Oblivious Relay Feedback
draft-rdb-ohai-feedback-to-proxy-05
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draft-rdb-ohai-feedback-to-proxy-05
ohai T. Reddy
Internet-Draft Nokia
Intended status: Standards Track D. Wing
Expires: 30 March 2023 Citrix
M. Boucadair
Orange
R. Polli
Team Digitale, Italian Government
26 September 2022
Oblivious Relay Feedback
draft-rdb-ohai-feedback-to-proxy-05
Abstract
To provide equitable service to clients, servers often rate-limit
incoming requests, for example, based upon the source IP address.
However, oblivious HTTP removes the ability for the server to
distinguish amongst clients so the server can only rate-limit traffic
from the oblivious relay. This harms all clients behind that
oblivious relay.
This specification enables a server to convey rate-limit information
to an oblivious relay, which can use it to apply rate-limit policies
on clients. Cooperating oblivious relays can thus provide more
equitable service to their distinguishable clients without impacting
on all clients behind that oblivious relay.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 30 March 2023.
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Copyright Notice
Copyright (c) 2022 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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Providing RateLimit Information to an Oblivious Proxy . . . . 4
4. The ohttp-target Quota Policy Parameter . . . . . . . . . . . 4
4.1. ohttp-target Parameter . . . . . . . . . . . . . . . . . 4
4.2. Processing the ohttp-target Parameter . . . . . . . . . . 5
5. Prevent Client identification . . . . . . . . . . . . . . . . 6
6. The attack-severity Quota Policy Parameter . . . . . . . . . 7
7. Use of The ohttp-target Quota Policy Parameters: An
Example . . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Ohttp-Outside-Encap Header . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9.1. Client and Oblivous Proxy Collusion . . . . . . . . . . . 11
9.2. Attack Categories . . . . . . . . . . . . . . . . . . . . 11
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10.1. RateLimit Parameter Value Registrations . . . . . . . . 12
10.2. Registration of new HTTP Header Field . . . . . . . . . 12
10.2.1. Ohttp-Outside-Encap Header . . . . . . . . . . . . . 12
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
12.1. Normative References . . . . . . . . . . . . . . . . . . 13
12.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
Oblivious HTTP [OHTTP] requires three parties to exchange HTTP
messages: the client, the relay, and the target (formally, the
Oblivious Gateway Resource and Oblivious Target Resource). Oblivious
HTTP enables a client to send requests to a target in such a way that
the target cannot tell whether two requests came from the same
client, and the relay cannot see the contents of the requests.
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Since clients are located behind a relay, a target cannot distinguish
between well-behaving and malicious clients: an unexpected behavior
from one or more clients can then impact on all the intermediated
clients, as described in Section 8.2.1 of [OHTTP]. This can be
problematic when the target implements rate limiting policies based
on an information masked by the intermediary, such as the source IP
address.
This document defines a mechanism that allows Oblivious gateway and
target resource to provide rate-limit information to an Oblivious
relay via the RateLimit fields defined in [RATELIMIT]. This is
useful when such servers identify traffic anomalies or unexpected
request volumes. The Oblivious relay can then use this information
to apply rate-limit policies on clients.
While [RATELIMIT] provides enough information to generic clients to
shape their request policy and avoid being throttled out, this
specification allows an Oblivious gateway and target resource to
indicate their RateLimit information is intended for the Oblivious
relay (rather than to the client).
How an Oblivious relay can use this information to avoid being
throttled out or shape its request policy is outside the scope of
this specification.
2. Terminology
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.
The terms "content", "receiver", "request", and "response" are to be
interpreted as described in [HTTP].
The terms "Encapsulated request", "Encapsulated response", "Oblivious
relay resource", "Oblivious gateway resource", "Oblivious target
resource", and "Client" are to be interpreted as described in
[OHTTP].
The collective term "Oblivious resource" indicates either an
"Oblivious gateway resource" or an "Oblivious target resource".
