Internet Engineering Task Force (IETF)                          S. Ludin
Request for Comments: 8586                           Akamai Technologies
Category: Standards Track                                  M. Nottingham
ISSN: 2070-1721                                                   Fastly
                                                            N. Sullivan
                                                             Cloudflare
                                                             April 2019


          Loop Detection in Content Delivery Networks (CDNs)

Abstract

  This document defines the CDN-Loop request header field for HTTP.
  CDN-Loop addresses an operational need that occurs when an HTTP
  request is intentionally forwarded between Content Delivery Networks
  (CDNs), but is then accidentally or maliciously re-routed back into
  the original CDN causing a non-terminating loop.  The new header
  field can be used to identify the error and terminate the loop.

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/rfc8586.

Copyright Notice

  Copyright (c) 2019 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
  carefully, as they describe your rights and restrictions with respect
  to this document.  Code Components extracted from this document must
  include Simplified BSD License text as described in Section 4.e of
  the Trust Legal Provisions and are provided without warranty as
  described in the Simplified BSD License.



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Table of Contents

  1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
    1.1.  Relationship to Via . . . . . . . . . . . . . . . . . . .   2
    1.2.  Conventions and Definitions . . . . . . . . . . . . . . .   3
  2.  The CDN-Loop Request Header Field . . . . . . . . . . . . . .   3
  3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
  4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
  5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
    5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
    5.2.  Informative References  . . . . . . . . . . . . . . . . .   6
  Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

  In modern deployments of HTTP servers, it is common to interpose
  Content Delivery Networks (CDNs) in front of origin servers to
  improve latency perceived by end users, reduce operational costs, and
  improve scalability and reliability of services.

  Often, more than one CDN is in use by a given origin.  This happens
  for a variety of reasons, such as cost savings, arranging for
  failover should one CDN have issues, or direct comparison of the
  CDNs' services.

  As a result, it is possible for forwarding CDNs to be configured in a
  "loop" accidentally; because routing is achieved through a
  combination of DNS and forwarding rules, and site configurations are
  sometimes complex and managed by several parties.

  When this happens, it is difficult to debug.  Additionally, it
  sometimes isn't accidental; loops between multiple CDNs can be used
  as an attack vector (e.g., see [loop-attack]), especially if one CDN
  unintentionally strips the loop detection headers of another.

  This specification defines the CDN-Loop HTTP request header field to
  help detect such attacks and accidents among forwarding CDNs that
  have implemented it; the header field may not be modified by their
  customers.

1.1.  Relationship to Via

  HTTP defines the Via header field in Section 5.7.1 of [RFC7230] for
  "tracking message forwards, avoiding request loops, and identifying
  the protocol capabilities of senders along the request/response
  chain."





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  In theory, Via could be used to identify these loops.  However, in
  practice it is not used in this fashion, because some HTTP servers
  use Via for other purposes -- in particular, some implementations
  disable some HTTP/1.1 features when the Via header is present.

1.2.  Conventions and Definitions

  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 the Augmented Backus-Naur Form (ABNF)
  notation of [RFC5234] with a list extension, defined in Section 7 of
  [RFC7230], that allows for compact definition of comma-separated
  lists using a '#' operator (similar to how the '*' operator indicates
  repetition).  Additionally, it uses a token (OWS), uri-host, and port
  rules from [RFC7230] and the parameter rule from [RFC7231].

2.  The CDN-Loop Request Header Field

  The CDN-Loop request header field is intended to help a Content
  Delivery Network identify when an incoming request has already passed
  through that CDN's servers to detect loops.

  CDN-Loop  = #cdn-info
  cdn-info  = cdn-id *( OWS ";" OWS parameter )
  cdn-id    = ( uri-host [ ":" port ] ) / pseudonym
  pseudonym = token

  The cdn-id identifies the CDN using either a hostname under its
  control or a pseudonym.  Hostnames are preferred, to help avoid
  accidental collisions.  If a pseudonym is used, unintentional
  collisions are more likely, and therefore values should be carefully
  chosen to prevent them; for example, using a well-known value (such
  as the recognized name of the CDN in question), or a generated value
  with enough entropy to make collisions unlikely (such as a UUID
  [RFC4122]).

