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HTTP Proxy-Status Parameter for Next-Hop Aliases
RFC 9532

Document Type RFC - Proposed Standard (January 2024)
Author Tommy Pauly
Last updated 2024-02-05
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Francesca Palombini
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RFC 9532

Internet Engineering Task Force (IETF)                          T. Pauly
Request for Comments: 9532                                   Apple, Inc.
Category: Standards Track                                   January 2024
ISSN: 2070-1721

            HTTP Proxy-Status Parameter for Next-Hop Aliases


   This document defines the next-hop-aliases HTTP Proxy-Status
   Parameter.  This parameter carries the list of aliases and canonical
   names an intermediary received during DNS resolution as part of
   establishing a connection to the next hop.

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

Copyright Notice

   Copyright (c) 2024 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
   ( 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
     1.1.  Requirements
   2.  next-hop-aliases Parameter
     2.1.  Encoding Special Characters
   3.  Implementation Considerations
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Author's Address

1.  Introduction

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

   [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
   the blocking of malicious hosts.

   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 value of the next-hop-aliases parameter is a String
   [STRUCTURED-FIELDS] 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 and MAY include the
   original requested hostname itself.  The names ought to appear in the
   order in which they were received in DNS, for the sake of
   consistency.  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 parameter uses a comma
   (",") as a separator between names.  Note that if a comma is included
   in a name itself, the comma must be encoded as described in
   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";

   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 proxy can also include the name of the next hop as the first item
   in the list.  This is particularly useful for reverse proxies when
   clients would not otherwise know the name of the next hop, and the
   next-hop header is used to convey an IP address.

   For example, consider a proxy "" that
   receives the following records when performing DNS resolution for the
   next hop "":          CNAME       AAAA    2001:db8::2

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

   Proxy-Status:; next-hop="2001:db8::2";

   The next-hop-aliases parameter only applies when DNS was used to
   resolve the next hop's name, and it 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 they need to
   gracefully handle situations in which this parameter is not present.

   The proxy MAY send the empty string ("") as the value of next-hop-
   aliases parameter to indicate that no CNAME records were encountered
   when resolving the next hop's name.

2.1.  Encoding Special Characters

   DNS names commonly contain just 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 the DNS name "comma,".  This
   name would be encoded within a next-hop-aliases parameter as follows:

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

   It is also possible for a DNS name to include a period character
   (".") within a label instead of as a label separator.  In this case,
   the period character MUST first be escaped as "\.".  Since the "\"
   character itself will be percent-encoded, the name
   "dot\" would be encoded within a next-hop-aliases
   parameter as follows:

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

   Upon parsing this name, "dot%5C.label" MUST be treated as a single

   Similarly, the "\" character in a label MUST be escaped as "\\" and
   then percent-encoded.  Other uses of "\" MUST NOT appear in the label
   after percent-decoding.  For example, if there is a DNS name
   "backslash\", it would first be escaped as
   "backslash\\" and then percent-encoded as follows:

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

3.  Implementation Considerations

   In order to include the next-hop-aliases parameter, a proxy needs to
   have access to the chain of alias names and canonical names received
   in CNAME records.

   Implementations ought to note that the full chain of names might not
   be available in common DNS resolution APIs, such as getaddrinfo
   [POSIX]. getaddrinfo does have an option for AI_CANONNAME ([RFC3493],
   Section 6.1), but this will only return the last name in the chain
   (the canonical name), not the alias names.

   An implementation MAY include incomplete information in the next-hop-
   aliases parameter to accommodate cases where it is unable to include
   the full chain, such as only including the canonical name if the
   implementation can only use getaddrinfo as described above.

4.  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 the 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.

   Inspecting CNAME chains can be used to detect cloaking of trackers or
   malicious hosts.  However, the CNAME records could be omitted by a
   recursive or authoritative resolver that is trying to hide this form
   of cloaking.  In particular, recursive or authoritative resolvers can
   omit these records for both clients directly performing DNS name
   resolution and proxies performing DNS name resolution on behalf of a
   client.  A malicious proxy could also choose to not report these
   CNAME chains in order to hide the cloaking.  In general, clients can
   trust information included (or not included) in the next-hop-aliases
   parameter to the degree that the proxy and any resolvers used by the
   proxy are trusted.

5.  IANA Considerations

   This document registers the next-hop-aliases parameter in the "HTTP
   Proxy-Status Parameters" registry <
   http-proxy-status> as shown in Table 1.

    | Name             | Description                     | Reference |
    | next-hop-aliases | A string containing one or more | RFC 9532  |
    |                  | DNS aliases or canonical names  |           |
    |                  | used to establish a proxied     |           |
    |                  | connection to the next hop.     |           |

              Table 1: HTTP Proxy-Status Parameters Registry

6.  References

6.1.  Normative References

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

   [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, <>.

              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,

6.2.  Informative References

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

   [POSIX]    IEEE, "IEEE Standard for Information Technology--Portable
              Operating System Interface (POSIX(TM)) Base
              Specifications, Issue 7", IEEE Std 1003.1-2017,
              DOI 10.1109/IEEESTD.2018.8277153, January 2018,

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

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, DOI 10.17487/RFC3493, February 2003,

Author's Address

   Tommy Pauly
   Apple, Inc.