.

The response body must indicate the reason why the version is not supported and which versions can be used.

506 Variant Also Negotiates

This server response will be received if an error was made during the setup process where the selected parameter points to itself, which leads to a communication failure.

When is it used? When the server is configured incorrectly and is unable to process the request.

507 Insufficient Storage

Error 507 occurs when the server cannot accommodate the data due to insufficient space for the current request. For example, if the server is running many applications or storing a lot of information and the server is not configured to manage disk space, etc.

This status code can be used when the server is fully loaded and the user requests a resource that is already there. The problem is that the server does not have the space to store the data sent in the request to provide the requested resource.

508 Loop Detected

This server response code indicates that the request cannot be processed because it would result in a loop. The error occurs when the server encounters a persistent circular reference in the request, causing an infinite redirect loop.

509 Bandwidth Limit Exceeded

This status code indicates that the maximum data transfer capacity for a particular service or website has been exceeded. Bandwidth in the context of the Internet is the maximum amount of data that can be transferred over a network in a given time. Exceeding this limit may result in restricted access or temporary unavailability of the service.

This situation usually occurs when a website experiences a sudden increase in traffic that exceeds its designated bandwidth.

510 Not Extended

Due to lack of support for the requested extension, the server is unable to process the client’s request. This may be due to the following reasons:

• the server is using an outdated or incompatible version of software that does not support the required extension;
• the server is not configured to work with a certain type of extensions;
• It is also possible that there are not enough resources to perform these operations.

511 Network Authentication Required

The appearance of this code indicates a situation where a network resource requires the user to verify their identity for access, and such authentication is necessary to continue the interaction. This HTTP code emphasizes the importance of having valid user credentials to ensure secure and authorized interaction with the specified resource.

The point of the 511 error is that it acts as a gatekeeper, emphasizing the importance of proper authentication for secure information exchange within a network infrastructure.

520 Unknown Error

This HTTP status code indicates unexpected problems between the web server and the cloud service it is trying to access. This error typically occurs when the server is unable to provide an adequate response, leaving users without a clear explanation for the failure.

In other words, the server returns a response that exceeds the set limits and is considered invalid.

Determining the root cause of a 520 server error can be difficult, but common factors can include server overload, network issues, or misconfiguration. Common causes include:

• no response from the server (when DNS domains point to the wrong server),
• Invalid response from the server and requests blocked by the security system (to avoid requests being blocked, specify Cloudflare IP addresses in the exception list).

521 Web Server Is Down

This status code indicates that the web server hosting the website is not responding. This typically occurs when Cloudflare, acting as a reverse proxy, is unable to establish a connection with the primary server and does not receive a response within the allotted time frame. Possible causes include server overload, misconfiguration, or network issues.

When this error occurs, users may be temporarily unable to access the website.

526 Invalid SSL Certificate

Error code 526 occurs when the user’s browser encounters an SSL certificate that Cloudflare does not recognize as valid. This typically occurs when the certificate presented by the origin server is expired, self-signed, or was not issued by a trusted certificate authority. This results in Cloudflare being unable to establish a secure connection between the user’s browser and the origin server, causing this error.

529 Site is overloaded

HTTP status code 529 indicates a server-side problem when “The server is overloaded.” This error occurs when a server is unable to handle incoming requests due to overload or insufficient resources. This indicates performance issues, usually caused by a sudden increase in traffic or a lack of server capacity. Understanding this code is important for web developers and administrators, as it highlights the need for effective load management and server scaling.

To summarize

5xx response codes signal problems on the server side when it cannot process the request. These errors indicate server software failures, overload or lack of resources. Understanding and promptly eliminating such errors is important for maintaining the stability of the site and preventing the loss of users, because the correct operation of the server directly affects the user experience and the effectiveness of online business.

Olha Tyshchenko

Editor

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