Oblivious Proxy Feedback

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 7 September 2022.¶

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

Oblivious HTTP [I-D.ietf-ohai-ohttp] describes a method of encapsulation for binary HTTP messages [BINARY] using Hybrid Public Key Encryption (HPKE; [HPKE]). This protects the content of both requests and responses and enables a deployment architecture that can separate the identity of a requester from the request. This scheme requires that servers and proxies explicitly support it. The server is susceptible to attacks described below, but the server cannot take any mitigation action per client to protect itself from various attacks -- the server can only take mitigation actions per oblivious proxy. Rate-limiting traffic from an oblivious proxy impacts all clients behind that proxy -- both misbehaving clients and well-behaved clients.¶

Attacks against the Request and Target Resources can be classified into three primary categories:¶

1. A client sends a malformed encapsulated request causing decryption failure or decryption overload failure on the oblivious request resource. This causes the oblivious request resource to send an error status code back to the oblivious proxy.¶
2. A client sends an HTTP transaction that causes an HTTP error on the oblivious target tesource. This might be a malformed HTTP request, or request for a missing resource.,¶
3. HTTP flood: A botnet performing an HTTP flood attack against a victim's server. Because each bot in a botnet makes seemingly legitimate network requests the traffic is not spoofed and may appear "normal" in origin. This might be too many requests from a single client, too many requests from the clients behind the same oblivious proxy or too many requests from all clients on the Internet.¶

This document defines how an overload indication is communicated to an oblivious proxy so that this proxy can rate limit transactions by overzealous or misbehaving clients, allowing the oblivious proxy to continue servicing well-behaved clients to that same oblivious target tesource.¶

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.¶

This document makes use of the terms defined in [I-D.ietf-ohai-ohttp].¶

3. Feedback Header

The "Feedback" header field is defined in this specification. The Feedback header provides feedback information from the request resource or target resource to the proxy in the HTTP response. The proxy MUST remove the Feedback header before sending the HTTP response containing the encapsulated response to the client. If the feedback information is generated by the request resource before removing the protection (including being unable to remove encapsulation for any reason)(see Section 6.2 of [I-D.ietf-ohai-ohttp]), it will result in the Feedback Header added in the status code being sent without protection in response to the POST request from the client.¶

Figure 1 describes the syntax (Augmented Backus-Naur Form) of the header field, using the grammar defined in [RFC5234] and the rules defined in Section 3.2 of [RFC7230]. The field values of the header field conform to the same rules.¶

  Feedback = feedback-parameter *( OWS ";" OWS feedback-parameter)
  feedback-parameter   =
          feedback-parameter-name [ "=" feedback-parameter-value ]
  feedback-parameter-name = registered-token
  registered-token = token
  feedback-parameter-value = 1*DIGIT

• [[NOTE: CHECK IF WE CAN REUSE THE STRUCTURED FIELDS IN RFC 8941]]¶

Optional white space (OWS) is used as defined in Section 3.2.3 of [RFC7230] and token is used as defined in Section 3.2.6 of [RFC7230].¶

The overall processing of the parameters is discussed below:¶

1. The order of appearance of the parameters is not significant.¶
2. A given parameter MUST NOT appear more than once in the Feedback header.¶
3. Parameters are either optional or required, as explicited in their definitions.¶
4. Parameter names are case insensitive.¶
5. Proxies MUST ignore any parameters or values, that do not conform to the syntax defined in this specification. In particular, proxies must not attempt to fix malformed parameters or parameter values.¶
6. If the parameter is not recognized by the proxy, it MUST be ignored by the proxy.¶

4. Feedback Header Parameters

The feedback information includes the following parameters:¶

c-any-req:

The maximum number of HTTP requests allowed per second from any client interacting with the oblivious proxy. This is a optional parameter.¶

c-any-outstanding:

The maximum number of outstanding HTTP requests allowed from any client interacting with the oblivious proxy. This is an optional attribute.¶

p-req:

The maximum number of HTTP requests allowed per second from the proxy. This is an optional attribute.¶

p-outstanding:

The maximum number of outstanding HTTP requests allowed from the proxy. This is an optional attribute.¶

c-req:

The maximum number of HTTP requests allowed per second from the client which has sent a malformed request. This is an optional attribute.¶

c-outstanding:

The maximum number of outstanding HTTP requests allowed from the client which has sent a malformed request. This is an optional attribute.¶

td:

The time duration the OHAI target server wants this policy applied. A value of -1 indicates infinity. A value of 0 indicates all currently and previously-signaled feedback thresholds no longer apply. Value in seconds. This is a mandatory attribute.¶

