draft-ietf-httpbis-cdn-loop-02.txt   rfc8586.txt 
HTTP Working Group S. Ludin Internet Engineering Task Force (IETF) S. Ludin
Internet-Draft Akamai Technologies Request for Comments: 8586 Akamai Technologies
Intended status: Standards Track M. Nottingham Category: Standards Track M. Nottingham
Expires: August 8, 2019 Fastly ISSN: 2070-1721 Fastly
N. Sullivan N. Sullivan
Cloudflare Cloudflare
February 04, 2019 April 2019
CDN Loop Detection Loop Detection in Content Delivery Networks (CDNs)
draft-ietf-httpbis-cdn-loop-02
Abstract Abstract
This document defines the CDN-Loop request header field for HTTP. This document defines the CDN-Loop request header field for HTTP.
CDN-Loop addresses an operational need that occurs when an HTTP CDN-Loop addresses an operational need that occurs when an HTTP
request is intentionally forwarded between Content Delivery Networks request is intentionally forwarded between Content Delivery Networks
(CDNs), but is then accidentally or maliciously re-routed back into (CDNs), but is then accidentally or maliciously re-routed back into
the original CDN causing a non-terminating loop. The new header the original CDN causing a non-terminating loop. The new header
field can be used to identify the error and terminate the loop. 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.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Relationship to Via . . . . . . . . . . . . . . . . . . . 3 1.1. Relationship to Via . . . . . . . . . . . . . . . . . . . 2
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 . . . . . . . . . . . . . . . . . . . . . 5 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . 5 5.1. Normative References . . . . . . . . . . . . . . . . . . 5
5.2. Informative References . . . . . . . . . . . . . . . . . 6 5.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 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 latency perceived by end users, 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 direct comparison of the
services. CDNs' services.
As a result, it is not unknown for forwarding CDNs to be configured As a result, it is possible for forwarding CDNs to be configured in a
in a "loop" accidentally; because routing is achieved through 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 can be used sometimes isn't accidental; loops between multiple CDNs can be used
as an 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 HTTP request header field to This specification defines the CDN-Loop HTTP request header field to
help detect such attacks and accidents among implementing forwarding help detect such attacks and accidents among forwarding CDNs that
CDNs, by disallowing its modification by their customers. have implemented it; the header field may not be modified by their
customers.
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 Section 5.7.1 of [RFC7230] 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
use Via for other purposes - in particular, some implementations use Via for other purposes -- in particular, some implementations
disable some HTTP/1.1 features when the Via header is present. disable some HTTP/1.1 features when the Via header is present.
1.2. Conventions and Definitions 1.2. Conventions and Definitions
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
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 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 token, OWS, uri-host and port repetition). Additionally, it uses a token (OWS), uri-host, and port
rules from [RFC7230] and 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 detect loops. through that CDN's servers to detect loops.
CDN-Loop = #cdn-info CDN-Loop = #cdn-info
cdn-info = cdn-id *( OWS ";" OWS parameter ) cdn-info = cdn-id *( OWS ";" OWS parameter )
cdn-id = ( uri-host [ ":" port ] ) / pseudonym cdn-id = ( uri-host [ ":" port ] ) / pseudonym
pseudonym = token pseudonym = token
The cdn-id identifies the CDN using either a hostname under its The cdn-id identifies the CDN using either a hostname under its
control or a pseudonym. Hostnames are preferred, to help avoid control or a pseudonym. Hostnames are preferred, to help avoid
accidental collisions. If a pseudonym is used, unintentional accidental collisions. If a pseudonym is used, unintentional
collisions are more likely, and therefore values should be carefully collisions are more likely, and therefore values should be carefully
chosen to prevent them; for example, using a well-known value (such 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 as the recognized name of the CDN in question), or a generated value
with enough entropy to make collisions unlikely (such as a UUID with enough entropy to make collisions unlikely (such as a UUID
[RFC4122]). [RFC4122]).
Optionally, cdn-info can have semicolon-separated key/value Optionally, cdn-info 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.