The terms "quota policy", "service limit", "expiring limit", and
"RateLimit fields" are to be interpreted as described in [RATELIMIT].
This document uses the Integer type from [STRUCTURED-FIELDS].
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3. Providing RateLimit Information to an Oblivious Proxy
An Oblivious resource that uses RateLimit fields [RATELIMIT] to
return service limit information MAY add the "ohttp-target" quota
policy parameter defined in Section 4 to signal to the receiver that
the associated quota policy is intended for an Oblivious relay. For
example, when an Oblivious target identifies a high frequency or high
volume anomalies in the HTTP requests it would include the "ohttp-
target" parameter.
The term "Oblivious Relay Feedback" denotes both the mechanism
described in this specification and the complete set of RateLimit
fields together with the "ohttp-target" parameter.
To know whether the RateLimit fields provides Oblivious Relay
Feedback (see Section 3.1), an Oblivious relay MUST:
1. Identify the quota policy associated to the expiring limit.
2. Check whether the "ohttp-target" parameter is present and its
syntax is correct.
In the example shown in Figure 1, the expiring limit value is "100",
so the associated quota policy is the second one. This quota policy
includes the "ohttp-target" parameter: this indicates that the
RateLimit fields are intended for an Oblivious relay.
RateLimit-Limit: 100
RateLimit-Policy: 10;w=1, 100;w=60;ohttp-target=1
RateLimit-Remaining: 8
RateLimit-Reset: 15
Figure 1: An Example of Oblivious Proxy Feedback.
4. The ohttp-target Quota Policy Parameter
4.1. ohttp-target Parameter
The following quota policy parameter is defined for the RateLimit-
Policy field [RATELIMIT]:
ohttp-target: Indicates that the associated quota policy provides
Oblivious Relay Feedback. This parameter is OPTIONAL.
The "ohttp-target" parameter has the following syntax:
ohttp-target = sf-integer
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Its value MUST be an Integer (Section 3.3.1 of [STRUCTURED-FIELDS])
and indicates whether the quota policy is applicable to all the
clients that are serviced by the Oblivious relay or applicable only
to a specific client. The "ohttp-target" parameter MUST have one of
the following values:
1: Indicates that RateLimit fields are applicable to all the clients
that are serviced by the same Oblivious relay.
2: Indicates that RateLimit fields are applicable only to the
offending client. For example, this value is used if the client
is attacking the server (e.g., the client is using an abnormal
header that matches an attack pattern). The mechanisms Oblivious
relay can use to rate-limit the traffic from the client but at the
same time prevent the target to correlate requests from clients
are discussed in Section 5.
Other values MUST cause the parameter to be ignored.
The "ohttp-target" parameter MUST NOT appear more than once in a
quota policy. If the parameter is malformed or its value is invalid,
it MUST be ignored, and the receiving Oblivious relay MUST NOT
attempt to fix neither the parameter nor its value. That is, the
RateLimit fields must not be considered as providing Oblivious Relay
Feedback.
4.2. Processing the ohttp-target Parameter
An Oblivious relay receiving RateLimit fields providing Oblivious
Relay Feedback will do the following:
1. It MUST remove the RateLimit fields from the response, since they
are not intended to be forwarded to clients.
2. It MAY add a new set of RateLimit fields that are intended to be
forwarded to a client.
An Oblivious gateway resource receiving RateLimit fields providing
Oblivious Relay Feedback MUST proceed as follows:
1. Remove the RateLimit fields from the HTTP response, since they
are not intended to be forwarded to the client. It, then,
encapsulates the HTTP response.
2. Add the above RateLimit fields to the response containing the
encapsulated response sent to the Oblivious relay, so that the
Oblivious relay can access them.
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If the RateLimit fields along with the "ohttp-target" parameter are
generated by the Oblivious gateway resource before removing the
protection (including being unable to remove the encapsulation for
any reason)(Section 6.2 of [OHTTP]), it will result in the RateLimit
fields added in the response being sent without protection in
response to a POST request from a client.
While this specification does not mandate specific traffic shaping
actions for Oblivious proxies in addition to the ones indicated in
[RATELIMIT], an Oblivious relay failing to reshape traffic from a
specific client or from all the clients according to the received
Oblivious Relay Feedback can experience different levels of service
denial by the Oblivious gateway and target resources. There is no
explicit mechanism for an Oblivious relay to indicate to the server
that the rate-limit information was processed or was ignored.
5. Prevent Client identification
The following mechanisms can be used by an Oblivious relay to process
"ohttp-target" parameter with a value set to '2':
1. The Oblivious relay does not immediately act to rate-limit the
traffic from the client but starts maintaining a count of
responses to the client with "ohttp-target" parameter value set
to "2" marked as "potential malicious requests" and responses
without the parameter marked as "legitimate requests". The
Oblivious relay can rate-limit requests from the offending client
for a certain duration (that is implementation-specific) only
when the client has a high ratio of "potential malicious
requests" to "legitimate requests". In other words, the
Oblivious relay will rate-limit requests from a client if the
target has seen an attack pattern in multiple requests from that
same client. A malicious client sends malformed HTTP requests,
whereas a benign client sends valid HTTP requests. The malformed
HTTP requests are linkable whereas the valid HTTP requests are
not linkable. Most importantly, the target will not be able to
partition the anonymity set of legitimate clients.
2. The Oblivious relay enforces the delayed rate-limiting of the
offending client discussed in step 1 provided a large number of
clients are actively using the relay and the total number of
benign clients is significantly a large number compared to
offending clients (e.g., total clients is greater than hundred
thousand and benign clients is above eighty percent of the total
clients). This way the anonymity set of benign clients will be a
relatively larger number even after rate-limiting the offending
clients.
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3. The Oblivious relay acts to rate-limit the traffic of the client
provided it has a bad reputation score (e.g., many security
network security vendors offer client reputation score, for
instance, see [Reputation]).
4. The Oblivious relay provider hosts a honeypot of clients to send
good traffic to the server. The relay checks if it receives the
"ohttp-target" parameter set to '2' for these benign clients to
identify if the target is misbehaving. For example, if the
target is oblivious DoH server, these clients can send a mix of
queries to the top one million domains and sensitive domains
(e.g., Health, Ethnicity, Religion, Sexual Orientation, Political
Beliefs, Adult etc). If the Oblivious relay receives "ohttp-
target" parameter with value set to 2 for these clients, it would
identify the target is misbehaving and ignores the targeted rate-
limit for any other client it is serving.
5. The Oblivious relay can introduce synthetic encapsulated requests
to simulate equal distribution of sequence of requests to avoid
any request correlation by the target.
6. The attack-severity Quota Policy Parameter
The following quota policy parameter is defined for the RateLimit-
Policy field defined in [RATELIMIT]:
attack-severity: Is used by the Oblivious resource to convey the
likeliness that an HTTP request is malicious. This parameter is
OPTIONAL.
attack-severity = sf-string
Note that sf-string is defined in Section 3.3.3 of
[STRUCTURED-FIELDS].
The value of the "attack-severity" parameter is a String
(Section 3.3.3 of [RFC8941]) that takes one of the values defined in
[SEVERITY]. This parameter MUST NOT appear more than once in a quota
policy. If the parameter is malformed or its value is invalid, the
parameter MUST be ignored, and the relays MUST NOT attempt to fix
neither the parameter nor the value.
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7. Use of The ohttp-target Quota Policy Parameters: An Example
The example depicted in Figure 2 illustrates the use of the "ohttp-
target" parameter. An oblivious target resource receives a malformed
request and uses the source IP address to identify that it was an
encapsulated request decapsulated by an oblivious gateway resource.
The Oblivious target resource generates a 400 response and adds the
RateLimit fields along with the "ohttp-target" quota policy
parameter. The oblivious gateway resource proceeds as follows:
1. Copy the RateLimit fields from the original response.
2. Remove them from the original response before encapsulating it.
3. Generate a single 200 response containing the encapsulated
response for the oblivious relay resource along with the copied
RateLimit fields.
