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Internet Engineering Task Force (IETF)                          S. LudinRequest for Comments: 8586                           Akamai TechnologiesCategory: Standards Track                                  M. NottinghamISSN: 2070-1721                                                   Fastly                                                             N. Sullivan                                                              Cloudflare                                                              April 2019Loop 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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://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 toBCP 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.Ludin, et al.                Standards Track                    [Page 1]

RFC 8586                   CDN Loop Detection                 April 2019Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Relationship to Via . . . . . . . . . . . . . . . . . . .21.2.  Conventions and Definitions . . . . . . . . . . . . . . .32.  The CDN-Loop Request Header Field . . . . . . . . . . . . . .33.  Security Considerations . . . . . . . . . . . . . . . . . . .44.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .55.  References  . . . . . . . . . . . . . . . . . . . . . . . . .55.1.  Normative References  . . . . . . . . . . . . . . . . . .55.2.  Informative References  . . . . . . . . . . . . . . . . .6   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .61.  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 inSection 5.7.1 of [RFC7230] for   "tracking message forwards, avoiding request loops, and identifying   the protocol capabilities of senders along the request/response   chain."Ludin, et al.                Standards Track                    [Page 2]

RFC 8586                   CDN Loop Detection                 April 2019   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 inBCP 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 inSection 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).Ludin, et al.                Standards Track                    [Page 3]

RFC 8586                   CDN Loop Detection                 April 2019   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 "(),/:;<=>?@[]{}".  SeeSection 3.2.6 of   [RFC7230].  Likewise, note the rules for when parameter values need   to be quoted inSection 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   (seeSection 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).Ludin, et al.                Standards Track                    [Page 4]

RFC 8586                   CDN Loop Detection                 April 2019   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 85865.  References5.1.  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>.Ludin, et al.                Standards Track                    [Page 5]

RFC 8586                   CDN Loop Detection                 April 2019   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase inRFC2119 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: sludin@akamai.com   Mark Nottingham   Fastly   Email: mnot@fastly.com   Nick Sullivan   Cloudflare   Email: nick@cloudflare.comLudin, et al.                Standards Track                    [Page 6]