Oblivious Relay Feedback

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Table of Contents

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.¶

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].¶

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

4. The ohttp-target Quota Policy Parameter

ohttp-target = sf-integer

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.¶
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.¶

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.¶

+----+            +----------+       +----------+    +----------+
| 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        |                  |               |
  |                     |                  |               |

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".¶

=============== 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...

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

Optional white space (OWS) is used as defined in Section 5.6.3 of [RFC9110].¶

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.¶

10. IANA Considerations

+=================+=================+================+===============+
| 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  |
+-----------------+-----------------+----------------+---------------+

Authors' Addresses