| Internet-Draft | Proxy-Status | August 2021 |
| Nottingham & Sikora | Expires 17 February 2022 | [Page] |
This document defines the Proxy-Status HTTP field to convey the details of intermediary response handling, including generated errors.¶
RFC EDITOR: please remove this section before publication¶
Discussion of this draft takes place on the HTTP working group mailing list (ietf-http-wg@w3.org), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/.¶
Working Group information can be found at https://httpwg.org/; source code and issues list for this draft can be found at https://github.com/httpwg/http-extensions/labels/proxy-status.¶
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HTTP intermediaries -- including both forward proxies and gateways (also known as "reverse proxies") -- have become an increasingly significant part of HTTP deployments. In particular, reverse proxies and Content Delivery Networks (CDNs) form part of the critical infrastructure of many Web sites.¶
Typically, HTTP intermediaries forward requests towards the origin server and then forward their responses back to clients. However, if an error occurs before a response is obtained from upstream, the response is often generated by the intermediary itself.¶
HTTP accommodates these types of errors with a few status codes; for example, 502 Bad Gateway and 504 Gateway Timeout. However, experience has shown that more information is necessary to aid debugging and communicate what's happened to the client. Additionally, intermediaries sometimes want to convey additional information about their handling of a response, even if they did not generate it.¶
To enable these uses, Section 2 defines a new HTTP response field to allow intermediaries to convey details of their handling of a response. Section 2.1 enumerates the information that can be added to the field by intermediaries, which can be extended as per Section 2.2. Section 2.3 defines a set of error types for use when a proxy encounters an issue when obtaining a response for the request; these can likewise be extended as per Section 2.4.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This specification uses Structured Fields [RFC8941] to specify syntax and parsing, and ABNF [RFC5234] as a shorthand for that syntax. The terms sf-list, sf-item, sf-string, sf-token, sf-integer and key refer to the structured types defined therein.¶
Note that in this specification, "proxy" is used to indicate both forward and reverse proxies, otherwise known as gateways. "Next hop" indicates the connection in the direction leading to the origin server for the request.¶
The Proxy-Status HTTP response field allows an intermediary to convey additional information about its handling of a response and its associated request.¶
It is a List ([RFC8941], Section 3.1):¶
Proxy-Status = sf-list¶
Each member of the list represents an intermediary that has handled the response. The first member of the list represents the intermediary closest to the origin server, and the last member of the list represents the intermediary closest to the user agent.¶
For example:¶
indicates that this response was handled first by FooProxy (a reverse proxy adjacent to the origin server) and then ExampleCDN.¶
Intermediaries determine when it is appropriate to add the Proxy-Status field to a response. Some might decide to append to it to all responses, whereas others might only do so when specifically configured to, or when the request contains a header that activates a debugging mode.¶
Each member of the list identifies the intermediary that inserted the value, and MUST have a type of either sf-string or sf-token. Depending on the deployment, this might be a product or service name (e.g., ExampleProxy or "Example CDN"), a hostname ("proxy-3.example.com"), an IP address, or a generated string.¶
Parameters on each member convey additional information about that intermediary's handling of the response and its associated request; see Section 2.1. While all of these parameters are OPTIONAL, intermediaries are encouraged to provide as much information as possible (but see Section 4 for security considerations in doing so).¶
When adding a value to the Proxy-Status field, intermediaries SHOULD preserve the existing members of the field, to allow debugging of the entire chain of intermediaries handling the request.¶
Proxy-Status MAY be sent in HTTP trailers. For example, if an intermediary is streaming a response and the upstream connection suddenly terminates, Proxy-Status can only be appended to the trailers of the outgoing message, since the headers have already been sent. As with all trailers, it might be silently discarded along the path to the user agent, so this SHOULD NOT be done unless it is not possible to send it in headers, and an intermediary MUST NOT send Proxy-Status as a trailer field unless it has also sent a corresponding Proxy-Status header field in the same message, so that the trailer value's ordering relative to other intermediaries is preserved.¶
Origin servers MUST NOT generate the Proxy-Status field.¶
This section lists parameters that can be used on the members of the Proxy-Status field. Unrecognised parameters MUST be ignored.¶
The error parameter's value is an sf-token that is a Proxy Error Type. When present, it indicates that the proxy encountered an issue when obtaining a response.¶
Unless a Proxy Error Type specifies otherwise, the presences of error often, but not always, indicates that response was generated by the proxy, not the origin server or any other upstream server. For example, a proxy might attempt to correct an error, or part of a response might be forwarded before the error is encountered.¶
