Yahoo! YSlow recommends removing Entity tags – also known as ETag. Unfortunately removing the ETag response header is not an easy task on Windows Server IIS web servers. Here I show you how to properly remove ETag HTTP response headers with an Outbound rewrite rule…

Remove ETag response headers in IIS

How to remove the ETag response header in IIS as Yahoo! YSlow recommends?

Entity Tags (ETags) are commonly used in Web applications to effectively leverage the use of web farms, which is a non-fancy term for HTTP/S load balancing. In web farms, a common practice is to set what is called ETags headers, as it helps enhance performance in web farm scenarios.

You might expect you can easily remove Entity tags, or Etag headers , in IIS by using <remove name="ETag" /> in the customHeaders node of the web.config configuration file.

For example:

<httpProtocol>
  <customHeaders>
    <remove name="ETag"/>
  </customHeaders>
</httpProtocol>

or by setting its value to an empty string:

<httpProtocol>
  <customHeaders>
    <remove name="ETag"/>
    <add name="ETag" value=" "/>
  </customHeaders>
</httpProtocol>
<httpProtocol>
  <customHeaders>
    <remove name="ETag"/>
    <add name="ETag" value="""" />
  </customHeaders>
</httpProtocol>

but these customHeaders are ignored…

Luckily you can use an IIS URL Rewrite Outbound Rule in Windows Server to rewrite, and remove, the ETags response header. And here is how.

This may interest you:   7 Snippets to use .htaccess as a Web Application Firewall

Outbound rewrite rule to properly remove ETag headers in IIS

You have to use an Outbound Rule to remove Etag headers. Use the following URL Rewrite Outbound Rule in your web.config, to remove the ETag header:

<outboundRules>
  <rule name="Remove ETag">
    <match serverVariable="RESPONSE_ETag" pattern=".+" />
    <action type="Rewrite" value="" />
  </rule>
</outboundRules>

It’s pretty straight forward what this rule does, no need to explain.

Thanks to NathanFox.net for sharing this information.

Disable Etag header in applicationHost.config

If you have administrator access to the IIS web server and you want to completely disable Etag headers, then you can do so in your IIS applicationHost.config configuration file.

Since IIS 8.0 you have an updated IIS_schema.xml file, with the following contents:

<sectionSchema name="system.webServer/staticContent">
  <element name="clientCache">
    <attribute name="cacheControlMode" type="enum" defaultValue="NoControl">
      <enum name="NoControl" value="0" />
      <enum name="DisableCache" value="1" />
      <enum name="UseMaxAge" value="2" />
      <enum name="UseExpires" value="3" />
    </attribute>
    <attribute name="cacheControlMaxAge" type="timeSpan" defaultValue="1.00:00:00" />
    <attribute name="httpExpires" type="string" />
    <attribute name="cacheControlCustom" type="string" />
    <attribute name="setEtag" type="bool" defaultValue="true" />
  </element>

You can find IIS_schema.xml in the folder C:\Windows\System32\inetsrv\config\schema. This means you can add the following in your C:\Windows\System32\inetsrv\config\applicationHost.config:

<clientCache setEtag="false" />

And for a website level, you can add it to your web.config file as well.

When you’ve already have an entry for clientCache, you can just add in the setEtag attribute within the element:

<staticContent>
  <clientCache
    cacheControlMode="UseMaxAge"
    cacheControlMaxAge="14.00:00:00"
    setEtag="false" />
</staticContent>

(Source, source 2,