What does the HTTP header If-None-Match: * mean?

http http-headers http-1.1
AlexV picture AlexV · Jan 21, 2010 · Viewed 44.7k times · Source

What does the following HTTP 1.1 header mean?

If-None-Match: *

I understand it when using a strong or weak ETag or even a list of ETags, but I don't understant it's use when using star (*).

Edit: Would be nice to have some pseudocode (PHP code good also) that would show how/what to answer to "If-None-Match: *".

Answer

St.Woland picture St.Woland · Jan 29, 2010

The answer is: it depends.

Suppose we have received

If-None-Match: *
If-Modified-Since: <yesterday date>

And the page has been altered today.

First, we take a look at the * which tells us: "Return 304 if the resource is there and condition (2) is met". Fine, the resource exists, BUT condition (2) states: "Only return 304, if the date is later than current". So this condition is not met, and the page will be delivered completely.

If we hadn't received If-Modified-Since, the response would have been 304.

If the resource hadn't existed upon request, we'd have returned the appropriate code (as if there was no If-None-Match).

304 should only be returned in response for GET and HEAD requests, and all cache-related response headers have to be there. For all other types of request your server needs to be answering 412 (Precondition failed).

I hope it helps ;)

Related questions

HTTP If-None-Match vs. If-Match

I'm currently building up a PHP script that will answer HTTP "304 Not Modified" when required. (See question #2086712 for what I do so far). Currently I answer to the following: If-Modified-Since If-None-Match But I found out that 3 more headers can trigger …

Read More
How does a HTTP 1.1 server respond to a HTTP 1.0 request?

How does HTTP 1.1 server should respond to a HTTP 1.0 request for headers like Pragma : no-cache which are supported in HTTP 1.0 but not in HTTP 1.1

Read More
application/x-www-form-urlencoded or multipart/form-data?

In HTTP there are two ways to POST data: application/x-www-form-urlencoded and multipart/form-data. I understand that most browsers are only able to upload files if multipart/form-data is used. Is there any additional guidance when to use one of …

Read More