Conforming Content Delivery Networks SHOULD add a cdn-info to this Conforming Content Delivery Networks SHOULD add a cdn-info to this
header field in all requests they generate or forward (creating the header field in all requests they generate or forward (creating the
header field if necessary). header field if necessary).
As with all HTTP header fields defined using the "#" rule, the CDN- As with all HTTP header fields defined using the "#" rule, the
Loop header field can be added to by comma-separating values, or by CDN-Loop header field can be added to by comma-separating values, or
creating a new header field with the desired value. by creating a new header field with the desired value.
For example: For example:
GET /image.jpg HTTP/1.1 GET /image.jpg HTTP/1.1
Host: cdn-customer.example Host: cdn-customer.example
User-Agent: ExampleBrowser/5 User-Agent: ExampleBrowser/5
CDN-Loop: foo123.foocdn.example, barcdn.example; trace="abcdef" CDN-Loop: foo123.foocdn.example, barcdn.example; trace="abcdef"
CDN-Loop: AnotherCDN; abc=123; def="456" CDN-Loop: AnotherCDN; abc=123; def="456"
Note that the pseudonym syntax does not allow whitespace, DQUOTE or Note that the pseudonym syntax does not allow whitespace, DQUOTE, or
any of the characters "(),/:;<=>?@[]{}". See [RFC7230], any of the characters "(),/:;<=>?@[]{}". See Section 3.2.6 of
Section 3.2.6. Likewise, note the rules for when parameter values [RFC7230]. Likewise, note the rules for when parameter values need
need to be quoted in [RFC7231], Section 3.1.1. to be quoted in Section 3.1.1 of [RFC7231].
The effectiveness of this mechanism relies on all intermediaries The effectiveness of this mechanism relies on all intermediaries
preserving the header field, since removing (or allowing it to be preserving the header field, since removing (or allowing it to be
removed, e.g., by customer configuration) would prevent downstream removed, e.g., by customer configuration) would prevent downstream
CDNs from using it to detect looping. In general, unknown header CDNs from using it to detect looping. In general, unknown header
fields are not removed by intermediaries, but there may be need to 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 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 to be removed under any circumstances. The header field SHOULD
NOT be used for other purposes. 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 attacking a service provider by configuring a customer who is attacking a service provider by configuring a
forwarding loop by accident or malice. For it to function, CDNs forwarding loop by accident or malice. For it to function, CDNs
cannot allow customers to modify or remove it in their configuration cannot allow customers to modify or remove it in their configuration
(see Section 2). (see Section 2).
Note that a CDN that allows customers to remove or modify the CDN- Note that a CDN that allows customers to remove or modify the
Loop header field (i.e., they do not implement this specification) CDN-Loop header field (i.e., they do not implement this
remains an attack vector against both implementing and non- specification) remains an attack vector against both implementing and
implementing CDNs. non-implementing CDNs.
A CDN's use of the CDN-Loop header field might expose its presence. 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 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 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. 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 behavior 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 field (either by It is possible to sign the contents of the header field (either by
putting the signature directly into the field's content, or using putting the signature directly into the field's content or using
another header field), but such use is not defined (or required) by another header field), but such use is not defined (or required) by
this specification. this specification.
Depending on how it is used, CDN-Loop can expose information about Depending on how it is used, CDN-Loop can expose information about
the internal configuration of the CDN; for example, the number of the internal configuration of the CDN; for example, the number of
hops inside the CDN, and the hostnames of nodes. 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
o Status: standard o Status: standard
o Reference: (this document) o Reference: RFC 8586
5. References 5. References
5.1. Normative References 5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
skipping to change at page 6, line 24 skipping to change at page 6, line 14
[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", DOI 10.14722/ndss.2016.23442, Content Delivery Networks", 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 [RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122, Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005, DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/info/rfc4122>. <https://www.rfc-editor.org/info/rfc4122>.
Authors' Addresses Authors' Addresses
Stephen Ludin Stephen Ludin
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