Why both no-cache and no-store should be used in H

Question

I'm told to prevent user-info leaking, only "no-cache" in response is not enough. "no-store" is also necessary.

Cache-Control: no-cache, no-store

After reading this spec http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html, I'm still not quite sure why.

My current understanding is that it is just for intermediate cache server. Even if "no-cache" is in response, intermediate cache server can still save the content to non-volatile storage. The intermediate cache server will decide whether using the saved content for following request. However, if "no-store" is in the response, the intermediate cache sever is not supposed to store the content. So, it is safer.

Is there any other reason we need both "no-cache" and "no-store"?

Answer 1

For chrome, no-cache is used to reload the page on a re-visit, but it still caches it if you go back in history (back button). To reload the page for history-back as well, use no-store. IE needs must-revalidate to work in all occasions.

So just to be sure to avoid all bugs and misinterpretations I always use

Cache-Control: no-store, no-cache, must-revalidate

if I want to make sure it reloads.

Answer 2

If you want to prevent all caching (e.g. force a reload when using the back button) you need:

• no-cache for IE

• no-store for Firefox

There's my information about this here:

http://blog.httpwatch.com/2008/10/15/two-important-differences-between-firefox-and-ie-caching/

Answer 3

Note that Internet Explorer from version 5 up to 8 will throw an error when trying to download a file served via https and the server sending Cache-Control: no-cache or Pragma: no-cache headers.

See http://support.microsoft.com/kb/812935/en-us

The use of Cache-Control: no-store and Pragma: private seems to be the closest thing which still works.

Answer 4

From the HTTP 1.1 specification:

no-store:

The purpose of the no-store directive is to prevent the inadvertent release or retention of sensitive information (for example, on backup tapes). The no-store directive applies to the entire message, and MAY be sent either in a response or in a request. If sent in a request, a cache MUST NOT store any part of either this request or any response to it. If sent in a response, a cache MUST NOT store any part of either this response or the request that elicited it. This directive applies to both non- shared and shared caches. "MUST NOT store" in this context means that the cache MUST NOT intentionally store the information in non-volatile storage, and MUST make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it. Even when this directive is associated with a response, users might explicitly store such a response outside of the caching system (e.g., with a "Save As" dialog). History buffers MAY store such responses as part of their normal operation. The purpose of this directive is to meet the stated requirements of certain users and service authors who are concerned about accidental releases of information via unanticipated accesses to cache data structures. While the use of this directive might improve privacy in some cases, we caution that it is NOT in any way a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.

Answer 5

I must clarify that no-cache does not mean do not cache. In fact, it means "revalidate with server" before using any cached response you may have, on every request.

must-revalidate, on the other hand, only needs to revalidate when the resource is considered stale.

If the server says that the resource is still valid then the cache can respond with its representation, thus alleviating the need for the server to resend the entire resource.

no-store is effectively the full do not cache directive and is intended to prevent storage of the representation in any form of cache whatsoever.

I say whatsoever, but note this in the RFC 2616 HTTP spec:

History buffers MAY store such responses as part of their normal operation

But this is ommitted from the newer RFC 7234 HTTP spec in potentially an attempt to make no-store stronger, see:

http://tools.ietf.org/html/rfc7234#section-5.2.1.5

Answer 6

Under certain circumstances, IE6 will still cache files even when Cache-Control: no-cache is in the response headers.

The W3C states of no-cache:

If the no-cache directive does not specify a field-name, then a cache MUST NOT use the response to satisfy a subsequent request without successful revalidation with the origin server.

In my application, if you visited a page with the no-cache header, then logged out and then hit back in your browser, IE6 would still grab the page from the cache (without a new/validating request to the server). Adding in the no-store header stopped it doing so. But if you take the W3C at their word, there's actually no way to control this behavior:

History buffers MAY store such responses as part of their normal operation.

General differences between browser history and the normal HTTP caching are described in a specific sub-section of the spec.