Oblivious Relay Feedback

Oblivious HTTP 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 . 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 . 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 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.

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 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 . The terms "Encapsulated request", "Encapsulated response", "Oblivious relay resource", "Oblivious gateway resource", "Oblivious target resource", and "Client" are to be interpreted as described in . 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 . This document uses the Integer type from .

An Oblivious resource that uses RateLimit fields to return service limit information MAY add the "ohttp-target" quota policy parameter defined in 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: Identify the quota policy associated with the expiring limit. Check whether the "ohttp-target" parameter is present and its syntax is correct. In the example shown in , 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.

The following quota policy parameter is defined for the RateLimit-Policy field : Indicates that the associated quota policy provides Oblivious Relay Feedback. This parameter is OPTIONAL. The "ohttp-target" parameter has the following syntax:

Its value MUST be an Integer (Section 3.3.1 of ) 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: Indicates that RateLimit fields are applicable to all the clients that are serviced by the same Oblivious relay. In order to offer better privacy, a relay can rate-limit all of the clients it is serving provided it will continue to forward a high volume of messages from a large number of clients even after applying the rate-limiting. If the olivious Relay Resource attempts to divide the rate limit fairly among the active clients, the timing pattern of requests can possibly be strongly correlated by the to gateway to de-anonymize clients. As discussed in Section 5 of , the relay can delay requests before forwarding them to migitate the attack as it will likely increase the anonymity set into which each request is attributed. 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). Naive application of the rate limit can allow the target to take advantage of the differential treatment applied by the relay to de-anonymize the client so the Oblivious relay MUST follow the mitigation mechanisms discussed in . 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.

An Oblivious relay receiving RateLimit fields providing Oblivious Relay Feedback will do the following: It MUST remove all of the RateLimit fields from the response, since they are not intended to be forwarded to clients. An Oblivious gateway resource receiving RateLimit fields providing Oblivious Relay Feedback MUST proceed as follows: 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. 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. 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 ), 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 , 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.

The following mitigations MUST be used by an Oblivious relay to process "ohttp-target" parameter with a value set to '2' to prevent the target to correlate requests from clients': 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 very high ratio of "potential malicious requests" to "legitimate requests" and the condition explained in Step 2 is met. For instance, if the ratio of "potential malicious requests" to legitimate requests" is 500 to 5, which means the ratio is very high. 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. The target can see a matching attack pattern in multiple requests from that same offending client or multiple offending clients. However, the target will not be able to partition the anonymity set of legitimate clients. If a large number of clients are actively using the relay to send large volume of messages and the total number of benign clients is significantly larger than the total number of offending clients (e.g., total clients are greater than 100,000 and benign clients are above 80% of the total clients), the Oblivious relay will enforce the delayed rate-limiting of the offending client discussed in step 1. This way the anonymity set of clients sending large volume of messages will be a relatively larger number even after rate-limiting the offending clients. When a relay forwards a large volume of messages from a large number of clients after rate-limiting the offending clients, it assures better privacy by providing larger sets of messages that need to be matched. For example, if a relay chooses to rate limit or block all offending clients, this means that any client requests which are not treated this way are known to be non-abusive by the gateway but it will not be able to correlate requests from a large number of clients sending large volume of messages. The following mechanisms can be optionally used by an Oblivious relay to process "ohttp-target" parameter with a value set to '2': The Oblivious relay acts to rate-limit the traffic of the client provided it has a bad reputation score for sending attack traffic (e.g., many security network security vendors offer client reputation score, for instance, see ). The Oblivious relay provider hosts a honeypot of clients or leverages a honeypot service to send benign flows 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 benign 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 any these clients, it would identify the target is misbehaving and ignores the targeted rate-limit for any other client it is serving. The other advantage of using a honeypot is that it increases the size of the the anonymity set of clients using a gateway. The Oblivious relay can introduce synthetic encapsulated requests to simulate equal distribution of sequence of requests to avoid any request correlation by the target. The last two optional mechanisms need to be used with caution as they would increase the load on the relay, gateway, and target resources.

The following quota policy parameter is defined for the RateLimit-Policy field defined in : Is used by the Oblivious resource to convey the likeliness that an HTTP request is malicious. This parameter is OPTIONAL.

Note that sf-string is defined in Section 3.3.3 of . The value of the "attack-severity" parameter is a String (Section 3.3.3 of ) that takes one of the values defined in . 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.

The example depicted in 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: Copy the RateLimit fields from the original response. Remove them from the original response before encapsulating it. Generate a single 200 response containing the encapsulated response for the oblivious relay resource along with the copied RateLimit fields.

...encrypted content... ]]>

The "Ohttp-Outside-Encap" HTTP header field is defined in this specification (). Its purpose is to signal which HTTP headers will be removed by the Oblivious gateway. It is intended only for use in requests to an Oblivious taget. 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. describes the syntax. The syntax of this header field conforms to . It is a List (, Section 3.1):

An example is illustrated below:

The security considerations for the Oblivious HTTP protocol (Section 8 of ) as well as the ones for RateLimit fields (Section 6 of ) apply. The following sub-sections discuss security considerations specific to this specification.

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 ). The Oblivious target and gateway resources SHOULD convey Oblivious Relay Feedback only to trusted Oblivious proxies.

Attacks against the Oblivious Gateway and Target Resources can be classified into three primary categories: 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. 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. 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.

This specification requests IANA to add the following parameters to the "Hypertext Transfer Protocol (HTTP) RateLimit Parameters" registry defined in .

This section describes a header field for registration in the Permanent Message Header Field Registry . Ohttp-Outside-Encap http Standard IETF RFC XXXX This header field is only used for Oblivious HTTP.

Thanks to Lucas Pardue, Rich Salz, Martin Thomson, Christopher A. Wood, Ben Schwartz and Brandon Williams for the discussion and comments.