304 Not Modified

The HTTP304 Not Modifiedredirection response status code indicates that there is no need to retransmit the requested resources.

This response code is sent when the request is aconditional GET orHEAD request with anIf-None-Match or anIf-Modified-Since header and the condition evaluates to 'false'.It confirms that the resource cached by the client is still valid and that the server would have sent a200 OK response with the resource if the condition evaluated to 'true'.SeeHTTP caching for more information.

The response must not contain a body and must include the headers that would have been sent in an equivalent200 response, such as:

Note:Manydeveloper tools' network panels of browsers create extraneous requests leading to304 responses, so that access to the local cache is visible to developers.

Status

http

304 Not Modified

Examples

304 response to conditional requests

The examples below showGET requests made usingcurl with conditional request headers.The--http1.1 flag is used to force the HTTP/1.1 protocol for readability.

The first request uses anIf-Modified-Since condition with a future date of 21st November 2050.This must evaluate tofalse, because the resource can't have been updated after a time that hasn't happened yet:

bash

curl --http1.1 -I --header 'If-Modified-Since: Tue, 21 Nov 2050 08:00:00 GMT' \ https://developer.mozilla.org/en-US/

This will result in the following HTTP request:

http

GET /en-US/ HTTP/1.1Host: developer.mozilla.orgUser-Agent: curl/8.7.1Accept: */*If-Modified-Since: Tue, 21 Nov 2050 08:00:00 GMT

The response would be200 OK with the current version of the resource if the resource had been updated after the timestamp in theIf-Modified-Since header.Instead, we get a304 response that includesETag,Age andExpires headers, telling us our cached version of the resource is still current:

http

HTTP/1.1 304 Not ModifiedDate: Wed, 28 Aug 2024 09:52:35 GMTExpires: Wed, 28 Aug 2024 10:01:53 GMTAge: 3279ETag: "b20a0973b226eeea30362acb81f9e0b3"Cache-Control: public, max-age=3600Vary: Accept-EncodingX-cache: hitAlt-Svc: clear

Now run anothercurl command using theetag value from the previous response with theIf-None-Match condition (since thisetag is the current version of the resource on the server we expect to receive a304 Not Modified response):

bash

curl --http1.1 -I --header 'If-None-Match: "b20a0973b226eeea30362acb81f9e0b3"' \ https://developer.mozilla.org/en-US/

This will result in the following HTTP request:

http

GET /en-US/ HTTP/1.1Host: developer.mozilla.orgUser-Agent: curl/8.7.1Accept: */*If-None-Match: "b20a0973b226eeea30362acb81f9e0b3"

Because theetag value matches at the time of the request, the entity tag fails the condition, and a304 response is returned:

http

HTTP/1.1 304 Not ModifiedDate: Wed, 28 Aug 2024 10:36:35 GMTExpires: Wed, 28 Aug 2024 11:02:17 GMTAge: 662ETag: "b20a0973b226eeea30362acb81f9e0b3"Cache-Control: public, max-age=3600Vary: Accept-EncodingX-cache: hitAlt-Svc: clear