  Optionally, cdn-info can have semicolon-separated key/value
  parameters to accommodate additional information for the CDN's use.

  Conforming Content Delivery Networks SHOULD add a cdn-info to this
  header field in all requests they generate or forward (creating the
  header field if necessary).





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  As with all HTTP header fields defined using the "#" rule, the
  CDN-Loop header field can be added to by comma-separating values, or
  by creating a new header field with the desired value.

  For example:

  GET /image.jpg HTTP/1.1
  Host: cdn-customer.example
  User-Agent: ExampleBrowser/5
  CDN-Loop: foo123.foocdn.example, barcdn.example; trace="abcdef"
  CDN-Loop: AnotherCDN; abc=123; def="456"

  Note that the pseudonym syntax does not allow whitespace, DQUOTE, or
  any of the characters "(),/:;<=>?@[]{}".  See Section 3.2.6 of
  [RFC7230].  Likewise, note the rules for when parameter values need
  to be quoted in Section 3.1.1 of [RFC7231].

  The effectiveness of this mechanism relies on all intermediaries
  preserving the header field, since removing (or allowing it to be
  removed, e.g., by customer configuration) would prevent downstream
  CDNs from using it to detect looping.  In general, unknown header
  fields are not removed by intermediaries, but there may be a need to
  add CDN-Loop to an implementation's list of header fields that are
  not to be removed under any circumstances.  The header field SHOULD
  NOT be used for other purposes.

3.  Security Considerations

  The threat model that the CDN-Loop header field addresses is a
  customer who is attacking a service provider by configuring a
  forwarding loop by accident or malice.  For it to function, CDNs
  cannot allow customers to modify or remove it in their configuration
  (see Section 2).

  Note that a CDN that allows customers to remove or modify the
  CDN-Loop header field (i.e., they do not implement this
  specification) remains an attack vector against both implementing and
  non-implementing CDNs.

  A CDN's use of the CDN-Loop header field might expose its presence.
  For example, if CDN A is configured to forward its requests to CDN B
  for a given origin, CDN B's presence can be revealed if it behaves
  differently based upon the presence of the CDN-Loop header field.

  The CDN-Loop header field can be generated by any client, and
  therefore its contents cannot be trusted.  CDNs who modify their
  behavior based upon its contents should assure that this does not
  become an attack vector (e.g., for Denial of Service).



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  It is possible to sign the contents of the header field (either by
  putting the signature directly into the field's content or using
  another header field), but such use is not defined (or required) by
  this specification.

  Depending on how it is used, CDN-Loop can expose information about
  the internal configuration of the CDN; for example, the number of
  hops inside the CDN, and the hostnames of nodes.

4.  IANA Considerations

  This document registers the "CDN-Loop" header field in the "Permanent
  Message Header Field Names" registry.

  o  Header Field Name: CDN-Loop

  o  Protocol: http

  o  Status: standard

  o  Reference: RFC 8586

5.  References

5.1.  Normative References

  [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>.

  [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
             Specifications: ABNF", STD 68, RFC 5234,
             DOI 10.17487/RFC5234, January 2008,
             <https://www.rfc-editor.org/info/rfc5234>.

  [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
             Protocol (HTTP/1.1): Message Syntax and Routing",
             RFC 7230, DOI 10.17487/RFC7230, June 2014,
             <https://www.rfc-editor.org/info/rfc7230>.

  [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
             Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
             DOI 10.17487/RFC7231, June 2014,
             <https://www.rfc-editor.org/info/rfc7231>.






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  [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>.

5.2.  Informative References

  [loop-attack]
             Chen, J., Jiang, J., Zheng, X., Duan, H., Liang, J., Li,
             K., Wan, T., and V. Paxson, "Forwarding-Loop Attacks in
             Content Delivery Networks", February 2016,
             <http://www.icir.org/vern/papers/cdn-loops.NDSS16.pdf>.

  [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
             Unique IDentifier (UUID) URN Namespace", RFC 4122,
             DOI 10.17487/RFC4122, July 2005,
             <https://www.rfc-editor.org/info/rfc4122>.

Authors' Addresses

  Stephen Ludin
  Akamai Technologies

  Email: [email protected]


  Mark Nottingham
  Fastly

  Email: [email protected]


  Nick Sullivan
  Cloudflare

  Email: [email protected]
















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