< draft-ietf-httpbis-cdn-loop-01.txt   draft-ietf-httpbis-cdn-loop-02.txt >
HTTP Working Group S. Ludin HTTP Working Group S. Ludin
Internet-Draft Akamai Technologies Internet-Draft Akamai Technologies
Intended status: Standards Track M. Nottingham Intended status: Standards Track M. Nottingham
Expires: April 27, 2019 Fastly Expires: August 8, 2019 Fastly
N. Sullivan N. Sullivan
Cloudflare Cloudflare
October 24, 2018 February 04, 2019
CDN Loop Prevention CDN Loop Detection
draft-ietf-httpbis-cdn-loop-01 draft-ietf-httpbis-cdn-loop-02
Abstract Abstract
This specification defines the CDN-Loop request header field for This document defines the CDN-Loop request header field for HTTP.
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 Status of This Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Relationship to Via . . . . . . . . . . . . . . . . . . . 2 1.1. Relationship to Via . . . . . . . . . . . . . . . . . . . 3
1.2. Conventions and Definitions . . . . . . . . . . . . . . . 3 1.2. Conventions and Definitions . . . . . . . . . . . . . . . 3
2. The CDN-Loop Request Header Field . . . . . . . . . . . . . . 3 2. The CDN-Loop Request Header Field . . . . . . . . . . . . . . 3
3. Security Considerations . . . . . . . . . . . . . . . . . . . 4 3. Security Considerations . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . 5 5.1. Normative References . . . . . . . . . . . . . . . . . . 5
5.2. Informative References . . . . . . . . . . . . . . . . . 5 5.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction 1. Introduction
In modern deployments of HTTP servers, it is common to interpose In modern deployments of HTTP servers, it is common to interpose
Content Delivery Networks (CDNs) in front of origin servers to Content Delivery Networks (CDNs) in front of origin servers to
improve end-user perceived latency, reduce operational costs, and improve end-user perceived latency, reduce operational costs, and
improve scalability and reliability of services. improve scalability and reliability of services.
Often, more than one CDN is in use by a given origin. This happens 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 for a variety of reasons, such as cost savings, arranging for
failover should one CDN have issues, or to directly compare their failover should one CDN have issues, or to directly compare their
services. services.
As a result, it is not unknown for forwarding CDNs to be configured As a result, it is not unknown for forwarding CDNs to be configured
in a "loop" accidentally; because routing is achieved through a in a "loop" accidentally; because routing is achieved through a
combination of DNS and forwarding rules, and site configurations are combination of DNS and forwarding rules, and site configurations are
sometimes complex and managed by several parties. sometimes complex and managed by several parties.
When this happens, it is difficult to debug. Additionally, it When this happens, it is difficult to debug. Additionally, it
sometimes isn't accidental; loops between multiple CDNs be used as an sometimes isn't accidental; loops between multiple CDNs can be used
attack vector (e.g., see [loop-attack]), especially if one CDN as an attack vector (e.g., see [loop-attack]), especially if one CDN
unintentionally strips the loop detection headers of another. unintentionally strips the loop detection headers of another.
This specification defines the CDN-Loop request header field for HTTP This specification defines the CDN-Loop HTTP request header field to
to enable secure interoperability of forwarding CDNs. Having a help detect such attacks and accidents among implementing forwarding
header that is guaranteed not to be modified by other CDNs that are CDNs, by disallowing its modification by their customers.
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 1.1. Relationship to Via
HTTP defines the Via header field in [RFC7230], Section 5.7.1 for HTTP defines the Via header field in [RFC7230], Section 5.7.1 for
"tracking message forwards, avoiding request loops, and identifying "tracking message forwards, avoiding request loops, and identifying
the protocol capabilities of senders along the request/response the protocol capabilities of senders along the request/response
chain." chain."
In theory, Via could be used to identify these loops. However, in In theory, Via could be used to identify these loops. However, in
practice it is not used in this fashion, because some HTTP servers practice it is not used in this fashion, because some HTTP servers
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
This specification uses the Augmented Backus-Naur Form (ABNF) This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with a list extension, defined in Section 7 of notation of [RFC5234] with a list extension, defined in Section 7 of
[RFC7230], that allows for compact definition of comma-separated [RFC7230], that allows for compact definition of comma-separated
lists using a '#' operator (similar to how the '*' operator indicates lists using a '#' operator (similar to how the '*' operator indicates
repetition). Additionally, it uses the OWS rule from [RFC7230] and repetition). Additionally, it uses the token, OWS, uri-host and port
the parameter rule from [RFC7231]. rules from [RFC7230] and the parameter rule from [RFC7231].
2. The CDN-Loop Request Header Field 2. The CDN-Loop Request Header Field
The CDN-Loop request header field is intended to help a Content The CDN-Loop request header field is intended to help a Content
Delivery Network identify when an incoming request has already passed Delivery Network identify when an incoming request has already passed
through that CDN's servers, to prevent loops. through that CDN's servers, to detect loops.
