Definition of ETag header (https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag):
The ETag HTTP response header is an identifier for a specific version
of a resource. It allows caches to be more efficient, and saves
bandwidth, as a web server does not need to send a full response if
the content has not changed. On the other side, if the content has
changed, etags are useful to help prevent simultaneous updates of a
resource from overwriting each other ("mid-air collisions").
Definition of Cache-Control header (https://developer.mozilla.org/de/docs/Web/HTTP/Headers/Cache-Control):
The Cache-Control general-header field is used to specify directives
for caching mechanisms in both requests and responses.
So the ETag header tells the browser for a resource to send a single HTTP request to the server and ask if the file hash has changed. If yes, download a new one. Great. So if the ETag header is set why should I need Cache-Control any more (beside of the Expires header which may help to avoid this single request)?
So if I have to set the Cache-Control header anyway it can only be harmful right? I think the most appropriate value would be:
Cache-Control: must-revalidate
But I am not sure if this triggers unecessary additional actions.
After some research, I found a great tutorial on Medium by Alex Barashkov: "Best practices for cache control settings for your website".
Alex writes:
I recommend you apply Cache-Control: no-cache to html files. Applying
“no-cache” does not mean that there is no cache at all, it simply
tells the browser to validate resources on the server before use it
from the cache. That’s why we need to use it with Etag, so browsers
will send a simple request and load the extra 80 bytes to verify the
state of the file.
Presence of ETag header does not tell the browser to do anything. Browser decides what to do based on the Cache-Control header it receives in the request and cached response. If it decides that resource is stale or needs to be re-validated, then it can use the ETag value to create a conditional request to the server and either get a new resource (status code 200), or a notification that things have not changed (status code 304)
Both headers are necessary for your cache to work optimally.
I have a server which hosts resources for several users on the same hostname. For example:
http://example.com/alice/blog.html
http://example.com/bob/cat.jpg
http://example.com/carol/todo.txt
I would like to allow users to specify their own response headers for resources within their directories, similar to what is done on AWS S3. For example, Carol may want her TODO list readable from scripts on another domain, so she might want Access-Control-Allow-Origin: * set for todo.txt.
While I want this feature to be as flexible as possible, I cannot allow just any response headers to be specified, as some response headers have side effects for the entire origin or hostname. For example, Set-Cookie could be used for one person's directory, but the user agent could then make a request to someone else's directory with the cookie value. As another example, a user could set Strict-Transport-Security, potentially locking out other users from using normal HTTP.
What other HTTP response headers have the potential for side effects for the entire origin, rather than just the resource that was requested? My list so far:
Alt-Svc
Public-Key-Pins
Server
Set-Cookie
Strict-Transport-Security
Rather than blocking response headers that could affect the entire domain I would recommend a slightly different approach and specify a white list of response headers that are definitely okay to use. There could be new, experimental or browser-specific headers that are non-standard but potentially affect the entire domain for a user with a specific browser.
I would suggest that the following headers are safe to use and should be everything your user needs to modify:
Access-Control-Allow-Origin
Access-Control-Allow-Credentials
Access-Control-Expose-Headers
Access-Control-Max-Age
Access-Control-Allow-Methods
Access-Control-Allow-Headers
Age
Allow
Cache-Control
Content-Disposition
Content-Encoding
Content-Language
Content-Length
Content-Location
Content-Range
Content-Type
Date
ETag
Expires
Last-Modified
Link
Location
Pragma
Retry-After
Transfer-Encoding
For static content such as files and html pages I would not set Content-Range or Content-Length manually. The server should set many of these headers automatically. Nevertheless overriding them might make sense for some users. Transfer-Encoding can be used to add gzip or deflate during transfer if your server supports it, but must not be used with HTTP/2.
Also Location, Allow and Retry-After only make sense for certain status codes. You might want to omit them
There are so many questions asking "how to make sure the pages are not cached", and answers like "this is how to instruct the both clients and proxy servers not to cache". I'm instead looking for a way to achieve "allow proxy cache but not clients (i.e. browsers) cache".
In fact, I found that setting no cache-related headers can achieve this, but I'm not sure if this is the right way to do it and there's no explicit way of instructing it.
In the HTTP jargon these caches are referred as shared or public (proxy) and private caches (browser).
You should use a response header similar to this:
cache-control: public, max-age=0, s-maxage=${seconds}
Being ${seconds} the TTL of the cached elements. The key here is using the directive s-maxage.
If a response includes an s-maxage directive, then for a shared cache (but not for a private cache), the maximum age specified by this directive overrides the maximum age specified by either the max-age directive or the Expires header. The s-maxage directive also implies the semantics of the proxy-revalidate directive (see section 14.9.4), i.e., that the shared cache must not use the entry after it becomes stale to respond to a subsequent request without first revalidating it with the origin server. The s- maxage directive is always ignored by a private cache.
