When running YSlow or similar performance analysis tools, you might encounter warnings about misconfigured ETags for static files like JPG, PNG, or GIF. ETags (Entity Tags) are part of HTTP headers used for cache validation. A misconfiguration can lead to unnecessary server requests, impacting website performance.

IIS7 generates ETags that include the server's local file path by default. This becomes problematic in load-balanced environments where the same file might have different paths across servers, causing cache misses. Here's a typical ETag header you might see:

ETag: "a5d6e7f89b0c1d2e3f4g5h6i7j8k9l0m"

To properly configure ETags in IIS7, we need to modify the server's behavior through the web.config file. Here's how to implement a solution:

Option 1: Remove ETags Completely

If you don't need ETag validation, you can remove them entirely:

<configuration>
  <system.webServer>
    <staticContent>
      <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="365.00:00:00" />
      <removeETag />
    </staticContent>
  </system.webServer>
</configuration>

Option 2: Generate Consistent ETags

For environments where you need ETags but want consistency across servers:

<configuration>
  <system.webServer>
    <httpProtocol>
      <customHeaders>
        <add name="ETag" value="" />
      </customHeaders>
    </httpProtocol>
  </system.webServer>
</configuration>

After implementing these changes, verify the headers using browser developer tools or command-line tools like cURL:

curl -I https://yourdomain.com/image.jpg

Look for the ETag header in the response to confirm your changes took effect.

While fixing ETags, consider these complementary optimizations:

• Implement proper Cache-Control headers
• Set appropriate Expires headers
• Consider using a CDN for static assets

If your changes don't seem to take effect:

1. Clear the IIS configuration cache: iisreset
2. Check for conflicting settings in parent web.config files
3. Verify file permissions on the web.config file

When YSlow reports "ETags are misconfigured" for static assets like images, it's flagging an inefficiency in how IIS7 generates entity tags. ETags help browsers validate cached resources, but improper configuration can actually hurt performance by causing unnecessary 304 responses.

The default ETag format in IIS7 includes the server-specific W/ weak validator and machine-specific identifiers. For load-balanced environments, this means:

• Different ETags generated across servers for same file
• Prevents proper cache validation in CDN scenarios
• Violates HTTP/1.1 spec recommendations for static content

Add this XML to your web.config's system.webServer section:

<configuration>
  <system.webServer>
    <httpProtocol>
      <customHeaders>
        <remove name="ETag" />
      </customHeaders>
    </httpProtocol>
    <staticContent>
      <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="365.00:00:00" />
    </staticContent>
  </system.webServer>
</configuration>

For static assets that rarely change, you might prefer removing ETags completely and relying solely on Cache-Control headers. Add this rewrite rule:

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

After implementation:

1. Clear IIS cache (iisreset /noforce)
2. Check response headers with Chrome DevTools or Fiddler
3. Re-run YSlow to confirm warning disappears

If you need ETags for dynamic content while maintaining cluster consistency:

<system.web>
  <machineKey validationKey="AutoGenerate" 
              decryptionKey="AutoGenerate"
              validation="SHA1" />
</system.web>

This ensures consistent ETags across servers while maintaining proper cache validation semantics.