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HTTP ETag
From Wikipedia, the free encyclopedia
| HTTP |
| Persistence · Compression · SSL |
| Headers |
| ETag · Cookie · Referrer |
| Status codes |
| 301 Moved permanently |
| 302 Found |
| 303 See Other |
| 403 Forbidden |
| 404 Not Found |
An ETag (entity tag) is an HTTP response header returned by an HTTP/1.1 compliant web server used to determine change in content at a given URL. When a new HTTP response contains the same ETag as an older HTTP response, the contents are considered to be the same without further downloading. The header is useful for intermediary devices that perform caching, as well as for client web browsers that cache results. One method of generating the ETag is based on the last modified time of the file and the size of the file, another is using a checksum.
If the ETag is generated incorrectly, it can lead to updated files not being redownloaded by the user agent, or files that are already in the cache being downloaded again.
[edit] References
- ETag in HTTP/1.1 specification
- Apache HTTP Server Documentation - FileETag Directive
- Old SQUID Development projects - ETag support (completed in 2001)
- Concerning Etags and Datestamps by Lars R. Clausen (2004)
- ETag in Lighttpd
[edit] See also
- List of HTTP headers (includes example of 'ETag'/'If-None-Match'-headers)
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