HTTP M. Nottingham
Internet-Draft Fastly
Intended status: Standards Track P. Sikora
Expires: November 7, 2019 Google
May 6, 2019

The Proxy-Status HTTP Header Field


This document defines the Proxy-Status HTTP header field to convey the details of errors generated by HTTP intermediaries.

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RFC EDITOR: please remove this section before publication

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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, the response is 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.

To address this, Section 2 defines a new HTTP response header field to convey such information, using the Proxy Status Types defined in Section 3. Section 4 explains how to define new Proxy Status Types.

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 Headers [I-D.ietf-httpbis-header-structure] to specify syntax. The terms sh-param-list, sh-item, sh-string, sh-token and sh-integer 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.

2. The Proxy-Status HTTP Header Field

The Proxy-Status HTTP response header field allows an intermediary to indicate the nature and details of an error condition it encounters when servicing a request.

It is a Structured Headers [I-D.ietf-httpbis-header-structure] Parameterised List, where each item in the list indicates an error condition. Typically, it will have only one param-item (the error condition that triggered generation of the response it occurs within), but more than one value is not prohibited.

Each param-item’s primary-id is a Proxy Status Type, a registered value that indicates the nature of the error.

Each param-item can have zero to many parameters. Section 2.1 lists parameters that can be used with all Proxy Status Types; individual types can define additional parameters to use with them. All parameters are optional; see Section 6 for their potential security impact.

For example:

HTTP/1.1 504 Gateway Timeout
Proxy-Status: connection_timeout; proxy=SomeCDN; origin=abc; tries=3

indicates the specific nature of the timeout as a connect timeout to the origin with the identifier “abc”, and that is was generated by the intermediary that identifies itself as “FooCDN.” Furthermore, three connection attempts were made.


HTTP/1.1 429 Too Many Requests
Proxy-Status: http_request_error; proxy=SomeReverseProxy

indicates that this 429 Too Many Requests response was generated by the intermediary, not the origin.

Each Proxy Status Type has a Recommended HTTP Status Code. When generating a HTTP response containing Proxy-Status, 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) when another status code might be used.

Section 3 lists the Proxy Status Types defined in this document; new ones can be defined using the procedure outlined in Section 4.

Proxy-Status MAY be sent in HTTP trailers, but – as with all trailers – it might be silently discarded along the path to the user agent, this SHOULD NOT be done unless it is not possible to send it in headers. For example, if an intermediary is streaming a response and the upstream connection suddenly terminates, Proxy-Status can be appended to the trailers of the outgoing message (since the headers have already been sent).

Note that there are various security considerations for intermediaries using the Proxy-Status header field; see Section 6.

Origin servers MUST NOT generate the Proxy-Status header field.

2.1. Generic Proxy Status Parameters

This section lists parameters that are potentially applicable to most Proxy Status Types.

3. Proxy Status Types

This section lists the Proxy Status Types defined by this document. See Section 4 for information about defining new Proxy Status Types.

3.1. DNS Timeout

3.2. DNS Error

3.3. Destination Not Found

3.4. Destination Unavailable

3.5. Destination IP Prohibited

3.6. Destination IP Unroutable

3.7. Connection Refused

3.8. Connection Terminated

3.9. Connection Timeout

3.10. Connection Read Timeout

3.11. Connection Write Timeout

3.12. Connection Limit Reached

3.13. HTTP Response Status

3.14. HTTP Incomplete Response

3.15. HTTP Protocol Error

3.16. HTTP Response Header Block Too Large

3.17. HTTP Response Header Too Large

3.18. HTTP Response Body Too Large

3.19. HTTP Response Transfer-Coding Error

3.20. HTTP Response Content-Coding Error

3.21. HTTP Response Timeout

3.22. TLS Handshake Error

3.23. TLS Untrusted Peer Certificate

3.24. TLS Expired Peer Certificate

3.25. TLS Unexpected Peer Certificate

3.26. TLS Unexpected Peer Identity

3.27. TLS Missing Proxy Certificate

3.28. TLS Rejected Proxy Certificate

3.29. TLS Error

3.30. HTTP Request Error

3.31. HTTP Request Denied

3.32. HTTP Upgrade Failed

3.33. Proxy Internal Error

3.34. Proxy Loop Detected

4. Defining New Proxy Status Types

New Proxy Status Types can be defined by registering them in the HTTP Proxy Status Types registry.

Registration requests are reviewed and approved by a Designated Expert, 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 for details on where to send registration requests.

5. IANA Considerations

Upon publication, please create the HTTP Proxy Status Types registry at and populate it with the types defined in Section 3; see Section 4 for its associated procedures.

6. Security Considerations

One of the primary security concerns when using Proxy-Status is leaking information that might aid an attacker.

As a result, care needs to be taken when deciding to generate a Proxy-Status header. Note that intermediaries are not required to generate a Proxy-Status header 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.

Special care needs to be taken in generating proxy and origin parameters, as they can expose information about the intermediary’s configuration and back-end topology.

7. References

7.1. Normative References

[I-D.ietf-httpbis-header-structure] Nottingham, M. and P. Kamp, "Structured Headers for HTTP", Internet-Draft draft-ietf-httpbis-header-structure-09, December 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[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, July 2014.
[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, June 2017.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC8499] Hoffman, P., Sullivan, A. and K. Fujiwara, "DNS Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, January 2019.

7.2. Informative References

[I-D.ietf-httpbis-cdn-loop] Ludin, S., Nottingham, M. and N. Sullivan, "CDN Loop Detection", Internet-Draft draft-ietf-httpbis-cdn-loop-02, February 2019.

Authors' Addresses

Mark Nottingham Fastly EMail: URI:
Piotr Sikora Google EMail: