HTTP Working Group S. Ludin
Internet-Draft Akamai Technologies
Intended status: Standards Track M. Nottingham
Expires: April 27, 2019 Fastly
N. Sullivan
Cloudflare
October 24, 2018
CDN Loop Prevention
draft-ietf-httpbis-cdn-loop-01
Abstract
This specification defines the CDN-Loop request header field for
HTTP.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 27, 2019.
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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 . . . . . . . . . . . . . . . . . . . . . 4
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . 5
5.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
In modern deployments of HTTP servers, it is common to interpose
Content Delivery Networks (CDNs) in front of origin servers to
improve end-user perceived latency, 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 to directly compare their
services.
As a result, it is not unknown 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 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 request header field for HTTP
to enable secure interoperability of forwarding CDNs. Having a
header that is guaranteed not to be modified by other CDNs that are
used by a shared customer helps give each CDN additional confidence
that any purpose (debugging, data gathering, enforcement) that they
use this header for is free from tampering due to how that customer
configured the other CDNs.
1.1. Relationship to Via
HTTP defines the Via header field in [RFC7230], Section 5.7.1 for
"tracking message forwards, avoiding request loops, and identifying
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the protocol capabilities of senders along the request/response
chain."
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 the OWS rule 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 prevent loops.
CDN-Loop = #cdn-id
cdn-id = token *( OWS ";" OWS parameter )
Conforming Content Delivery Networks SHOULD add a value to this
header field to all requests they generate or forward (creating the
header if necessary).
The token identifies the CDN as a whole. Chosen token values SHOULD
be unique enough that a collision with other CDNs is unlikely.
Optionally, the token can have semicolon-separated key/value
parameters, to accommodate additional information for the CDN's use.
As with all HTTP headers defined using the "#" rule, the CDN-Loop
header can be added to by comma-separating values, or by creating a
new header field with the desired value.
For example:
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CDN-Loop: FooCDN, barcdn; host="foo123.bar.cdn"
CDN-Loop: baz-cdn; abc="123"; def="456", anotherCDN
Note that the token syntax does not allow whitespace, DQUOTE or any
of the characters "(),/:;<=>?@[]{}". See [RFC7230], Section 3.2.6.
Likewise, note the rules for when parameter values need to be quoted
in [RFC7231], Section 3.1.1.
To be effective, intermediaries - including Content Delivery Networks
- MUST NOT remove this header field, or allow it to be removed (e.g.,
through configuration) and servers (including intermediaries) SHOULD
NOT use it for other purposes.
3. Security Considerations
The threat model that the CDN-Loop header field addresses is a
customer who is attempting to attack a service provider by
configuring a forwarding loop by accident or malice. For it to
function, CDNs cannot allow it to be modified by customers (see
Section 2).
The CDN-Loop header field can be generated by any client, and
therefore its contents cannot be trusted. CDNs who modify their
behaviour based upon its contents should assure that this does not
become an attack vector (e.g., for Denial-of-Service).
It is possible to sign the contents of the header (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.
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: (this document)
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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>.
[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", ISBN 1-891562-41-X,
DOI 10.14722/ndss.2016.23442, February 2016,
<http://www.icir.org/vern/papers/cdn-loops.NDSS16.pdf>.
Authors' Addresses
Stephen Ludin
Akamai Technologies
Email: sludin@akamai.com
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Mark Nottingham
Fastly
Email: mnot@fastly.com
Nick Sullivan
Cloudflare
Email: nick@cloudflare.com
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