Tomcat – IE 11 ignoring Cache Control header in response

cache-controlcachingexpires-headertomcat

I am setting Expires filter in web.xml of Tomcat. The header comes correctly in response but still IE is not caching. It is always making the fresh request to Tomcat.

<filter>
<filter-name>ExpiresFilter</filter-name>
<filter-class>org.apache.catalina.filters.ExpiresFilter</filter-class>
<init-param>
<param-name>ExpiresByType image/png</param-name>
<param-value>access plus 2 hours</param-value>
</init-param>

</filter>
<filter-mapping>
        <filter-name>ExpiresFilter</filter-name>
        <url-pattern>/*</url-pattern>
        <dispatcher>REQUEST</dispatcher>
</filter-mapping>

Below are the headers from respone

Cache-Control: max-age=7199

Expires: Mon, 14 Nov 2016 16:08:22 GMT

Content-Type: image/png

Transfer-Encoding: chunked

Date: Mon, 14 Nov 2016 14:08:22 GMT

Does anyone know, what am I missing here?

Best Answer

There is Microsoft's own documentation on cache-control.

Short answer: you have to use Pragma: no-cache, Cache-Control: no-cache and Expires all together.

Update 2019-01-24

The above instructions are for disabling caching, while the answer to being able to force caching can be found in this fine SO answer: Make IE to cache resources but always revalidate

Introduction

The correct minimum set of headers that works across all mentioned clients (and proxies):

Cache-Control: no-cache, no-store, must-revalidate
Pragma: no-cache
Expires: 0

The Cache-Control is per the HTTP 1.1 spec for clients and proxies (and implicitly required by some clients next to Expires). The Pragma is per the HTTP 1.0 spec for prehistoric clients. The Expires is per the HTTP 1.0 and 1.1 specs for clients and proxies. In HTTP 1.1, the Cache-Control takes precedence over Expires, so it's after all for HTTP 1.0 proxies only.

If you don't care about IE6 and its broken caching when serving pages over HTTPS with only no-store, then you could omit Cache-Control: no-cache.

Cache-Control: no-store, must-revalidate
Pragma: no-cache
Expires: 0

If you don't care about IE6 nor HTTP 1.0 clients (HTTP 1.1 was introduced in 1997), then you could omit Pragma.

Cache-Control: no-store, must-revalidate
Expires: 0

If you don't care about HTTP 1.0 proxies either, then you could omit Expires.

Cache-Control: no-store, must-revalidate

On the other hand, if the server auto-includes a valid Date header, then you could theoretically omit Cache-Control too and rely on Expires only.

Date: Wed, 24 Aug 2016 18:32:02 GMT
Expires: 0

But that may fail if e.g. the end-user manipulates the operating system date and the client software is relying on it.

Other Cache-Control parameters such as max-age are irrelevant if the abovementioned Cache-Control parameters are specified. The Last-Modified header as included in most other answers here is only interesting if you actually want to cache the request, so you don't need to specify it at all.

How to set it?

Using PHP:

header("Cache-Control: no-cache, no-store, must-revalidate"); // HTTP 1.1.
header("Pragma: no-cache"); // HTTP 1.0.
header("Expires: 0"); // Proxies.

Using Java Servlet, or Node.js:

response.setHeader("Cache-Control", "no-cache, no-store, must-revalidate"); // HTTP 1.1.
response.setHeader("Pragma", "no-cache"); // HTTP 1.0.
response.setHeader("Expires", "0"); // Proxies.

Using ASP.NET-MVC

Response.Cache.SetCacheability(HttpCacheability.NoCache);  // HTTP 1.1.
Response.Cache.AppendCacheExtension("no-store, must-revalidate");
Response.AppendHeader("Pragma", "no-cache"); // HTTP 1.0.
Response.AppendHeader("Expires", "0"); // Proxies.

Using ASP.NET Web API:

// `response` is an instance of System.Net.Http.HttpResponseMessage
response.Headers.CacheControl = new CacheControlHeaderValue
{
    NoCache = true,
    NoStore = true,
    MustRevalidate = true
};
response.Headers.Pragma.ParseAdd("no-cache");
// We can't use `response.Content.Headers.Expires` directly
// since it allows only `DateTimeOffset?` values.
response.Content?.Headers.TryAddWithoutValidation("Expires", 0.ToString());

Using ASP.NET:

Response.AppendHeader("Cache-Control", "no-cache, no-store, must-revalidate"); // HTTP 1.1.
Response.AppendHeader("Pragma", "no-cache"); // HTTP 1.0.
Response.AppendHeader("Expires", "0"); // Proxies.

Using ASP.NET Core v3

// using Microsoft.Net.Http.Headers
Response.Headers[HeaderNames.CacheControl] = "no-cache, no-store, must-revalidate";
Response.Headers[HeaderNames.Expires] = "0";
Response.Headers[HeaderNames.Pragma] = "no-cache";

Using ASP:

Response.addHeader "Cache-Control", "no-cache, no-store, must-revalidate" ' HTTP 1.1.
Response.addHeader "Pragma", "no-cache" ' HTTP 1.0.
Response.addHeader "Expires", "0" ' Proxies.

