Let’s talk about the complete guide to HTTP status codes. Have you learned it?

1. Overview of HTTP Status Codes

1. Concept

When we enter a URL in the browser and press Enter, the browser sends an HTTP request to the site's server. The server receives and processes the request and then returns the relevant resources along with the HTTP header. You can view the HTTP request status code in the browser's Network:

Wikipedia explains HTTP status codes:

The HTTP status code is a 3-digit code used to indicate the status of the Hypertext Transfer Protocol response of a web server. It is defined by the RFC 2616 specification and extended by RFC 2518, RFC 2817, RFC 2295, RFC 2774 and RFC 4918. All status codes are divided into five categories, and the first digit of the status code represents one of the five statuses of the response. The message phrases shown are typical, but any readable alternatives can be provided. Unless otherwise specified, status codes are part of the HTTP/1.1 standard (RFC 7231).

An HTTP status code is a 3-digit code (such as 200 OK or 404 Not Found) sent from a web server to let us and search engines know if there were any errors in the request or if there were any problems while the server was trying to process the request.

2. Classification

An application is usually a client that sends a request to a server, and the server responds. The status code lets us know whether the HTTP request was successful, failed, or something else. HTTP status codes are usually divided into five categories:

3. Importance

HTTP status codes are important for diagnosing application problems, such as if the web server is not working properly and cannot serve the page. Finding these problems quickly is important for providing a good experience for developers and search engines. So why are HTTP status codes and errors important for search engine optimization (SEO)?

Search engine bots see HTTP status codes when they crawl your site. In some cases, these can affect whether and how a page is indexed, as well as how search engines view the health of your site.

Generally speaking, 1XX and 2XX status codes don't have much of an impact on SEO. They indicate that everything is working fine on the site and enable search engine bots to continue to perform operations. However, they do not improve the site's ranking. In most cases, it's the higher-level codes that are important for SEO. 4XX and 5XX responses prevent bots from crawling and indexing the page. Too many errors can also indicate a low-quality site, which can lower the site's ranking.

The relationship between 3XX status codes and SEO is much more complicated. To understand their impact, you need to understand the difference between permanent redirects and temporary redirects, which I will not explain here, but will explain in detail later. In short, using permanent redirects for pages that have been moved will have better SEO.

You can use Google Search Console to view 3XX, 4XX, and 5XX status codes in the Coverage report:

This report shows four types of pages on your site:

• Returns an error page;
• Pages with warnings but valid;
• Valid pages;
• Pages excluded from indexing;

We can find pages with HTTO status codes of 3XX, 4XX, and 5XX in the three categories other than valid pages. The specific situation depends on the status code type. For example, 301 redirect may appear in the last category.

2. HTTP status code list

4XX and 5XX status codes may appear under Error.

1. 1xx Informational

The 1XX status codes were introduced in HTTP/1.1. They are informational status codes that are temporary and indicate that the request has been accepted and needs to be processed. These status codes do not provide much useful information, and we may never see 1XX related status codes.

(1) 100 Continue

The server has received the browser's request headers and is now ready to send the request body. This makes the request process more efficient because it prevents the browser from sending a body request, even if the headers have been rejected.

(2) 101 Switching Protocols

The server has accepted the client's request and will inform the client through the Upgrade message header to use a different protocol to complete the request. After sending the last blank line of this response, the server will switch to the protocol defined in the Upgrade message header.

(3) 102 Processing

The server has received and is processing the request and has not yet responded. This prevents the client from timing out and assuming the request was lost.

(4) 103 Early hints

Some response headers are returned before the server responds to the HTTP message. This status code is used to allow user agents to preload resources while the server prepares the response.

2. 2xx Success

The 2xx status code indicates that the client's request was successfully received, understood, and accepted.

(1) 200 OK

Indicates that the request sent by the client was processed normally by the server. From an SEO perspective, the 200 OK status code is a perfect status code for a functional page, and all linked pages are working properly. 200 means that the search engine crawler can successfully crawl the page and put it in its search engine.

（2）201 Created

The server fulfills the browser's request, thereby creating one or more new resources.

(3) 202 Accepted

The server has accepted the browser's request, but it is still processing it. The request may or may not eventually be responded to.

(4) 203 Non-authoritative Information

This status code may appear when using a proxy. It means that the proxy server received a 200 status code from the origin server, but modified the response before passing it to the browser.

(5) 204 No Content

This status code indicates that the request sent by the client has been processed normally by the server, but no content is returned, and the response message does not contain the body of the entity. It is generally used when only information needs to be sent from the client to the server, and the server does not need to send content to the client.

