Rule from network under performance category
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The Vary header lets browser clients define how the server identifies each request. This allows the server to control how caching is handled on all it's clients.
The "Accept-Encoding" is a quick way to tell clients and caching servers how to handle requests where GZIP is on and off, treating them as the same or unique.
When "Accept-Encoding" is configured as one of the options in Vary the GZIP and NON-GZIP request would be treated as unique, allowing browsers that do not support GZIP to still access the site.
To fix simply define the header, with at a minimum the "Accept-Encoding" option.
If using Apache (and most probably a shared environment) create/edit the
.htaccess file in the root of the site with the following content:
<IfModule mod_headers.c> <FilesMatch ".(js|css|xml|gz|html)$"> Header append Vary: Accept-Encoding </FilesMatch> </IfModule>
To define the header in your location or server block use the following:
Define the setting in web.config:
<system.webServer> <httpProtocol> <customHeaders> <remove name="Vary"></remove> <add name="Vary" value="Accept-Encoding"></add> </customHeaders> </httpProtocol> </system.webServer>
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