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+----+ +----------+ +----------+ +----------+
| C | | Relay | | Gateway | | Target |
| | | Resource | | Resource | | Resource |
+-+--+ +----+-----+ +-----+----+ +-----+----+
| | | |
| Encapsulated | | |
+------------------->| | |
| Request | | |
| | Encapsulated | |
| +------------------>| |
| | Request | |
| | | Request | .---------.
| | +-------------->| | Identify|
| | | +-+malformed|
| | | | | request |
| | | 400 response | '---------'
| | |<--------------+
| | | |
| | 200 response with | |
| | RateLimit-Limit, | |
| | RateLimit-Policy | |
| | fields and the | |
| | ohttp-target | |
| | parameter | |
|<------------------+ |
.--------------------. | Encapsulated 400 | |
| Process | | response | |
| ohttp-target +-+ | |
| and rate-limit | | | |
| requests from the | | | |
| offending client | | | |
'--------------------' | | |
| | |
| | | |
| Encapsulated 400 | | |
|<--------------------+ | |
| response | | |
| | | |
Figure 2: An Example of Ratelimit Feedback to Proxy
The response that is generated by the Oblivious gateway resource is
depicted in Figure 3. This response includes an unregistered,
informative "comment" quota policy parameter providing the rationale
for the "attack- severity".
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=============== NOTE: '\' line wrapping per RFC 8792 ================
HTTP/1.1 200 OK
Date: Wed, 27 March 2022 04:45:07 GMT
Cache-Control: private, no-store
RateLimit-Limit: 10
RateLimit-Policy: 10;ohttp-target=2;attack-severity="high";\
comment="abnormal header matching a WAF rule"
Content-Type: message/ohttp-res
Content-Length: 38 <content is the encapsulated 400 response>
...encrypted content...
Figure 3: Example of a Response
8. Ohttp-Outside-Encap Header
The "Ohttp-Outside-Encap" header is defined in this specification
(Section 10.2.1). Its purpose is to signal which HTTP headers will
be removed by the Oblivious gateway.
When an Oblivious gateway resource sends an HTTP request to an
Oblivious taget, it adds the "Ohttp-Outside-Encap" header to indicate
which headers will be removed from the response.
On receipt of an HTTP response from the Oblivious target resource,
the Oblivious gateway resource copies the header fields signaled in
the associated request and removes those headers from the HTTP
response. The Oblivious gateway then encapsulates the HTTP response.
The Oblivious gateway resource adds the copied header fields and
values to the response containing the encapsulated response, so that
the Oblivious relay can access and act on them.
The "Ohttp-Outside-Encap" header is useful in deployments where the
Oblivious gateway resource and Oblivious target resource are managed
by separate entities.
Figure 4 describes the syntax using Augmented Backus-Naur Form (ABNF)
of the header field, using the grammar defined in [RFC5234] and the
rules defined in Section 5 of [RFC9110]. The field values of the
header field conform to the same rules.
Ohai-Outside-Encap = header-field *( OWS "|" OWS header-field)
header-field = token
Figure 4: Ohttp-Outside-Encap Header Syntax
Optional white space (OWS) is used as defined in Section 5.6.3 of
[RFC9110].
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An example is illustrated below:
Ohttp-Outside-Encap: RateLimit-Limit|RateLimit-Remaining|RateLimit-Reset|RateLimit-Policy
9. Security Considerations
The security considerations for the Oblivious HTTP protocol
(Section 8 of [OHTTP]) as well as the ones for RateLimit fields
(Section 6 of [RATELIMIT]) apply. The following sub-sections discuss
security considerations specific to this specification.