TBD: Use of any other parameters like min-encap-request-size and max-encap-request-size to defend from garbled encapsulated requests.¶

TBD: Recommended lifetime of Feedback (3600 seconds) ?¶

Note that we plan to use short parameter names in future versions of the draft as recommended by [I-D.ietf-httpbis-bcp56bis].¶

The above parameters are in the form of a name=value pair. The feedback information header MUST include the td parameter and atleast one of the parameters c-any-req, c-any-outstanding, p-req, p-outstanding, c-req or c-outstanding.¶

Example: A target resource receives an malformed message and generate an HTTP response with a 400 status code, it adds the "Feedback" header to the 400 response and sends the 400 response to the request resource. The request resource copies the "Feedback" header from the 400 response, removes the "Feedback" header from the 400 response and encapsulates the 400 response. The request resource sends a single 200 response along with the copied "Feedback" header in the 200 response and encapsulated 400 response as the response content.¶

+---------+                             +-----------+                        +----------+         +-----------+
| Client  |                             | Proxy     |                        | Request  |         | Target    |
|         |                             | Resource  |                        | Resource |         | Resource  |
+---------+                             +-----------+                        +----------+         +-----------+
     |                                  |                                         |                   |
     | Encapsulated Request             |                                         |                   |
     |--------------------------------->|                                         |                   |
     |                                  |                                         |                   |
     |                                  | Encapsulated Request                    |                   |
     |                                  |---------------------------------------->|                   |
     |                                  |                                         |                   |
     |                                  |                                         | Request           |
     |                                  |                                         |------------------>|
     |                                  |                                         |                   | -----------------------------\
     |                                  |                                         |                   |-| Identify malformed request |
     |                                  |                                         |                   | |----------------------------|
     |                                  |                                         |                   |
     |                                  |                                         |      400 response |
     |                                  |                                         |<------------------|
     |                                  |                                         |                   |
     |                                  | 200 response with Feedback Header       |                   |
     }                                  | and Encapsulated 400 response           |                   |
     |                                  | as the response content                 |                   |
     |                                  |<----------------------------------------|                   |
     | -----------------------------\   |                                         |                   |
     | | Process Feedback Header    |   |                                         |                   |
     | } take mitigation action     |---|                                         |                   |
     | |----------------------------|   |                                         |                   |
     |                                  |                                         |                   |
     |  Encapsulated 400 response       |                                         |                   |
     |<---------------------------------|                                         |                   |
     |                                  |                                         |                   |

The response constructed by the oblivious request resource is depicted below:¶

  HTTP/1.1 200 OK
  Date: Wed, 27 March 2022 04:45:07 GMT
  Cache-Control: private, no-store
  Feedback: c-any-req=1000; p-any-outstanding=20000; td=600
  Content-Type: message/ohttp-res
  Content-Length: 38 <content is the encapsulated 400 response>

5. Request or Target Resource Generating Feedback Header

When an overlaod is experienced by the request or target resource it adds the Feedback header and parameters to request load adjustement. For example, when a HTTP server itself identifies high frequency or high volume anomalies in the traffic directed to the server it would include the Feedback header. Ideally the Feedback header provides enough detail to the oblivious proxy to avoid the server rate limiting the oblivious proxy's IP address.¶

6. Proxy Processing of Feedback Header

When presented with a response that contains a Feedback Header, the proxy can process the parameters in the headers and take appropriate action. There is no mechanism for proxy to indicate to server that feedback information was processed or was ignored. The proxy can honor the rate indicated by the request resource/resource target. To that aim, the proxy may take appropriate additional actions such as (1) rate-limiting the requests from a client not to exceed requests per second (c-req) value (2) rate-limit the outstanding HTTP requests from a client not to exceed outstanding requests (c-outstanding) value.¶

If the proxy ignores the feedback information, there is a risk that the overload may still be encountered by the request and target resources. More severe actions may be then taken at the server, e.g., block all the requests from this proxy for a given time duration.¶

7. Security Considerations

The security considerations for the Oblivious HTTP protocol are discussed in Section 8 of [I-D.ietf-ohai-ohttp]. The target and request resources SHOULD convey the Feedback header to trusted oblivious proxy. However, if this oblivious proxy is not trusted, security risks discussed below may arise:¶

• If oblivious proxy and clients attacking the server are managed by an attacker, the attacker can use the Feedback information to identify the server has detected the attack and possibly change the attack strategy.¶
• The oblivious proxy can colloude with the attacking clients and leak the Feedback information to the clients.¶

8. IANA Considerations

Authors' Addresses