CDN-Loop: Loop Detection in Content Delivery Networks (CDNs)
A request header that lets a CDN notice when a request has come back around to it, so loops can be stopped.
Ludin, S., Nottingham, M., and N. Sullivan, "Loop Detection in Content Delivery Networks (CDNs)", RFC 8586, DOI 10.17487/RFC8586, April 2019, <https://www.rfc-editor.org/info/rfc8586>.
@misc{rfc8586,
series = {Request for Comments},
number = 8586,
howpublished = {RFC 8586},
publisher = {RFC Editor},
doi = {10.17487/RFC8586},
url = {https://www.rfc-editor.org/info/rfc8586},
author = {S. Ludin and M. Nottingham and N. Sullivan},
title = {{Loop Detection in Content Delivery Networks (CDNs)}},
pagetotal = 6,
year = 2019,
month = apr,
} RFC text
Contents
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.
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."
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).
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).
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>.
[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: sludin@akamai.com
Mark Nottingham
Fastly
Email: mnot@fastly.com
Nick Sullivan
Cloudflare
Email: nick@cloudflare.com
AI ELI5
On this page
§1Why CDNs Can Loop
A Content Delivery Network (CDN) sits in front of an origin server to cut latency and cost and to improve scalability and reliability. Often one origin uses more than one CDN, for cost savings, for failover, or to compare the services directly.
Because routing depends on a mix of DNS and forwarding rules, often managed by several parties, CDNs can end up forwarding to each other in a loop by accident. Such loops are hard to debug. They are also sometimes deliberate: the RFC notes that loops between CDNs can be used as an attack vector, especially if one CDN unintentionally strips another CDN's loop detection headers.
RFC 8586 defines the CDN-Loop HTTP request header field to detect these cases among CDNs that implement it. Customers must not be able to modify the header field.
Imagine two mail carriers who each think the other should deliver a letter, so it goes back and forth forever. Sometimes that happens by mistake, and sometimes someone does it on purpose to cause trouble.
§1.1Why Not Use Via
HTTP already has a Via header field, defined in RFC 7230, for tracking message forwards, avoiding request loops and identifying the capabilities of senders along the chain. In theory it could find these loops.
In practice it is not used that way. Some HTTP servers use Via for other purposes, and in particular some implementations disable some HTTP/1.1 features when the header is present. A dedicated header avoids that problem.
There was already a general sign-in sheet for packages, but some offices change how they work when they see it. So a separate, special stamp is safer.
§2The CDN-Loop Header Syntax
The header is a comma-separated list of cdn-info entries. Each entry is a cdn-id, optionally followed by semicolon-separated key/value parameters for the CDN's own use. The cdn-id is either a hostname (with an optional port) or a pseudonym, which is an HTTP token.
Hostnames under the CDN's control are preferred because they help avoid accidental collisions. A pseudonym is more likely to collide, so it should be chosen carefully: a well-known value such as the recognized name of the CDN, or a generated value with enough entropy, such as a UUID. The pseudonym syntax does not allow whitespace, double quotes or delimiter characters such as parentheses, commas, slashes, colons, semicolons and brackets.
As with other headers defined as comma-separated lists, a CDN can add its entry by comma-separating onto an existing value or by creating a new header field line.
Each company signs the package with its own name, or a secret nickname if it prefers. The name has to be unusual enough that nobody else uses the same one.
§2.1What a CDN Does With It
Conforming CDNs SHOULD add a cdn-info to this header field in all requests they generate or forward, creating the header field if necessary. A CDN that receives a request already carrying its own identifier can conclude that the request has passed through its servers before, which means a loop.
The mechanism only works if all intermediaries preserve the header. Removing it, or letting a customer remove it through configuration, stops downstream CDNs from detecting looping. Unknown header fields are generally left alone by intermediaries, but implementations may need to add CDN-Loop to a list of fields that are never removed. The header field SHOULD NOT be used for other purposes.
Every company stamps each package it handles. If a company sees its own stamp already on a package that just arrived, it knows the package has gone in a circle and can stop it.
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"§3Security Considerations
The threat model is a customer attacking a service provider by configuring a forwarding loop, by accident or on purpose. For the defense to work, CDNs cannot let customers modify or remove the header in their configuration. A CDN that does allow this remains an attack vector against both implementing and non-implementing CDNs.
There are other cautions. Using the header might reveal that a CDN is present, for example if CDN B behaves differently when it sees the header from CDN A. Any client can generate the header, so its contents cannot be trusted, and a CDN that changes its behavior based on the contents must make sure that does not become an attack vector such as Denial of Service. Signing the contents is possible but not defined or required. Depending on use, the header can also expose internal details such as the number of hops inside a CDN and the hostnames of nodes.
The stamp only helps if customers cannot erase it. Also, anyone can fake a stamp, and a stamp can show which companies were involved, so companies should be careful what they trust and what they reveal.
§4IANA Registration
The document registers the CDN-Loop header field in the Permanent Message Header Field Names registry. The protocol is http, the status is standard, and the reference is RFC 8586.
The new stamp was added to the official list of header names so nobody else picks the same name for something different.