RFC 9209 Proxy-Status Header June 2022
Nottingham & Sikora Standards Track [Page]
Stream:
Internet Engineering Task Force (IETF)
RFC:
9209
Category:
Standards Track
Published:
ISSN:
2070-1721
Authors:
M. Nottingham
Fastly
P. Sikora
Google

RFC 9209

The Proxy-Status HTTP Response Header Field

Abstract

This document defines the Proxy-Status HTTP response field to convey the details of an intermediary's response handling, including generated errors.

Status of This Memo

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/rfc9209.

Table of Contents

1. Introduction

HTTP intermediaries (see Section 3.7 of [HTTP]) -- 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 websites.

Typically, HTTP intermediaries forward requests towards the origin server (inbound) and then forward their responses back to clients (outbound). However, if an error occurs before a response is obtained from an inbound server, 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 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 per Section 2.4.

1.1. Notational Conventions

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 document uses the following terminology from Section 3 of [STRUCTURED-FIELDS] to specify syntax and parsing: List, String, Token, Integer, and Byte Sequence.

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.

3. IANA Considerations

IANA has created the "HTTP Proxy-Status Parameters" registry and the "HTTP Proxy Error Types" registry at <https://www.iana.org/assignments/http-proxy-status> and has populated them with the types defined in Sections 2.1 and 2.3 respectively; see Sections 2.2 and 2.4 for their associated procedures.

Additionally, the following entry has been added to the "Hypertext Transfer Protocol (HTTP) Field Name Registry":

Field name:
Proxy-Status
Status:
permanent
Specification document(s):
RFC 9209
Comments:

4. Security Considerations

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 backend topology can be exposed, allowing attackers to directly target backend services that are not prepared for high traffic volume or malformed inputs. Some information might only be suitable to reveal to authorized parties.

As a result, care needs to be taken when deciding to generate a Proxy-Status field and what information to include in it. 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, as is the generation of the field itself. Also, the field's content is not verified; an intermediary can claim certain actions (e.g., sending a request over an encrypted channel) but fail to actually do that.

5. References

5.1. Normative References

[HTTP]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, , <https://www.rfc-editor.org/info/rfc9110>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC7301]
Friedl, S., Popov, A., Langley, A., and E. Stephan, "Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, , <https://www.rfc-editor.org/info/rfc7301>.
[RFC8126]
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <https://www.rfc-editor.org/info/rfc8126>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8499]
Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, , <https://www.rfc-editor.org/info/rfc8499>.
[RFC8914]
Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D. Lawrence, "Extended DNS Errors", RFC 8914, DOI 10.17487/RFC8914, , <https://www.rfc-editor.org/info/rfc8914>.
[STRUCTURED-FIELDS]
Nottingham, M. and P-H. Kamp, "Structured Field Values for HTTP", RFC 8941, DOI 10.17487/RFC8941, , <https://www.rfc-editor.org/info/rfc8941>.
[TLS]
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <https://www.rfc-editor.org/info/rfc8446>.

5.2. Informative References

[RFC8586]
Ludin, S., Nottingham, M., and N. Sullivan, "Loop Detection in Content Delivery Networks (CDNs)", RFC 8586, DOI 10.17487/RFC8586, , <https://www.rfc-editor.org/info/rfc8586>.

Authors' Addresses

Mark Nottingham
Fastly
Prahran
Australia
Piotr Sikora
Google