Using Ruby on Rails:

headers["Cache-Control"] = "no-cache, no-store, must-revalidate" # HTTP 1.1.
headers["Pragma"] = "no-cache" # HTTP 1.0.
headers["Expires"] = "0" # Proxies.

Using Python/Flask:

response = make_response(render_template(...))
response.headers["Cache-Control"] = "no-cache, no-store, must-revalidate" # HTTP 1.1.
response.headers["Pragma"] = "no-cache" # HTTP 1.0.
response.headers["Expires"] = "0" # Proxies.

Using Python/Django:

response["Cache-Control"] = "no-cache, no-store, must-revalidate" # HTTP 1.1.
response["Pragma"] = "no-cache" # HTTP 1.0.
response["Expires"] = "0" # Proxies.

Using Python/Pyramid:

request.response.headerlist.extend(
    (
        ('Cache-Control', 'no-cache, no-store, must-revalidate'),
        ('Pragma', 'no-cache'),
        ('Expires', '0')
    )
)

Using Go:

responseWriter.Header().Set("Cache-Control", "no-cache, no-store, must-revalidate") // HTTP 1.1.
responseWriter.Header().Set("Pragma", "no-cache") // HTTP 1.0.
responseWriter.Header().Set("Expires", "0") // Proxies.

Using Clojure (require Ring utils):

(require '[ring.util.response :as r])
(-> response
  (r/header "Cache-Control" "no-cache, no-store, must-revalidate")
  (r/header "Pragma" "no-cache")
  (r/header "Expires" 0))

Using Apache .htaccess file:

<IfModule mod_headers.c>
    Header set Cache-Control "no-cache, no-store, must-revalidate"
    Header set Pragma "no-cache"
    Header set Expires 0
</IfModule>

Using HTML:

<meta http-equiv="Cache-Control" content="no-cache, no-store, must-revalidate">
<meta http-equiv="Pragma" content="no-cache">
<meta http-equiv="Expires" content="0">

HTML meta tags vs HTTP response headers

Important to know is that when an HTML page is served over an HTTP connection, and a header is present in both the HTTP response headers and the HTML <meta http-equiv> tags, then the one specified in the HTTP response header will get precedence over the HTML meta tag. The HTML meta tag will only be used when the page is viewed from a local disk file system via a file:// URL. See also W3 HTML spec chapter 5.2.2. Take care with this when you don't specify them programmatically because the webserver can namely include some default values.

Generally, you'd better just not specify the HTML meta tags to avoid confusion by starters and rely on hard HTTP response headers. Moreover, specifically those <meta http-equiv> tags are invalid in HTML5. Only the http-equiv values listed in HTML5 specification are allowed.

Verifying the actual HTTP response headers

To verify the one and the other, you can see/debug them in the HTTP traffic monitor of the web browser's developer toolset. You can get there by pressing F12 in Chrome/Firefox23+/IE9+, and then opening the "Network" or "Net" tab panel, and then clicking the HTTP request of interest to uncover all detail about the HTTP request and response. The below screenshot is from Chrome:

Chrome developer toolset HTTP traffic monitor showing HTTP response headers on stackoverflow.com

I want to set those headers on file downloads too

First of all, this question and answer are targeted on "web pages" (HTML pages), not "file downloads" (PDF, zip, Excel, etc). You'd better have them cached and make use of some file version identifier somewhere in the URI path or query string to force a redownload on a changed file. When applying those no-cache headers on file downloads anyway, then beware of the IE7/8 bug when serving a file download over HTTPS instead of HTTP. For detail, see IE cannot download foo.jsf. IE was not able to open this internet site. The requested site is either unavailable or cannot be found.

ETag vs Header Expires

They are slightly different - the ETag does not have any information that the client can use to determine whether or not to make a request for that file again in the future. If ETag is all it has, it will always have to make a request. However, when the server reads the ETag from the client request, the server can then determine whether to send the file (HTTP 200) or tell the client to just use their local copy (HTTP 304). An ETag is basically just a checksum for a file that semantically changes when the content of the file changes.

The Expires header is used by the client (and proxies/caches) to determine whether or not it even needs to make a request to the server at all. The closer you are to the Expires date, the more likely it is the client (or proxy) will make an HTTP request for that file from the server.

So really what you want to do is use BOTH headers - set the Expires header to a reasonable value based on how often the content changes. Then configure ETags to be sent so that when clients DO send a request to the server, it can more easily determine whether or not to send the file back.

One last note about ETag - if you are using a load-balanced server setup with multiple machines running Apache you will probably want to turn off ETag generation. This is because inodes are used as part of the ETag hash algorithm which will be different between the servers. You can configure Apache to not use inodes as part of the calculation but then you'd want to make sure the timestamps on the files are exactly the same, to ensure the same ETag gets generated for all servers.