The presence of the If-Modified-Since header causes the CORS OPTIONS request to experience failure

When using a custom header (If-Modified-Since) in the code below, it causes ajax to fail with a status of HTTP/1.1 405 Not Allowed for the OPTIONS preflight request. However, if the header is removed, the request works as expected (I am unsure if OPTIONS is not sent at all or if developer consoles just do not display it). Why does this happen? (Similar situation occurs with jquery hosted on googleapis).

http = new XMLHttpRequest();
http.open('GET', 'http://code.jquery.com/jquery-2.1.4.min.js');
http.setRequestHeader('If-Modified-Since', 'Sat, 29 Oct 1994 19:43:31 GMT')
http.send();

Furthermore, is there a method to include this header in the xhr request to the jquery cdn in order to receive a 304 response status?

javascriptajaxhttp-headerscorsbrowser-cache

Answer №1

It appears that the Web server is rejecting the use of the OPTIONS method. To verify this, look for an Allow header in the response (as per the HTTP specification), and see what methods are listed. If the Allow header does not include OPTION—for example, if it only lists GET, HEAD, PUT, then any OPTIONS requests will fail. Therefore, if your operation relies on using OPTIONS, it may not be compatible with this server.

According to CORS specifications, sending the If-Modified-Since header (or any other headers except Accept, Accept-Language, Content-Language, or specific Content-Type cases) triggers a preflight request, which necessitates using the OPTIONS method. However, it's important to note that not all Web servers are required to support the OPTIONS method, as mandated by CORS or any other standards. In cases where a server does not support OPTIONS, you can expect the type of response you're currently experiencing.

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