Original text:
<del> Requirements for Adaptive DNS Discovery draft-box-add-requirements-01 </del>  Abstract Adaptive DNS Discovery is chartered to define mechanisms that allow clients to discover and select encrypted DNS resolvers.  This <del> document describes one common use case, that of discovering the encrypted DNS resolver that corresponds to the Do53 resolver offered by a network.  It lists requirements that any proposed discovery mechanisms should address. </del>  1. The Adaptive DNS Discovery (ADD) Working Group is chartered to define mechanisms that allow clients to automatically discover and select encrypted DNS resolvers in a wide variety of network environments. <del> This document currently focusses on one common use case, that of discovering the encrypted DNS resolver that corresponds to the Do53 resolver offered by a network.  Additional use cases can be added in future versions.  As well as describing the use case, it lists requirements that any proposed discovery mechanisms should address. They can do this either by providing a solution, or by explicitly stating why it is not in scope. </del>  1.1. Do53: Unencrypted DNS over UDP port 53, or TCP port 53 RFC1035. <del> Equivalent: See equivalence. </del>  3. It is often the case that a client possesses no specific configuration for how to operate DNS, and at some point joins a <del> network that it has no previous knowledge about.  In such a case the usual existing behaviour is to dynamically discover the network's recommended Do53 resolver and use it.  This long-standing practice </del>  works in nearly all networks, but presents a number of privacy and security risks that were the motivation for the development of encrypted DNS. <del> The network's recommended unencrypted resolver may have a number of </del>  properties that differ from a generic resolver.  It may be able to answer names that are not known globally, it may exclude some names (for positive or negative reasons), and it may provide address least some of the time.  However they would like to use an encrypted DNS protocol rather than Do53. <del> Using an encrypted and authenticated resolver that is equivalent to the one provisioned by the network can provide several benefits that are not possible if only unencrypted DNS is used: </del>  Prevent other devices on the network from observing client DNS messages Verify that answers come from the selected DNS resolver To meet this case there should be a means by which the client can <del> learn how to contact an encrypted DNS resolver that provides equivalent responses as the ones served by the network's recommended unencrypted resolver.  It is not a requirement that these two resolvers are the same physical or logical machine.  Often they will be, but they could equally be separated, perhaps by hundreds of miles.  However it is deployed, the key is that they are equivalent. </del>  3.1. <del> Given two resolvers A and B, equivalence is the claim that A and B can provide the same upper-layer DNS function to the client.  This does not include the DNS transport protocol (e.g.  Do53 or DNS-over- HTTPS) which can differ between equivalent resolvers.  To provide equivalence it is frequently likely to be the case that A and B are operated by the same administrative domain, but this document does not require that. </del>  <del> There are two possible ways to claim equivalence. </del>  <del> The local network can claim that one or more encrypted DNS resolvers (B, C, etc) are equivalent to the Do53 resolver (A) it has offered.  This is known as network-identified. </del>  <del> During communication with the (often unencrypted) resolver (A), this resolver can claim that one or more encrypted DNS resolvers (B, C, etc) are equivalent.  This is known as resolver-identified. </del>  <del> Network-identified is preferred since it comes from the same source of information, and removes the need to talk to the Do53 resolver at all.  However it cannot be the sole mechanism, at least for several years, since there is a large installed base of local network equipment that is difficult to upgrade with new features.  Hence the second mechanism must support being able to announce an equivalent resolver using only existing widely-deployed DNS features. </del>  3.2. Many networks offer a Do53 resolver on an address that is not globally meaningful, e.g.  RFC1918, link-local or unique local <del> addresses.  To support the discovery of Encrypted DNS in these </del>  environments, a means is needed for the discovery process to work <del> from a locally-addressed Do53 resolver to an Encrypted DNS resolver </del>  that is accessible either at the same (local) address, or at a different global address.  Both options need to be supported. <del> 4. </del>  DNS servers are often provisioned by a network as part of DHCP options RFC2132, IPv6 Router Advertisement (RA) options RFC8106, resolver IP addresses, to be used for traditional unencrypted DNS. A solution is required that enhances the set of information delivered <del> to include details of one or more equivalent encrypted DNS resolvers, or states that there are none. </del>  <del> 5. </del>  To support cases where the network is unable to identify an encrypted resolver, it should be possible to learn the details of one or more <del> equivalent encrypted DNS resolvers by communicating with the network- recommended unencrypted Do53 resolver.  This should involve an exchange that uses standard DNS messages that can be handled, or forwarded, by existing deployed software. </del>  <del> It is frequently the case that Do53 resolvers announced by home networks are difficult to upgrade to support encrypted operation.  