Enable browser caching for static resources

Question

To improve the site performance, I'm adding following http headers in IIS 7.5.

Expires: Sun, 29 Mar 2020 00:00:00 GMT

and

Cache-Control: Public

I'm adding these headers for images folder in site's virtual directory. When I access the site, I see that for each image present in this folder; those response headers were:

Accept-Ranges:bytes
Cache-Control:no-cache, no-store,Public
Content-Length:4445
Content-Type:image/png
Date:Fri, 06 Jun 2014 09:18:36 GMT
ETag:"16874c2af55ecf1:0"
Expires:-1,Sun, 29 Mar 2020 00:00:00 GMT
Last-Modified:Wed, 23 Apr 2014 13:08:48 GMT
max-age:604800
Pragma:no-cache
Server:Microsoft-IIS/7.5
X-Powered-By:ASP.NET

I need browser to take these images from its cache instead of requesting again from server. How should I achieve it?

Somehow you use the ETag find what is place it and disable it for the images. — Aristos
– Aristos, Commented Jun 6, 2014 at 9:27
@Aristos, I removed ETag using solution provided by KristoferG at forums.iis.net/t/1177192.aspx. But still it is not getting cached. — benjamin54
– benjamin54, Commented Jun 6, 2014 at 9:42
See also this answer stackoverflow.com/questions/11393649/… if this not work then check if your images pass from some kind of http handler that add the no-cache. Also check if you send them via secure protocol, this can make them not been cached. — Aristos
– Aristos, Commented Jun 6, 2014 at 9:56
The site is configured as https. Can this produce a problem? — benjamin54
– benjamin54, Commented Jun 6, 2014 at 10:02
Yes probably this is the reason, but I do not have a fast answer on that, other than use the ETag that is only make a one trip round to the server and if the ETag has no change the image is not loading again, for this session. — Aristos
– Aristos, Commented Jun 6, 2014 at 10:19

Heretic Monkey · Accepted Answer · 2021-05-21 15:57:41Z

Your header shows that you add a new value but you need to replace the existing one

Cache-Control:no-cache, no-store,Public
Expires:-1,Sun, 29 Mar 2020 00:00:00 GMT

no-cache, no-store stands for no cache and -1 says that the content is already expired.

Instead of doing it from the code you can easily set it in the root web.config file as

  <location path="images">
    <system.webServer>
      <staticContent>
        <clientCache cacheControlMode="UseExpires" 
                     httpExpires="Sun, 29 Mar 2020 00:00:00 GMT" /> 
      </staticContent>
    </system.webServer>
  </location>
</configuration>

where images is a name of your directory

or add dedicated web.config file directly in the target directory

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <system.webServer>
    <staticContent>
      <clientCache cacheControlMode="UseExpires" 
                   httpExpires="Sun, 29 Mar 2020 00:00:00 GMT" /> 
    </staticContent>
  </system.webServer>
</configuration>

You can also use cacheControlMode="UseMaxAge" and set specific time of expiration

Example to set expiration in 7 days

<clientCache cacheControlMode="UseMaxAge" 
             cacheControlMaxAge="7.00:00:00" />

Read more http://msdn.microsoft.com/en-us/library/ms689443.aspx

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