Internet-Draft Proxy-Status August 2021
Nottingham & Sikora Expires 17 February 2022 [Page]
Intended Status:
Standards Track
M. Nottingham
P. Sikora

The Proxy-Status HTTP Response Header Field


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

Note to Readers

RFC EDITOR: please remove this section before publication

Discussion of this draft takes place on the HTTP working group mailing list (, which is archived at

Working Group information can be found at; source code and issues list for this draft can be found at

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This Internet-Draft will expire on 17 February 2022.

Table of Contents

1. Introduction

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.

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 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.

3. IANA Considerations

Upon publication, please create the HTTP Proxy-Status Parameters registry and the HTTP Proxy Error Types registry at 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.

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 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.

5. References

5.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <>.
Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Nottingham, M. and P-H. Kamp, "Structured Field Values for HTTP", RFC 8941, DOI 10.17487/RFC8941, , <>.
Friedl, S., Popov, A., Langley, A., and E. Stephan, "Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, , <>.
Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D. Lawrence, "Extended DNS Errors", RFC 8914, DOI 10.17487/RFC8914, , <>.
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <>.

5.2. Informative References

Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, , <>.
Ludin, S., Nottingham, M., and N. Sullivan, "Loop Detection in Content Delivery Networks (CDNs)", RFC 8586, DOI 10.17487/RFC8586, , <>.

Authors' Addresses

Mark Nottingham
Prahran VIC
Piotr Sikora