HTTP Proxy-Status Parameter for Next-Hop Aliases
A Proxy-Status parameter that lets a proxy report the full chain of DNS alias names it saw on the way to the next hop.
Pauly, T., "HTTP Proxy-Status Parameter for Next-Hop Aliases", RFC 9532, DOI 10.17487/RFC9532, January 2024, <https://www.rfc-editor.org/info/rfc9532>.
@misc{rfc9532,
series = {Request for Comments},
number = 9532,
howpublished = {RFC 9532},
publisher = {RFC Editor},
doi = {10.17487/RFC9532},
url = {https://www.rfc-editor.org/info/rfc9532},
author = {T. Pauly},
title = {{HTTP Proxy-Status Parameter for Next-Hop Aliases}},
pagetotal = 7,
year = 2024,
month = jan,
} RFC text
Contents
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
Abstract
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
https://www.rfc-editor.org/info/rfc9532.
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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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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
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
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",
"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.
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 "proxy.example.net" that receives the
following records when performing DNS resolution for the next hop
"host.example.com":
host.example.com. CNAME tracker.example.com.
tracker.example.com. CNAME service1.example.com.
service1.example.com. AAAA 2001:db8::1
The proxy could include the following proxy status in its response:
Proxy-Status: proxy.example.net; next-hop="2001:db8::1";
next-hop-aliases="tracker.example.com,service1.example.com"
This indicates that "proxy.example.net", which used the IP address
"2001:db8::1" as the next hop for this request, encountered the names
"tracker.example.com" and "service1.example.com" 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 "reverseproxy.example.net" that
receives the following records when performing DNS resolution for the
next hop "host.example.com":
host2.example.com. CNAME service2.example.com.
service2.example.com. AAAA 2001:db8::2
The proxy could include the following proxy status in its response:
Proxy-Status: reverseproxy.example.net; next-hop="2001:db8::2";
next-hop-aliases="host2.example.com,service2.example.com"
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,name.example.com". This
name would be encoded within a next-hop-aliases parameter as follows:
Proxy-Status: proxy.example.net; next-hop="2001:db8::1";
next-hop-aliases="comma%2Cname.example.com,service1.example.com"
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\.label.example.com" would be encoded within a next-hop-aliases
parameter as follows:
Proxy-Status: proxy.example.net; next-hop="2001:db8::1";
next-hop-aliases="dot%5C.label.example.com,service1.example.com"
Upon parsing this name, "dot%5C.label" MUST be treated as a single
label.
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\name.example.com", it would first be escaped as
"backslash\\name.example.com" and then percent-encoded as follows:
Proxy-Status: proxy.example.net; next-hop="2001:db8::1";
next-hop-aliases="backslash%5C%5Cname.example.com,s1.example.com"
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 <https://www.iana.org/assignments/
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
[CONNECT-UDP]
Schinazi, D., "Proxying UDP in HTTP", RFC 9298,
DOI 10.17487/RFC9298, August 2022,
<https://www.rfc-editor.org/info/rfc9298>.
[DNS] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[PROXY-STATUS]
Nottingham, M. and P. Sikora, "The Proxy-Status HTTP
Response Header Field", RFC 9209, DOI 10.17487/RFC9209,
June 2022, <https://www.rfc-editor.org/info/rfc9209>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>.
6.2. Informative References
[COOKIES] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[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,
<https://ieeexplore.ieee.org/document/8277153>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[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,
<https://www.rfc-editor.org/info/rfc3493>.
Author's Address
Tommy Pauly
Apple, Inc.
Email: tpauly@apple.com
AI ELI5
On this page
§1Why Report Alias Names
The Proxy-Status HTTP response field lets an intermediary tell clients how it handled a request. Its next-hop parameter can carry a hostname, an IP address or an alias of the next hop, but only one of them. It cannot describe a whole chain of aliases found during DNS resolution.
That chain matters most to clients of forward proxies, where the client asks the proxy to connect to a specific target using the CONNECT method or UDP proxying. CNAME records can be used to cloak hosts that track users or do malicious things behind more innocent-looking names. Web browsers use the chain of DNS names to shape behavior such as cookie policies or blocking of malicious hosts.
Sometimes a place has a nickname that hides its real name. The receipt only had room for one name, so you could not see the nicknames. Now there is room for all of them.
§2The next-hop-aliases Parameter
The value of next-hop-aliases is a String (as defined for HTTP structured fields) containing one or more DNS names in a comma-separated list. The list holds every alias name and canonical name received in CNAME records while resolving the next hop's hostname, and it MAY also include the original requested hostname itself.
The names ought to appear in the order they were received in DNS. The first name is the value of the CNAME record for the original hostname, and the last name is the one that finally resolved to one or more addresses.
The parameter can be sent without next-hop. A proxy MAY send the empty string as the value to say that no CNAME records were met. The parameter applies only when DNS was used to resolve the next hop's name, so clients need to handle its absence gracefully.
A proxy acting as a reverse proxy can put the next hop's own name first in the list. This helps when next-hop carries only an IP address and the client would otherwise not know the name.
The helper writes the names in the order the lookup found them, like a trail of clues, and writes nothing but an empty note if there were no nicknames.
host.example.com. CNAME tracker.example.com.
tracker.example.com. CNAME service1.example.com.
service1.example.com. AAAA 2001:db8::1
Proxy-Status: proxy.example.net; next-hop="2001:db8::1";
next-hop-aliases="tracker.example.com,service1.example.com"§2.1Encoding Special Characters
DNS names usually contain only letters, digits and hyphens, but they may contain any character, including a comma. To keep a comma inside a name from breaking the list, every character that is not a URI unreserved character MUST be percent-encoded.
A period inside a label, as opposed to a separator between labels, MUST first be escaped as \.. Because the backslash is then percent-encoded, dot\.label.example.com becomes dot%5C.label.example.com, and on parsing, dot%5C.label MUST be treated as a single label. A backslash in a label MUST be escaped as \\ and then percent-encoded. Other uses of the backslash MUST NOT appear after percent-decoding.
If a name has a comma in it, writing it plainly would look like two names. So the comma gets a secret code, like writing the word for it instead.
§3Getting the Chain From the Resolver
To send the parameter, a proxy needs access to the chain of alias and canonical names from the CNAME records. The RFC warns that common resolution APIs such as getaddrinfo may not expose the full chain. Its AI_CANONNAME option returns only the last name, the canonical name, and not the aliases before it.
An implementation MAY therefore send incomplete information, for example only the canonical name, when it cannot see the whole chain.
Some tools only tell the helper the final name, not the nicknames along the way. The helper is allowed to write just what it knows.
§4How Far to Trust the List
The parameter carries no DNSSEC information and does not imply that DNSSEC was used. The client can trust it only as far as it trusts the proxy. It is meant as a hint and SHOULD NOT be used to make security decisions about the identity of a resource reached through the proxy.
Inspecting CNAME chains can reveal cloaked trackers or malicious hosts, but a recursive or authoritative resolver that wants to hide cloaking could omit the CNAME records, and a malicious proxy could simply not report them. In general, clients can trust what is included or left out only to the degree that the proxy and its resolvers are trusted.
The note is only as honest as the helper who wrote it. If the helper or the phone book hides a nickname, you will not see it.
§5IANA Registration
The document registers next-hop-aliases in the HTTP Proxy-Status Parameters registry. The registered description is a string containing one or more DNS aliases or canonical names used to establish a proxied connection to the next hop.
The new note field gets written in the official list of allowed fields so everyone knows it exists.