(6) 205 Reset Content

The server successfully processed the request and did not return any content. However, unlike the 204 response, the response returning this status code requires the requester to reset the document view. This response is mainly used to reset the form immediately after accepting user input so that the user can easily start another input.

(7) 206 Partial Content

This status code indicates that the client made a range request and the server executed this part of the GET request. The response message contains the entity content of the range specified by Content-Range.

(8) 226 IM Used

The server has successfully processed the browser's GET method to retrieve an updated version of a cached resource. Typically, the response is returned when the requested resource has one or more minor modifications.

3. 3xx Redirection

A 3XX response result indicates that the browser needs to perform some special processing to properly handle the request.

(1) 300 Multiple Choices

Sometimes, a server may respond with multiple possible resources to satisfy a browser's request. The 300 status code means that the browser now needs to choose between them. This can happen when there are multiple file type extensions available.

(2) 301 Moved Permanently

Permanent redirection. A new permanent URI has been assigned to the target resource. The new URI is specified in the Location header field of the HTTP response header. If the user has saved the original URI as a bookmark, the bookmark will be re-saved according to the new URI in Location. At the same time, the search engine will replace the old URL with the redirected URL while crawling the new content.

(3) 302 Found

Temporary redirection. The requested resource is assigned a new URI, and the user is expected to use the new URI to access the resource (this time). Similar to the 301 Moved Permanently status code, but the resource represented by 302 is not permanently redirected, but only temporarily. This means that the URI corresponding to the moved resource may change in the future.

If the user saves the URI as a bookmark, they will not update the bookmark as they would if the 301 status code appeared, but will still keep the URI corresponding to the page that returned the 302 status code. At the same time, the search engine will crawl the new content and keep the old URL. Because the server returns a 302 code, the search engine believes that the new URL is only temporary.

Use scenarios:

Users who are not logged in will be redirected to the login page when visiting the user center.

Visit the 404 page and redirect to the home page.

(4) 303 See Other

Since the resource corresponding to the request has another URI, the GET method should be used to obtain the requested resource.

The 303 status code has similar functions to the 302 Found status code, but the 303 status code explicitly indicates that the client should use the GET method to obtain the resource. The 303 status code is usually returned as the result of a PUT or POST operation. It indicates that the redirect link does not point to the newly uploaded resource, but to another page, such as a message confirmation page or an upload progress page. The method for requesting a redirect page should always use GET.

Notice:

When a 301, 302, or 303 response status code is returned, almost all browsers will change POST to GET and delete the body of the request message, and then the request will be automatically sent again.

The 301 and 302 standards prohibit changing the POST method to the GET method, but in reality everyone does this.

(5) 304 Not Modified

Related to browser cache. This status code indicates that when the client sends a conditional request, the server allows the request to access the resource, but the condition is not met. When the 304 status code is returned, it does not contain any body part of the response. Although 304 is classified in the 3XX category, it has nothing to do with redirection.

Conditional request (Http conditional request): Use the Get method to request, and the request message contains any header (if-match, if-none-match, if-modified-since, if-unmodified-since, if-range).

Status code 304 is not an error, but tells the client that there is a cache and to use the data in the cache directly. The returned page only contains the header information, but no content, which improves the performance of the web page to a certain extent.

(6) 305 Use Proxy

The server requires a proxy in order to return the requested resource. This response code is not currently used because most current browsers do not support it due to security issues.

(7) 307 Temporary Redirect

Temporary redirection. This status code has the same meaning as 302 Found. Although the 302 standard prohibits POST from being converted to GET, it is still used in practice. 307 will comply with browser standards and will not be converted from POST to GET. However, different browsers will still have different situations when processing requests. The specification requires the browser to continue to POST content to the address of Location. The specification requires the browser to continue to POST content to the address of Location.

(8) 308 Permanent Redirect

A permanent redirect redirects current and future requests to the new URL.

4. 4xx Client errors

(1) 400 Bad Request

There is a syntax error in the request message. When an error occurs, you need to modify the request content and send the request again.

(2) 401 Unauthorized

The request sent needs to have authentication information that passes HTTP authentication (BASIC authentication, DIGEST authentication). If a request has been made before, it means that the user authentication has failed. The response returned with 401 must contain a WWW-Authenticate header applicable to the requested resource to challenge the user information. When the browser receives a 401 response for the first time, a dialog window for authentication will pop up.

(3) 402 Payment Required

Reserved for digital payment systems. However, it is not widely used.

(4) 403 Forbidden

The client request has been denied because the client does not have permission to access the content. Unlike 401, the server knows the identity of the client, but because they do not have permission to view the content, the server refuses to provide a proper response.

