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I am confused by how the W/ appears in the etag when I have not added it. I am using Node.js http server module and have a Nginx as reverse proxy. I am getting the browser see the Etag generated by the Node.js server but with a W/ tagged to it.
Can someone explain where that W/ comes from? Does the browser insert that based upon its determination that it is a weak etag? I want the browser to get it as I sent it. Without the W/ prefix.
Here is the Etag header as seen in the browser.
etag: W/"asv1534746804282-d62serveav"
When trying to compare with if-none-match, I have to strip the W/. Also, with the 304 status response, do I again have to send the Etag?
EDIT: I added the W/ myself so that Nginx leaves it unmodified. I hope my assumption is correct. It appears to be.
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An entity tag (ETag) is an HTTP header used for Web cache validation and conditional request from browsers to resources. The value of an ETag is an identifier that represents a specific version of the resource. Additionally, ETags help prevents simultaneous updates of a resource from overwriting each other.
Etags are just a Http request header just like any other from curls point of view. So you can just use: curl -v -H 'If-None-Match: 123456789' http:your.server.com/your/path/here.
Etag Working Style in StepsThe server generates an Etag value as a response header value. Requestor takes a 200 (Ok) status code for the resource and uses the Etag value for the If-None-Match request header. A requestor requests the same resource after a time.
If you ask nginx to dynamically gzip your content it will convert your ETags into weak ones. This is required by the specification, since a strong ETag can only be used for content that is byte-for-byte identical.
A validator is weak if it is shared by two or more representations of a given resource at the same time, unless those representations have identical representation data. For example, if the origin server sends the same validator for a representation with a gzip content coding applied as it does for a representation with no content coding, then that validator is weak.
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