Section 2.3 lists the Proxy Error Types defined in this document; new ones can be defined using the procedure outlined in Section 2.4.¶
For example:¶
indicates that this 504 response was generated by SomeCDN, due to a connection timeout when going forward.¶
Or:¶
indicates that this 429 Too Many Requests response was generated by the intermediary, not the origin.¶
When sending the error parameter, the most specific Proxy Error Type SHOULD be sent, provided that it accurately represents the error condition. If an appropriate Proxy Error Type is not defined, there are a number of generic error types (e.g., proxy_internal_error, http_protocol_error) that can be used. If they are not suitable, consider registering a new Proxy Error Type (see Section 2.4).¶
Each Proxy Error Type has a Recommended HTTP Status Code. When generating a HTTP response containing error, its HTTP status code SHOULD be set to the Recommended HTTP Status Code. However, there may be circumstances (e.g., for backwards compatibility with previous behaviours, a status code has already been sent) when another status code might be used.¶
Proxy Error Types can also define any number of extra parameters for use with that type. Their use, like all parameters, is optional. As a result, if an extra parameter is used with a Proxy Error Type for which it is not defined, it will be ignored.¶
The next-hop parameter's value is an sf-string or sf-token that identifies the intermediary or origin server selected (and used, if contacted) for this response. It might be a hostname, IP address, or alias.¶
For example:¶
The next-protocol parameter's value indicates the ALPN protocol identifier [RFC7301] used by the intermediary to connect to the next hop. This is only applicable when that connection was actually established.¶
The value MUST be either an sf-token or sf-binary, representing a TLS Application-Layer Protocol Negotiation (ALPN) Protocol ID (see https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml#alpn-protocol-ids). If the protocol identifier is able to be expressed as an sf-token using ASCII encoding, that form MUST be used.¶
For example:¶
The received-status parameter's value indicates the HTTP status code that the intermediary received from the next hop server.¶
The value MUST be an sf-integer.¶
For example:¶
New Proxy-Status Parameters can be defined by registering them in the HTTP Proxy-Status Parameters registry.¶
Registration requests are reviewed and approved by Expert Review, as per [RFC8126], Section 4.5. A specification document is appreciated, but not required.¶
The Expert(s) should consider the following factors when evaluating requests:¶
Registration requests should use the following template:¶
See the registry at https://iana.org/assignments/http-proxy-status for details on where to send registration requests.¶
This section lists the Proxy Error Types defined by this document. See Section 2.4 for information about defining new Proxy Error Types.¶
Note that implementations might not produce all Proxy Error Types. The set of types below is designed to map to existing states in implementations, and so may not be applicable to some.¶
Note that additional information about the error can be recorded in the details parameter (as is the case for all errors).¶
Note that additional information about the error can be recorded in the details parameter (as is the case for all errors).¶
Extra Parameters:¶
Extra Parameters:¶
Extra Parameters:¶
Extra Parameters:¶
Extra Parameters:¶
Extra Parameters:¶
Extra Parameters:¶
Extra Parameters:¶
Note that additional information about the error can be recorded in the details parameter (as is the case for all errors).¶
Note that additional information about the error can be recorded in the details parameter (as is the case for all errors).¶
Note that additional information about the error can be recorded in the details parameter (as is the case for all errors).¶
New Proxy Error Types can be defined by registering them in the HTTP Proxy Error Types registry.¶
Registration requests are reviewed and approved by Expert Review, as per [RFC8126], Section 4.5. A specification document is appreciated, but not required.¶
The Expert(s) should consider the following factors when evaluating requests:¶
Registration requests should use the following template:¶
If the Proxy Error Type might occur in responses that are not generated by the intermediary -- for example, when the error is detected during response content processing and a Proxy-Status trailer field is appended -- that SHOULD be explained in the Notes.¶
See the registry at https://iana.org/assignments/http-proxy-status for details on where to send registration requests.¶
Upon publication, please create the HTTP Proxy-Status Parameters registry and the HTTP Proxy Error Types registry at https://iana.org/assignments/http-proxy-status and populate them with the types defined in Section 2.1 and Section 2.3 respectively; see Section 2.2 and Section 2.4 for its associated procedures.¶
One of the primary security concerns when using Proxy-Status is leaking information that might aid an attacker. For example, information about the intermediary's configuration and back-end topology can be exposed.¶
As a result, care needs to be taken when deciding to generate a Proxy-Status field. Note that intermediaries are not required to generate a Proxy-Status field in any response, and can conditionally generate them based upon request attributes (e.g., authentication tokens, IP address).¶
Likewise, generation of all parameters is optional.¶