(5) 404 Not Found

The requested resource could not be found on the server, but may be available in the future. In addition, it can also be used when the server rejects the request and does not want to explain the reason.

(6) 405 Method Not Allowed

The server recognizes the HTTP request method used by the browser, but needs to use a different method to provide the requested resource. The server should always allow clients to access using the GET and HEAD methods.

The client can use the OPTIONS method (pre-check) to view the access methods allowed by the server, as follows

 Access - Control - Allow - Methods : GET , HEAD , PUT , PATCH , POST , DELETE

(7) 406 Not Acceptable

The server notifies the client that no resource meets the criteria requested by the browser.

(8)407 Proxy Authentication Required

The client must first authenticate with the proxy.

(9) 408 Request Timeout

This status code is generated when the server times out while waiting for a complete request from the browser. In other words, the server did not receive the complete request sent by the browser. One possible reason is that network congestion causes packet loss between the browser and the server.

(10) 409 Conflict

The server cannot process the browser's request because there is a conflict with a related resource. This sometimes happens due to multiple simultaneous edits.

(11) 410 Gone

The requested resource will no longer be available in the future.

(12) 411 Length Required

The requested resource requires the client to specify a certain length, which is defined by Content-Length.

(13)412 Precondition Failed

The server did not meet one or more preconditions indicated in the request-header fields.

(14) 413 Payload Too Large

The server is refusing to process the request because the request payload is greater than the server is able or willing to handle. While the server MAY close the connection to prevent the client from continuing the request, it SHOULD generate a Retry-After header field with an indication of how long the client may try again.

(15)414 Request-URI Too Long

The request made by the browser cannot be processed by the server because the client encoded too much data into the query string and then sent it as the GET method.

(16) 415 Unsupported Media Type

The request has been rejected by the server because it does not support the media format of the requested data.

(17) 416 Requested Range Not Satisfiable

The range specified in the Range header of the request cannot be satisfied. This may be because the given range exceeds the size of the target URI data.

(18) 417 Expectation Failed

The server cannot meet the requirements specified in the Expect field of the request header of the request.

(19) 421 Misdirected Request

The browser's request has been redirected to another server, which cannot generate a response.

(20) 422 Unprocessable Entity

The browser sent a request that contained a semantic error and could not be processed by the server.

(21) 423 Locked

Access to a required resource is denied because it is locked.

(22) 424 Failed Dependency

The request made by the browser failed because it depended on another request, and that request failed.

(23) 426 Upgrade Required

While a server refuses to perform a given request using the current protocol, it may be willing to perform the request after the client upgrades to a different protocol.

(24) 428 Precondition Required

The server requires that conditions be specified before processing a request.

(25) 429 Too Many Requests

This is generated by the server when a user sends too many requests in a given amount of time (rate limiting). This can sometimes be caused by a bot or script trying to access the site.

(26) 431 Request Header Fields Too Large

The server cannot process the request because a header field is too large. This could be a problem with a single header field or with all header fields.

(27) 451 Unavailable For Legal Reasons

The operator of the server has received a request that access to the requested resource be denied.

(28)499 Client closed request

When nginx is processing a request and the client closes the connection, nginx returns this status code.

5. 5xx Server Error

A 5XX response indicates an error on the server itself.

(1)500 Internal Server Error

An error occurred on the server side while executing the request. It could also be that there is a bug in the application or some temporary failure.

(2) 501 Not Implemented

The request cannot be processed because it is not supported by the server.

(3) 502 Bad Gateway

The server acting as a gateway or proxy received an invalid response from the upstream server. Note that 502 errors are usually not fixable by the client, but need to be fixed by the server or proxy server along the way.

(4) 503 Service Unavailable

The server is temporarily overloaded or down for maintenance and cannot handle the request right now.

Use scenarios:

When the server is down for maintenance, it actively responds to requests with 503;

Nginx sets a speed limit. If the speed limit is exceeded, 503 will be returned.

(5) 504 Gateway Timeout

The gateway or proxy server cannot get the expected response within the specified time. It is newly added in HTTP 1.1.

Use scenarios:

Code execution timeout;

The code has an infinite loop.

(6) 505 HTTP Version Not Supported

The HTTP version used in the request is not supported by the server.

(7)506 Variant Also Negotiates

Internal server configuration error.

(8) 507 Insufficient Storage

The server was unable to store the content necessary to fulfill the request.

(9) 508 Loop Detected

The server detected an infinite loop while processing a request.

(10) 510 Not Extended

The server needs to scale further to serve the request.

(11)511 Network Authentication Required

Clients need to authenticate to gain access.