A hop-by-hop header is a header which is designed to be processed and consumed by the proxy currently handling the request, as opposed to an end-to-end header.
* **`X-Cache`** in the response may have the value **`miss`** when the request wasn't cached and the value **`hit`** when it is cached
* **`Cache-Control`** indicates if a resource is being cached and when will be the next time the resource will be cached again: `Cache-Control: public, max-age=1800`
***`Vary`** is often used in the response to **indicate additional headers** that are treated as **part of the cache key** even if they are normally unkeyed.
* **`Age`** defines the times in seconds the object has been in the proxy cache.
*`Clear-Site-Data`: Header to indicate the cache that should be removed: `Clear-Site-Data: "cache", "cookies"`
*`Expires`: Contains date/time when the response should expire: `Expires: Wed, 21 Oct 2015 07:28:00 GMT`
*`Pragma: no-cache` same as `Cache-Control: no-cache`
*`Warning`: The **`Warning`** general HTTP header contains information about possible problems with the status of the message. More than one `Warning` header may appear in a response. `Warning: 110 anderson/1.3.37 "Response is stale"`
* Requests using these headers: **`If-Modified-Since`** and **`If-Unmodified-Since`** will be responded with data only if the response header**`Last-Modified`** contains a different time.
* Conditional requests using **`If-Match`** and **`If-None-Match`** use an Etag value so the web server will send the content of the response if the data \(Etag\) has changed. The `Etag` is taken from the HTTP response.
* The **Etag** value is usually **calculated based** on the **content** of the response. For example, `ETag: W/"37-eL2g8DEyqntYlaLp5XLInBWsjWI"` indicates that the `Etag` is the **Sha1** of **37 bytes**.
* **`Accept-Ranges`**: Indicates if the server supports range requests, and if so in which unit the range can be expressed. `Accept-Ranges: <range-unit>`
* **`Range`**: Indicates the part of a document that the server should return.
* **`If-Range`**: Creates a conditional range request that is only fulfilled if the given etag or date matches the remote resource. Used to prevent downloading two ranges from incompatible version of the resource.
* **`Content-Range`**: Indicates where in a full body message a partial message belongs.
* **`Content-Length`:** The size of the resource, in decimal number of bytes.
* **`Content-Type`**: Indicates the media type of the resource
* **`Content-Encoding`**: Used to specify the compression algorithm.
* **`Content-Language`**: Describes the human language\(s\) intended for the audience, so that it allows a user to differentiate according to the users' own preferred language.
* **`Content-Location`**: Indicates an alternate location for the returned data.
From a pentest point of view this information is usually "useless", but if the resource is **protected** by a 401 or 403 and you can find some **way** to **get** this **info**, this could be **interesting.**
For example a combination of **`Range`** and **`Etag`** in a HEAD request can leak the content of the page via HEAD requests:
* A request with the header `Range: bytes=20-20` and with a response containing `ETag: W/"1-eoGvPlkaxxP4HqHv6T3PNhV9g3Y"` is leaking that the SHA1 of the byte 20 is `ETag: eoGvPlkaxxP4HqHv6T3PNhV9g3Y`
* **`Allow`:** Lists the set of methods supported by a resource. `Allow: GET, POST, HEAD`
* **`Expect`**: The **`Expect`** HTTP request header indicates expectations that need to be fulfilled by the server in order to properly handle the request.
* No other expectations except `Expect: 100-continue` are specified currently. Informs recipients that the client is about to send a \(presumably large\) message body in this request and wishes to receive a [`100`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/100) \(Continue\) interim response.
## Downloads
* **`Content-Disposition`**: In a regular HTTP response, the **`Content-Disposition`** response header is a header indicating if the content is expected to be displayed _inline_ in the browser, that is, as a Web page or as part of a Web page, or as an _attachment_, that is downloaded and saved locally.
