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HTTP Headers

If None Match

HTTP Header

The HTTP If-None-Match request header enables conditional requests.
The server returns the requested resource using GET and HEAD methods with a 200 status only if it doesn’t have an ETag matching any of the values provided in the header.
For other HTTP methods, the request proceeds only if the resource’s ETag doesn’t match any listed value.

When the condition isn’t met for GET and HEAD requests, the server must respond with a 304 Not Modified status. It includes headers such as Cache-Control, Content-Location, Date, ETag, Expires, and Vary—the same headers that would be sent in a 200 response for the same request.
For methods that modify server-side data, such as PUT, a 412 Precondition Failed status indicates that the condition was not met.

The comparison between the provided ETag and stored ETag values uses a weak comparison algorithm. This means that two files are considered identical if their content is equivalent, regardless of byte-by-byte differences.
For example, two pages differing only in their footer’s creation date are deemed identical.

If If-Modified-Since is also used in a request, the server gives precedence to If-None-Match if it supports it.

Common scenarios for utilizing If-None-Match include:

  • For GET and HEAD requests, to refresh cached data with an ETag.

  • For other methods like PUT, where If-None-Match with the * value ensures a file is uploaded only if it doesn’t already exist. This prevents accidental overwrites and preserves data, addressing a variation of the lost update problem.

Syntax

The “If-None-Match” HTTP header is used for conditional requests, primarily involving cache validation. It allows a client to specify one or more ETag values. The server compares these values with the current ETag for the resource. If any match, the server can respond with a “304 Not Modified” status, indicating that the cached version is still valid.

Example syntax:

If-None-Match: <etag_value>
If-None-Match: <etag_value>, <etag_value>, ...
If-None-Match: *

Directives

<etag_value>

Entity tags are unique identifiers for requested resources. They are strings of ASCII characters enclosed in double quotes, such as "675af34563dc-tr34". Sometimes, they are prefixed with W/ to signify that a weak comparison algorithm should be used. This is generally not relevant with If-None-Match headers, which only employ the weak comparison method.

*

The asterisk is a special symbol representing any resource. It is primarily used during resource uploads, typically with PUT requests, to verify if a resource with the same identity has already been uploaded.

Example

http
If-None-Match: "bfc13a64729c4290ef5b2c2730249c88ca92d82d"

If-None-Match: W/"67ab43", "54ed21", "7892dd"

If-None-Match: *

How to Modify Header using Requestly

Requestly is a powerful Chrome extension that allows you to modify HTTP headers, including the If-None-Match header. This can help you control caching behavior and test how your application handles conditional requests. Steps to Modify the If-None-Match Header:

  1. Install and open the Requestly Chrome extension. You can find it on the Chrome Web Store.
  2. Create a new rule: Click on “Create Rule” and choose “Modify Headers” from the list of available rule types.
  3. Add a new header modification:
    • Under “Action”, select “Add” or “Override”.
    • In the “Header Name” field, enter If-None-Match.
    • In the “Header Value” field, enter the ETag value you want to use (e.g., “12345”).
  4. Set the URL condition: Specify the URL or pattern where this header change should apply (e.g., https://your-api.com/*).
  5. Save the rule.

Once set up, Requestly will inject the If-None-Match header with your specified value into all matching requests, allowing you to simulate cache validation and test how your server responds to conditional GET requests. Modifying the If-None-Match header is useful when you want to test caching mechanisms or verify how your server handles resource changes without repeatedly downloading full responses. It helps in efficient development and debugging of web applications.