CDN-Loop = #cdn-id CDN-Loop = #cdn-info
cdn-id = token *( OWS ";" OWS parameter ) cdn-info = cdn-id *( OWS ";" OWS parameter )
cdn-id = ( uri-host [ ":" port ] ) / pseudonym
pseudonym = token
Conforming Content Delivery Networks SHOULD add a value to this The cdn-id identifies the CDN using either a hostname under its
header field to all requests they generate or forward (creating the control or a pseudonym. Hostnames are preferred, to help avoid
header if necessary). 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]).
The token identifies the CDN as a whole. Chosen token values SHOULD Optionally, cdn-info can have semicolon-separated key/value
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. parameters, to accommodate additional information for the CDN's use.
As with all HTTP headers defined using the "#" rule, the CDN-Loop Conforming Content Delivery Networks SHOULD add a cdn-info to this
header can be added to by comma-separating values, or by creating a header field in all requests they generate or forward (creating the
new header field with the desired value. 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: For example:
CDN-Loop: FooCDN, barcdn; host="foo123.bar.cdn" GET /image.jpg HTTP/1.1
CDN-Loop: baz-cdn; abc="123"; def="456", anotherCDN 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 token syntax does not allow whitespace, DQUOTE or any Note that the pseudonym syntax does not allow whitespace, DQUOTE or
of the characters "(),/:;<=>?@[]{}". See [RFC7230], Section 3.2.6. any of the characters "(),/:;<=>?@[]{}". See [RFC7230],
Likewise, note the rules for when parameter values need to be quoted Section 3.2.6. Likewise, note the rules for when parameter values
in [RFC7231], Section 3.1.1. need to be quoted in [RFC7231], Section 3.1.1.
To be effective, intermediaries - including Content Delivery Networks The effectiveness of this mechanism relies on all intermediaries
- MUST NOT remove this header field, or allow it to be removed (e.g., preserving the header field, since removing (or allowing it to be
through configuration) and servers (including intermediaries) SHOULD removed, e.g., by customer configuration) would prevent downstream
NOT use it for other purposes. CDNs from using it to detect looping. In general, unknown header
fields are not removed by intermediaries, but there may be 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 3. Security Considerations
The threat model that the CDN-Loop header field addresses is a The threat model that the CDN-Loop header field addresses is a
customer who is attempting to attack a service provider by customer who is attacking a service provider by configuring a
configuring a forwarding loop by accident or malice. For it to forwarding loop by accident or malice. For it to function, CDNs
function, CDNs cannot allow it to be modified by customers (see cannot allow customers to modify or remove it in their configuration
Section 2). (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 The CDN-Loop header field can be generated by any client, and
therefore its contents cannot be trusted. CDNs who modify their therefore its contents cannot be trusted. CDNs who modify their
behaviour based upon its contents should assure that this does not behaviour based upon its contents should assure that this does not
become an attack vector (e.g., for Denial-of-Service). become an attack vector (e.g., for Denial-of-Service).
It is possible to sign the contents of the header (either by putting It is possible to sign the contents of the header field (either by
the signature directly into the field's content, or using another putting the signature directly into the field's content, or using
header field), but such use is not defined (or required) by this another header field), but such use is not defined (or required) by
specification. 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 4. IANA Considerations
This document registers the "CDN-Loop" header field in the Permanent This document registers the "CDN-Loop" header field in the Permanent
Message Header Field Names registry. Message Header Field Names registry.
o Header Field Name: CDN-Loop o Header Field Name: CDN-Loop
o Protocol: http o Protocol: http
skipping to change at page 5, line 38 skipping to change at page 6, line 24
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
5.2. Informative References 5.2. Informative References
[loop-attack] [loop-attack]
Chen, J., Jiang, J., Zheng, X., Duan, H., Liang, J., Li, Chen, J., Jiang, J., Zheng, X., Duan, H., Liang, J., Li,
K., Wan, T., and V. Paxson, "Forwarding-Loop Attacks in K., Wan, T., and V. Paxson, "Forwarding-Loop Attacks in
Content Delivery Networks", ISBN 1-891562-41-X, Content Delivery Networks", DOI 10.14722/ndss.2016.23442,
DOI 10.14722/ndss.2016.23442, February 2016, February 2016,
<http://www.icir.org/vern/papers/cdn-loops.NDSS16.pdf>. <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 Authors' Addresses
Stephen Ludin Stephen Ludin
Akamai Technologies Akamai Technologies
Email: sludin@akamai.com Email: sludin@akamai.com
Mark Nottingham Mark Nottingham
Fastly Fastly
Email: mnot@fastly.com Email: mnot@fastly.com
Nick Sullivan Nick Sullivan
Cloudflare Cloudflare
Email: nick@cloudflare.com Email: nick@cloudflare.com
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