9.1. Client and Oblivous Proxy Collusion
While Oblivious HTTP relies upon an Oblivious relay to prevent
leaking the client identity to the Oblivious resources, it might be
the case that the Oblivious relay colludes with clients in attacking
Oblivious resources. RateLimit fields might disclose operational
capacity information useful to design denial of service attacks or to
circumvent defensive measures put in place by the Oblivious resources
(Section 6.2 of [RATELIMIT]). The Oblivious target and gateway
resources SHOULD convey Oblivious Relay Feedback only to trusted
Oblivious proxies.
9.2. Attack Categories
Attacks against the Oblivious Gateway and Target Resources can be
classified into three primary categories:
1. A client deliberately sends a malformed encapsulated request
causing decryption failure or decryption overload failure on the
oblivious gateway resource. This causes the oblivious gateway
resource to send an error status code back to the oblivious
relay.
2. A client deliberately sends an HTTP request that causes an HTTP
error on the oblivious target resource. This might be a
malformed HTTP request, or request for a missing resource.
3. A botnet performing an application layer denial of service attack
(e.g. HTTP flood) against an Oblivious resource. Because each
bot in a botnet makes seemingly legitimate network requests the
traffic may appear "normal" in origin, nonetheless as a whole it
not only can saturate the Oblivious resources, but also makes
appear the Oblivious relay as an attacker. This might be too
many requests from a single client, too many requests from the
clients behind the same oblivious relay or too many requests from
all clients on the Internet.
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10. IANA Considerations
10.1. RateLimit Parameter Value Registrations
This specification requests IANA to add the following parameters to
the "Hypertext Transfer Protocol (HTTP) RateLimit Parameters"
registry defined in [RATELIMIT].
+=================+=================+================+===============+
| Field Name |Parameter Name |Description |Specification |
+=================+=================+================+===============+
| RateLimit-Policy|ohttp-target |ohttp ratelimit |Section 3 of |
| | | |this document |
| RateLimit-Policy|attack-severity |ohttp ratelimit |Section 5 of |
| | | |this document |
+-----------------+-----------------+----------------+---------------+
10.2. Registration of new HTTP Header Field
10.2.1. Ohttp-Outside-Encap Header
This section describes a header field for registration in the
Permanent Message Header Field Registry [RFC3864].
Header field name
Ohttp-Outside-Encap
Applicable protocol
http
Status
Standard
Author/Change controller
IETF
Specification document(s)
RFC XXXX
Related information
This header field is only used for Oblivious HTTP.
11. Acknowledgements
Thanks to Lucas Pardue, Rich Salz, Martin Thomson, Christopher A.
Wood and Brandon Williams for the discussion and comments.
12. References
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12.1. Normative References
[HTTP] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-19, 12 September 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
semantics-19>.
[OHTTP] Thomson, M. and C. A. Wood, "Oblivious HTTP", Work in
Progress, Internet-Draft, draft-ietf-ohai-ohttp-01, 15
February 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-ohai-ohttp-01>.
[RATELIMIT]
Polli, R. and A. M. Ruiz, "RateLimit Fields for HTTP",
Work in Progress, Internet-Draft, draft-ietf-httpapi-
ratelimit-headers-05, 6 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpapi-
ratelimit-headers-05>.
[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>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[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>.
[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>.
[RFC8941] Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>.
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
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[STRUCTURED-FIELDS]
Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/rfc/rfc8941>.
12.2. Informative References
[Reputation]
IANA, "Client reputation",
<https://identitydocs.akamai.com/getting-started/rba/2-
rba/page3-clientrep/>.
[SEVERITY] IANA, "Incident Object Description Exchange Format v2
(IODEF)", <https://www.iana.org/assignments/iodef2/
iodef2.xhtml#businessimpact-severity>.
Authors' Addresses
Tirumaleswar Reddy
Nokia
India
Email: kondtir@gmail.com
Dan Wing
Citrix Systems, Inc.
4988 Great America Pkwy
Santa Clara, CA 95054
United States of America
Email: danwing@gmail.com
Mohamed Boucadair
Orange
35000 Rennes
France
Email: mohamed.boucadair@orange.com
Roberto Polli
Team Digitale, Italian Government
Email: robipolli@gmail.com
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