Skip to main content

HTTP Proxy-Status Parameter for Next-Hop Aliases

Document Type Active Internet-Draft (httpbis WG)
Author Tommy Pauly
Last updated 2023-01-18
Replaces draft-pauly-httpbis-alias-proxy-status
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state WG Document
Document shepherd Mark Nottingham
IESG IESG state I-D Exists
Consensus boilerplate Yes
Telechat date (None)
Responsible AD (None)
Send notices to
HTTP                                                            T. Pauly
Internet-Draft                                               Apple, Inc.
Intended status: Standards Track                         18 January 2023
Expires: 22 July 2023

            HTTP Proxy-Status Parameter for Next-Hop Aliases


   This document defines an HTTP Proxy-Status Parameter that contains a
   list of aliases and canonical names received over DNS when
   establishing a connection to the next hop.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at

   Discussion of this document takes place on the HTTP Working Group
   mailing list (, which is archived at  Working Group
   information can be found at

   Source for this draft and an issue tracker can be found at

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 22 July 2023.

Pauly                     Expires 22 July 2023                  [Page 1]
Internet-Draft          DNS Aliases Proxy-Status            January 2023

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   3
   2.  next-hop-aliases Parameter  . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   The Proxy-Status HTTP response field [PROXY-STATUS] allows proxies to
   convey information about how a proxied request was handled in HTTP
   responses sent to clients.  It defines a set of parameters that
   provide information, such as the name of the next hop.

   [PROXY-STATUS] defines a next-hop parameter, which can contain a
   hostname, IP address, or alias of the next hop.  This parameter can
   contain only one such item, so it cannot be used to communicate a
   chain of aliases encountered during DNS resolution when connecting to
   the next hop.

   Knowing the full chain of names that were used during DNS resolution
   via CNAME records [DNS] is particularly useful for clients of forward
   proxies, in which the client is requesting to connect to a specific
   target hostname using the CONNECT method [HTTP] or UDP proxying
   [CONNECT-UDP].  CNAME records can be used to "cloak" hosts that
   perform tracking or malicious activity behind more innocuous
   hostnames, and clients such as web browsers use the chain of DNS
   names to influence behavior like cookie usage policies [COOKIES] or
   blocking of malicious hosts.

Pauly                     Expires 22 July 2023                  [Page 2]
Internet-Draft          DNS Aliases Proxy-Status            January 2023

   This document allows clients to receive the CNAME chain of DNS names
   for the next hop by including the list of names in a new next-hop-
   aliases Proxy-Status parameter.

1.1.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

2.  next-hop-aliases Parameter

   The next-hop-aliases parameter's value is a String that contains one
   or more DNS names in a comma-separated list.  The items in the list
   include all alias names and canonical names received in CNAME records
   [DNS] during the course of resolving the next hop's hostname using
   DNS, not including the original requested hostname itself.  The names
   SHOULD appear in the order in which they were received in DNS.  If
   there are multiple CNAME records in the chain, the first name in the
   next-hop-aliases list would be the value in the CNAME record for the
   original hostname, and the final name in the next-hop-aliases list
   would be the name that ultimately resolved to one or more addresses.

   The list of DNS names in next-hop-aliases use a comma (",") as a
   separator between names.  DNS names normally just contain
   alphanumeric characters and hyphens ("-"), although they are allowed
   to contain any character [RFC1035], Section 3.1, including a comma.
   To prevent commas or other special characters in names leading to
   incorrect parsing, any characters that appear in names in this list
   that do not belong to the set of URI Unreserved Characters [RFC3986],
   Section 2.3 MUST be percent-encoded as defined in [RFC3986],
   Section 2.1.

   For example, consider a proxy "" that receives the
   following records when performing DNS resolution for the next hop
   "":           CNAME        CNAME   AAAA    2001:db8::1

   The proxy could include the following proxy status in its response:

   Proxy-Status:; next-hop=2001:db8::1;

Pauly                     Expires 22 July 2023                  [Page 3]
Internet-Draft          DNS Aliases Proxy-Status            January 2023

   This indicates that, which used the IP address
   "2001:db8::1" as the next hop for this request, encountered the names
   "" and "" in the DNS
   resolution chain.  Note that while this example includes both the
   next-hop and next-hop-aliases parameters, next-hop-aliases can be
   included without including next-hop.

   The next-hop-aliases parameter only applies when DNS was used to
   resolve the next hop's name, and does not apply in all situations.
   Clients can use the information in this parameter to determine how to
   use the connection established through the proxy, but need to
   gracefully handle situations in which this parameter is not present.

3.  Security Considerations

   The next-hop-aliases parameter does not include any DNSSEC
   information or imply that DNSSEC was used.  The information included
   in the parameter can only be trusted to be valid insofar as the
   client trusts its proxy to provide accurate information.  This
   information is intended to be used as a hint, and SHOULD NOT be used
   for making security decisions about the identity of a resource
   accessed through the proxy.

4.  IANA Considerations

   This document registers the "next-hop-aliases" parameter in the "HTTP
   Proxy-Status Parameters" registry <

   Name:  next-hop-aliases

   Description:  A string containing one or more DNS aliases or
      canonical names used to establish a proxied connection to the next

   Reference:  This document

5.  References

5.1.  Normative References

              Schinazi, D., "Proxying UDP in HTTP", RFC 9298,
              DOI 10.17487/RFC9298, August 2022,

Pauly                     Expires 22 July 2023                  [Page 4]
Internet-Draft          DNS Aliases Proxy-Status            January 2023

   [DNS]      Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,

              Nottingham, M. and P. Sikora, "The Proxy-Status HTTP
              Response Header Field", RFC 9209, DOI 10.17487/RFC9209,
              June 2022, <>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

5.2.  Informative References

   [COOKIES]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

Author's Address

   Tommy Pauly
   Apple, Inc.

Pauly                     Expires 22 July 2023                  [Page 5]