Notice that this header does not exempt the browser from caching the resource (using it to serve a future request), but instead forces it to revalidate its content (if a last-modified or etag header is also returned)
I'm trying to understand how the cache-control http header works.
The cache-control header can have the no-cache value. I have checked the definition in w3c and it said:
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.
It tells no-cache value will trigger validation for every request.
What I want to know is, what is cache validation and what it does in the http protocol?
thanks for your help guys. now i understand validation means check if cache contain latest content from server.
my further question would be what issues no-cache will fix. please provide some scenario, like after applied no-cache in http header, what security issue will be fixed.
thanks guys
The no-cache directive is not intended for a security purpose. Security gets covered in rules that define which data/resources a cdn/proxy server is not permitted to cache. So, if security is required, the no-store directive should be used by the client/server. Look under :
paragraph 2 under section 13.4 on https://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
https://www.rfc-editor.org/rfc/rfc7234#section-3
The no-cache directive is used by the client when it is ready to accept a resource from a cache, provided there is a confirmation from the server that the cached resource is up to date (fresh). The proxy/cdn can use two methods to re-validate the resource's freshness :
If client sent an ETAG value, proxy/cdn can forward it to the server under an If-None-Match header. If server responds with '304 Not Modified', then the cached resource is fresh to serve.
Using an If-Modified-Since header with a date value that was received the last time the resource was downloaded from the server (was to be found under the Last-Modified header in server's last response).
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"?
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 omitted from the newer RFC 7234 HTTP spec in potentially an attempt to make no-store stronger, see:
https://www.rfc-editor.org/rfc/rfc7234#section-5.2.1.5
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.
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.
no-store should not be necessary in normal situations, and can harm both speed and usability. It is intended for use where the HTTP response contains information so sensitive it should never be written to a disk cache at all, regardless of the negative effects that creates for the user.
How it works:
Normally, even if a user agent such as a browser determines that a response shouldn't be cached, it may still store it to the disk cache for reasons internal to the user agent. This version may be utilised for features like "view source", "back", "page info", and so on, where the user hasn't necessarily requested the page again, but the browser doesn't consider it a new page view and it would make sense to serve the same version the user is currently viewing.
Using no-store will prevent that response being stored, but this may impact the browser's ability to give "view source", "back", "page info" and so on without making a new, separate request for the server, which is undesirable. In other words, the user may try viewing the source and if the browser didn't keep it in memory, they'll either be told this isn't possible, or it will cause a new request to the server. Therefore, no-store should only be used when the impeded user experience of these features not working properly or quickly is outweighed by the importance of ensuring content is not stored in the cache.
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.
This is incorrect. Intermediate cache servers compatible with HTTP 1.1 will obey the no-cache and must-revalidate instructions, ensuring that content is not cached. Using these instructions will ensure that the response is not cached by any intermediate cache, and that all subsequent requests are sent back to the origin server.
If the intermediate cache server does not support HTTP 1.1, then you will need to use Pragma: no-cache and hope for the best. Note that if it doesn't support HTTP 1.1 then no-store is irrelevant anyway.
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/
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.
If a caching system correctly implements no-store, then you wouldn't need no-cache. But not all do. Additionally, some browsers implement no-cache like it was no-store. Thus, while not strictly required, it's probably safest to include both.
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.
Originally we used no-cache many years ago and did run into some problems with stale content with certain browsers... Don't remember the specifics unfortunately.
We had since settled on JUST the use of no-store. Have never looked back or had a single issue with stale content by any browser or intermediaries since.
This space is certainly dominated by reality of implementations vs what happens to have been written in various RFCs. Many proxies in particular tend to think they do a better job of "improving performance" by replacing the policy they are supposed to be following with their own.
Just to make things even worse, in some situations, no-cache can't be used, but no-store can:
http://faindu.wordpress.com/2008/04/18/ie7-ssl-xml-flex-error-2032-stream-error/
To answer the question, there are two players here, the client (request) and the server (response).
Client:
The client can only request with ONE cache method. There are different methods and if not specified, will use default.
default: Inspect browser cache:
If cached and "fresh": Return from cache.
If cached, stale, but still "valid": Return from cache, and schedule a fetch to update cache (for next use).
If cached and stale: Fetch with conditions, cache, and return.
If not cached: Fetch, cache, and return.
no-store: Fetch and return.
reload: Fetch, cache, and return. (default-4)
no-cache: Inspect browser cache:
If cached: Fetch with conditions, cache, and return. (default-3)
If not cached: Fetch, cache, and return. (default-4)
force-cache: Inspect browser cache:
If cached: Return it regardless if stale.
If not cache: Fetch, cache, and return. (default-4)
only-if-cached: Inspect browser cache:
If cached: Return it regardless if stale.
If not cached: Throw network error.
Notes:
Still "valid" means the current age is within the stale-while-revalidate lifetime. It needs "revalidation", but is still acceptable to return.
"Fetch" here, for simplicity, is short for "non-conditional network
fetch".
"Fetch with conditions" means fetch using headers like
If-Modified-Since, or ETag so the server can respond with 304: (Not Modified).
https://fetch.spec.whatwg.org/#concept-request-cache-mode
Server::
Now that we understand what the client can do, the server responses make more sense.
Looking at the Cache-Control header, if the server returns:
no-store: Tells client to not use cache at all
no-cache: Tells client it should do conditional requests and ignore freshness
max-age: Tells client how long a cache is "fresh"
stale-while-revalidate: Tells client how long cache is "valid"
immutable: Cache forever
Now we can put it all together. That means the only possibilities are:
Non-conditional network fetch
Conditional network fetch
Return stale cache
Return stale but valid cache
Return fresh cache
Return any cache
Any combination of client, or server can dictate what method, or set of methods, to use. If the server returns no-store, it's not going to hit the cache, no matter what the client request type. If the client request was no-store, it doesn't matter what the server returns, it won't cache. If the client doesn't specify a request type, the server will dictate it with Cache-Control.
It makes no sense for a server to return both no-cache and no-store since no-store overrides everything. Yes, you've probably seen both together, and it's useless outside of broken browser implementations. Still, no-store has been part of spec since 1999: https://datatracker.ietf.org/doc/html/rfc2616#section-14.9.2
In real life usage, if your server supports 304: Not Modified, and you want to use client cache as a way to improve speed, but still want to force a network fetch, use no-cache. If don't support 304, and want to force a network fetch, use no-store. If you're okay with cache sometimes, use freshness and revalidation headers.
In reality, if you're mixing up no-cache and no-store on the client, very little would change. Then, just a couple of headers get sent and there will different internal responses handled by the browser. An issue can occur if you use no-cache and then forget to use it later. no-cache tells it to store the response in the cache, and a later request without it might trigger internal cache.
There are times when you may want to mix methods even on the same resource based on context. For example, you may want to use reload on a service worker and background sync, but use default for the web page itself. This is where you can manipulate the user agent (browser) cache to your liking. Just remember that the server generally has the final say as to how the cache should work.
To clarify some possible future confusion. The client can use the Cache-Control header on the request, to tell the server to not use its own cache system when responding. This is unrelated to the browser/server dynamic, and more about the server/database dynamic.
Also no-store technically means must not store to any non-volatile storage (disk) and release it from volatile storage (memory) ASAP. In practice, it means don't use a cache at all. The command actually goes both ways. A client request with no-store shouldn't write to disk or database and is meant to transient.
TL;DR: no-store overrides no-cache. Setting both is useless, unless we are talking out-of-spec or HTTP/1.0 browsers that don't support no-store (Maybe IE11?). Use no-cache for 304 support.
A pretty old topic but I'll share some recent ideas:
no-store: Must not attempt to store anything, and must also take action to delete any copy it might have.
no-cache: Never use a local copy without first validating with the origin server. It prevents all possibility of a cache hit, even with fresh resources.
So, answering the question, using only one of them is enough.
Also, some (not very) recent works prove that browsers are more Cache-Control compatible nowadays.
OWASP discusses this:
What's the difference between the cache-control directives: no-cache, and no-store?
The no-cache directive in a response indicates that the response must not be used to serve a subsequent request i.e. the cache must not display a response that has this directive set in the header but must let the server serve the request. The no-cache directive can include some field names; in which case the response can be shown from the cache except for the field names specified which should be served from the server. The no-store directive applies to the entire message and indicates that the cache must not store any part of the response or any request that asked for it.
Am I totally safe with these directives?
No. But generally, use both Cache-Control: no-cache, no-store and Pragma: no-cache, in addition to Expires: 0 (or a sufficiently backdated GMT date such as the UNIX epoch). Non-html content types like pdf, word documents, excel spreadsheets, etc often get cached even when the above cache control directives are set (although this varies by version and additional use of must-revalidate, pre-check=0, post-check=0, max-age=0, and s-maxage=0 in practice can sometimes result at least in file deletion upon browser closure in some cases due to browser quirks and HTTP implementations). Also, 'Autocomplete' feature allows a browser to cache whatever the user types in an input field of a form. To check this, the form tag or the individual input tags should include 'Autocomplete="Off" ' attribute. However, it should be noted that this attribute is non-standard (although it is supported by the major browsers) so it will break XHTML validation.
Source here.