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I'm bucketing User-Agents by device using something like varnish-devicedetect and storing the result in X-UA-Device on the request and the response.
I've seen several recommendations to vary on User-Agent. Any reason not to vary instead on X-UA-Device? Seems like it'd be nicer to downstream caches.
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The Vary HTTP response header describes the parts of the request message aside from the method and URL that influenced the content of the response it occurs in. Most often, this is used to create a cache key when content negotiation is in use.
What Does “Specify a Vary: Accept-Encoding Header” Actually Mean? Accept-encoding header is an HTTP header which must be included on every origin server response. Its main job is to inform the browsers if the client can handle the compressed version of the website.
Vary: Origin When a user agent receives a response to a non-CORS request for that resource (for example, as the result of a navigation request), the response will lack `Access-Control-Allow-Origin` and the user agent will cache that response.
Custom headers allow site owners to upload their own “title” image to their site, which can be placed at the top of certain pages. These can be customized and cropped by the user through a visual editor in the Appearance > Header section of the admin panel. You may also place text beneath or on top of the header.
Since X-UA-Device is not available on the client request or in any downstream proxys (it's generated inside Varnish) you have to vary on the raw User-Agent header.
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