The Cache-Status HTTP Response Header Field

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9211.

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To aid debugging (both by humans and automated tools), HTTP caches often append header fields to a response explaining how they handled the request. Unfortunately, the semantics of these header fields are often unclear, and both the semantics and syntax used vary between implementations.

This specification defines a new HTTP response header field, "Cache-Status", for this purpose with standardized syntax and semantics.

The Cache-Status HTTP response header field indicates how caches have handled that response and its corresponding request. The syntax of this header field conforms to [STRUCTURED-FIELDS].

Its value is a List. Each member of the List represents a cache that has handled the request. The first member represents the cache closest to the origin server, and the last member represents the cache closest to the user (possibly including the user agent's cache itself if it appends a value).

Caches determine when it is appropriate to add the Cache-Status header field to a response. Some might add it to all responses, whereas others might only do so when specifically configured to, or when the request contains a header field that activates a debugging mode. See Section 6 for related security considerations.

An intermediary SHOULD NOT append a Cache-Status member to responses that it generates locally, even if that intermediary contains a cache, unless the generated response is based upon a stored response (e.g., 304 (Not Modified) and 206 (Partial Content) are both based upon a stored response). For example, a proxy generating a 400 response due to a malformed request will not add a Cache-Status value, because that response was generated by the proxy, not the origin server.

When adding a value to the Cache-Status header field, caches SHOULD preserve the existing field value, to allow debugging of the entire chain of caches handling the request.

Each List member identifies the cache that inserted it, and this identifier MUST be a String or Token. Depending on the deployment, this might be a product or service name (e.g., "ExampleCache" or "Example CDN"), a hostname ("cache-3.example.com"), an IP address, or a generated string.

Each member of the list can have parameters that describe that cache's handling of the request. While these parameters are OPTIONAL, caches are encouraged to provide as much information as possible.

This specification defines the following parameters.

Cache-Status: ExampleCache; hit

Cache-Status: ExampleCache; hit; ttl=376

Cache-Status: ExampleCache; hit; ttl=-412

Cache-Status: ExampleCache; fwd=uri-miss

Cache-Status: ExampleCache; fwd=stale; fwd-status=304

Cache-Status: ExampleCache; fwd=uri-miss; collapsed

Cache-Status: ExampleCache; fwd=uri-miss; collapsed=?0

Cache-Status: OriginCache; hit; ttl=1100,
              "CDN Company Here"; hit; ttl=545

Cache-Status: ReverseProxyCache; hit
Cache-Status: ForwardProxyCache; fwd=uri-miss; collapsed; stored
Cache-Status: BrowserCache; fwd=uri-miss

New Cache-Status parameters can be defined by registering them in the "HTTP Cache-Status" registry.

Registration requests are reviewed and approved by a designated expert, per [RFC8126], Section 4.5. A specification document is appreciated but not required.

• Community feedback
• If the value is sufficiently well defined
• Generic parameters are preferred over vendor-specific, application-specific, or deployment-specific values. If a generic value cannot be agreed upon in the community, the parameter's name should be correspondingly specific (e.g., with a prefix that identifies the vendor, application, or deployment).

Name:

[a name for the Cache-Status parameter's key; see Section 3.1.2 of [STRUCTURED-FIELDS] for syntactic requirements]

Type:

[the Structured Type of the parameter's value; see Section 3.1.2 of [STRUCTURED-FIELDS]]

Description:

[a description of the parameter's semantics]

Reference:

[to a specification defining this parameter, if available]

See the registry at <https://www.iana.org/assignments/http-cache-status> for details on where to send registration requests.

IANA has created the "HTTP Cache-Status" registry at <https://www.iana.org/assignments/http-cache-status> and populated it with the types defined in Section 2; see Section 4 for its associated procedures.

Field name:

Cache-Status

Status:

permanent

Reference:

RFC 9211

Attackers can use the information in Cache-Status to probe the behavior of the cache (and other components) and infer the activity of those using the cache. The Cache-Status header field may not create these risks on its own, but it can assist attackers in exploiting them.

For example, knowing if a cache has stored a response can help an attacker execute a timing attack on sensitive data.

Additionally, exposing the cache key can help an attacker understand modifications to the cache key, which may assist cache poisoning attacks. See [ENTANGLE] for details.

The underlying risks can be mitigated with a variety of techniques (e.g., using encryption and authentication and avoiding the inclusion of attacker-controlled data in the cache key), depending on their exact nature. Note that merely obfuscating the key does not mitigate this risk.

To avoid assisting such attacks, the Cache-Status header field can be omitted, only sent when the client is authorized to receive it, or sent with sensitive information (e.g., the key parameter) only when the client is authorized.