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.
* 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**.
## Range requests
*`Accept-Ranges`: Indicates if the server supports range requests, and if so in which unit the range can be expressed.
*`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.
## Message body information
*`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`
