: Support of If-Modified-Since HTTP header I'm not sure if this is better in this forum or in the Server Fault forum... On Google Webmaster Guidelines I see this: "Make sure your web server supports

@Shelton105

This If-Modified-Since header is sent by client and not server: Server just has to respond accordingly. This request header "Allows a 304 Not Modified to be returned if content is unchanged"


A conditional GET method requests that the identified resource be
transferred only if it has been modified since the date given by the
If-Modified-Since header. The algorithm for determining this includes
the following cases:


If the request would normally result in anything other than a 200
(ok) status, or if the passed If-Modified-Since date is invalid, the
response is exactly the same as for a normal GET. A date which is
later than the server's current time is invalid.
If the resource has been modified since the If-Modified-Since
date, the response is exactly the same as for a normal GET.
If the resource has not been modified since a valid
If-Modified-Since date, the server shall return a 304 (not modified)
response.


The purpose of this feature is to allow efficient updates of cached
information with a minimum amount of transaction overhead.


It worth mentioning that wit ill automatically work for static files only (as such files are server directly by web server (Apache/IIS/etc) and it can detect the changes. For dynamically produced pages (by PHP/ASP.NET etc), you will need to implement your own logic inside the code that generates that page if you want to handle such request header efficiently.

You will not see this implemented for dynamically generated pages very often, unless you have very good full-page caching system in place -- either by Framework you use, by web server (IIS can do this) or maybe your proxy (like Varnish, for example).

Both Apache and Microsoft IIS do support these request headers (as they are that are used by GoDaddy on Shared Hosting plans).

www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.25 http://en.wikipedia.org/wiki/List_of_HTTP_header_fields

Vote Up Vote Down

@Bryan171

If you're running Apache, you need the mod_expires and mod_headers modules to be installed and enabled. Your web host may need to recompile Apache to include them if they're not installed.

With mod_expires and mod_headers active, you can send and control the Last-Modified header by editing your .htaccess file to include:

# Expire all files one week after they're accessed
ExpiresActive On
ExpiresDefault "access plus 1 week"


More information and settings available from the Apache mod_expires documentation.

By serving content with a Last-Modified header, your site will now support requests from clients that use If-Modified-Since headers.

Vote Up Vote Down