HTTP Working GroupS. Ludin
Internet-DraftAkamai Technologies
Intended status: Standards TrackM. Nottingham
Expires: April 27, 2019Fastly
N. Sullivan
October 24, 2018

CDN Loop Prevention


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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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 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].

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.

5. References

5.2. Informative References

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, <>.

Authors' Addresses

Stephen Ludin
Akamai Technologies
Mark Nottingham
Nick Sullivan