How to enable ETag support for static content (CSS, JS, images, etc.)

Question

ICSM Computer · Answer

1. ETag Support is Handled Automatically by IIS
For static files, IIS automatically generates ETags based on file metadata (like last modified date and file size).
However, you can control behavior and enhance caching using 
web.config.
2. Enable/Disable ETags via IIS Settings
IIS adds headers like:
ETag: "6396c9a2b1d6d81:0"
Last-Modified: Wed, 06 Aug 2025 06:30:00 GMT
These are enabled by default unless removed via custom headers.
3. Control Cache for Static Content in web.config
You can’t directly configure ETag values in web.config, but you can combine ETag behavior with long-term caching:
Example: Cache Static Files + Let IIS Handle ETag
andlt;configurationandgt;
  andlt;system.webServerandgt;
    andlt;staticContentandgt;
      andlt;clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="365.00:00:00" /andgt;
    andlt;/staticContentandgt;
  andlt;/system.webServerandgt;
andlt;/configurationandgt;
This will:
Add Cache-Control: public, max-age=...Keep ETag behavior managed by IIS
4. Disable ETag (Optional)
If you want to remove ETags (for performance tuning or to rely only on
Cache-Control):
In web.config
andlt;configurationandgt;
  andlt;system.webServerandgt;
    andlt;httpProtocolandgt;
      andlt;customHeadersandgt;
        andlt;remove name="ETag" /andgt;
      andlt;/customHeadersandgt;
    andlt;/httpProtocolandgt;
  andlt;/system.webServerandgt;
andlt;/configurationandgt;
⚠️ This removes the ETag header from the response — not recommended unless you're managing cache manually.
5. Advanced (Optional): Use Versioned URLs Instead
Another best practice is to use query string versioning or 
fingerprinted file names:
andlt;!-- Better cache busting with versioned links --andgt;
andlt;link rel="stylesheet" href="/css/site.css?v=20250806" /andgt;
This makes the browser treat the URL as a "new" file if the version changes, avoiding reliance on ETag entirely.
Summary
Goal
How
Enable ETag for static files
IIS does this by default
Control cache duration
Use andlt;clientCacheandgt; in web.config
Disable ETag
Remove ETag header via customHeaders
Version cache manually
Use versioned URLs (?v=...) in your markup

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How to enable ETag support for static content (CSS, JS, images, etc.)

ICSM Computer

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1 Answers

1. ETag Support is Handled Automatically by IIS

2. Enable/Disable ETags via IIS Settings

3. Control Cache for Static Content in `web.config`

Example: Cache Static Files + Let IIS Handle ETag

4. Disable ETag (Optional)

In `web.config`

5. Advanced (Optional): Use Versioned URLs Instead

Summary

Liked By

Goal	How
Enable ETag for static files	IIS does this by default
Control cache duration	Use `<clientCache>` in `web.config`
Disable ETag	Remove `ETag` header via `customHeaders`
Version cache manually	Use versioned URLs (`?v=...`) in your markup