In such cases it is possible that the only option for encrypted operation is to refer to a separate globally-addressed encrypted DNS resolver. </del>  If the local resolver has been upgraded to support encrypted DNS, the client may not initially be aware that its local resolver supports retaining the benefits of a local caching resolver with knowledge of the local topology.  An additional benefit of using a local resolver occurs with IoT devices.  A common usage pattern for such devices is for it to "call home" to a service that resides on the public Internet, where that accessible for inspection, it becomes quite difficult for the infrastructure to suspect anything. <del> 6. </del>  Encrypted (and authenticated) DNS improves the privacy and security of DNS queries and answers in the presence of malicious attackers. administrator. Cause clients to use a discovered resolver which has no <del> authenticated delegation from a client-known entity. Influence automatic discovery mechanisms such that a client uses one or more resolvers that are not otherwise involved with providing service to the client, such as: a network provider, a VPN server, a content provider being accessed, or a server that the client has manually configured. </del>  When discovering DNS resolvers on a local network, clients have no mechanism to distinguish between cases where an active attacker with discovery solutions designed around this threat model may have limited value. <del> 7. </del>  This section lists requirements that flow from the above sections. <del> 8. </del>  See priv-sec. <del> 9. </del>  This document has no IANA actions.

Reviewer comments:
This PR simplifies further by removing any mention of properties of the encrypted resolvers, or properties of the relationship between the Do53 and the encrypted resolver. It now purely focusses on the ability of an untrusted network or resolver to designate other encrypted resolvers. This is an assertion that the encrypted resolver is safe and appropriate to use, but little else. Clients will of course make their own decisions about whether to use designated resolvers or not. Other drafts will look at trusted networks, and communicating information that can be useful to clients as part of the untrusted decision-making.
comments
comments on the draft02
It was pointed out that this should be limited to IETF-standardised encrypted DNS protocols rather than potentially including DNSCrypt and being very open-ended.

Revised text:
 <ins> Requirements for Discovering Designated Resolvers draft-box-add-requirements-02 </ins> Abstract Adaptive DNS Discovery is chartered to define mechanisms that allow clients to discover and select encrypted DNS resolvers.  This  <ins> document describes one common use case, namely that of clients that connect to a network but where they cannot securely authenticate the identity of that network.  In such cases the client would like to learn which encrypted DNS resolvers are designated by that network or by the Do53 resolver offered by that network.  It lists requirements that any proposed discovery mechanisms should seek to address. </ins> 1. The Adaptive DNS Discovery (ADD) Working Group is chartered to define mechanisms that allow clients to automatically discover and select encrypted DNS resolvers in a wide variety of network environments.  <ins> This document describes one common use case, namely that of clients that connect to a network but where they cannot securely authenticate that network.  Whether the network required credentials before the client was permitted to join is irrelevant; the client still cannot be sure that it has connected to the network it was expecting. In such cases the client would like to learn which encrypted DNS resolvers are designated by that network, or by the Do53 resolver offered by that network.  It lists requirements that any proposed discovery mechanisms should seek to address.  They can do this either by providing a solution, or by explicitly stating why it is not in scope. </ins> 1.1. Do53: Unencrypted DNS over UDP port 53, or TCP port 53 RFC1035.  <ins> Designated: See designation. Designator: The network or resolver that issued the designation. </ins> 3. It is often the case that a client possesses no specific configuration for how to operate DNS, and at some point joins a  <ins> network that it cannot authenticate.  It may have no prior knowledge of the network, or it may have connected previously to a network that looked the same.  In either case the usual behaviour, because of lack of specific configuration, is to dynamically discover the network's designated Do53 resolver and use it.  This long-standing practice </ins> works in nearly all networks, but presents a number of privacy and security risks that were the motivation for the development of encrypted DNS.  <ins> The network's designated Do53 resolver may have a number of </ins> properties that differ from a generic resolver.  It may be able to answer names that are not known globally, it may exclude some names (for positive or negative reasons), and it may provide address least some of the time.  However they would like to use an encrypted DNS protocol rather than Do53.  <ins> Using an encrypted and authenticated resolver can provide several benefits that are not possible if only unencrypted DNS is used: </ins> Prevent other devices on the network from observing client DNS messages Verify that answers come from the selected DNS resolver To meet this case there should be a means by which the client can  <ins> learn how to contact a set of encrypted DNS resolvers that are designated by the network it has joined. </ins> 3.1.  <ins> Designation is the process by which a local network or a resolver can point clients towards a particular set of resolvers.  This is not a new concept, as networks have been able to dynamically designate Do53 resolvers for decades (see network).  However here we extend the concept in two ways: </ins>  <ins> To allow resolvers to designate other resolvers </ins>  <ins> The inclusion of support for encrypted DNS </ins>  <ins> The designated set could be empty, or it could list the contact details (such as DoH URI Template) of DNS resolvers that it recommends.  It is not required that there be any relationship between the resolvers in the set, simply that all of them are options that the designator asserts are safe and appropriate for the client to use without user intervention. There are two possible sources of designation. The local network can designate one or more encrypted DNS resolvers (B, C, etc) in addition to any Do53 resolver (A) it may offer.  This is known as network-identified. </ins>  <ins> During communication with the (often unencrypted) resolver (A), this resolver can designate one or more encrypted DNS resolvers (B, C, etc).  This is known as resolver-identified. Network-identified has the advantages that it derives from the same source of information as the network's Do53 announcement, and removes the need to talk to the Do53 resolver at all.  However it cannot be the sole mechanism, at least for several years, since there is a large installed base of local network equipment that is difficult to upgrade with new features.  Hence the second mechanism should support being able to designate resolvers using only existing widely-deployed DNS features. </ins> 3.2. Many networks offer a Do53 resolver on an address that is not globally meaningful, e.g.  RFC1918, link-local or unique local  <ins> addresses.  To support the discovery of encrypted DNS in these </ins> environments, a means is needed for the discovery process to work  <ins> from a locally-addressed Do53 resolver to an encrypted DNS resolver </ins> that is accessible either at the same (local) address, or at a different global address.  Both options need to be supported.  <ins> 3.3. After the client receives designation information, it must come to a decision on whether and when to use any of the designated resolvers. In the case of resolver-identified designation, it would be advantageous for a solution to enable the client to validate the source of the assertion in some way.  For example it may be possible to verify that the designation comes from an entity who already has full control of the client's Do53 queries.  Network-identified designation should not require this, unless the network-identified resolver in turn initiated a new resolver-identified designation.  It would be beneficial to extend such a verification process to defend against attackers that have only transient control of such queries. Clients may also seek to validate the identity of the designated resolver, beyond what is required by the relevant protocol.  Authors of solution specifications should be aware that clients may impose arbitrary additional requirements and heuristics as they see fit. 3.4. </ins> DNS servers are often provisioned by a network as part of DHCP options RFC2132, IPv6 Router Advertisement (RA) options RFC8106, resolver IP addresses, to be used for traditional unencrypted DNS. A solution is required that enhances the set of information delivered  <ins> to include details of one or more designated encrypted DNS resolvers, or states that there are none.  Such resolvers could be on the local network, somewhere upstream, or on the public Internet. </ins>  <ins> 3.5. </ins> To support cases where the network is unable to identify an encrypted resolver, it should be possible to learn the details of one or more  <ins> designated encrypted DNS resolvers by communicating with the network's designated Do53 resolver.  This should involve an exchange that uses standard DNS messages that can be handled, or forwarded, by existing deployed software. </ins>  <ins> Each resolver in the set may be at a different network location, which leads to several subcases for the mapping from Do53 to a particular designated resolver. 3.5.1. </ins> If the local resolver has been upgraded to support encrypted DNS, the client may not initially be aware that its local resolver supports retaining the benefits of a local caching resolver with knowledge of the local topology. <ins> Clients will be aware when the designated resolver has the same IP address as the Do53 (after looking up its name if required).  They can use this information in their decision-making as to the level of trust to place in the designated resolver.  In some networks it will not be possible to deploy encrypted DNS on the same IP address, e.g. because of the increased resource requirements of encrypted DNS. Discovery solutions should work in the presence of a change to a different local IP address. </ins> An additional benefit of using a local resolver occurs with IoT devices.  A common usage pattern for such devices is for it to "call home" to a service that resides on the public Internet, where that accessible for inspection, it becomes quite difficult for the infrastructure to suspect anything.  <ins> 3.5.2. It is frequently the case that Do53 resolvers announced by home networks are difficult to upgrade to support encrypted operation.  In such cases it is possible that the only option for encrypted operation is to refer to a separate globally-addressed encrypted DNS resolver, somewhere upstream.  Other networks may choose deploy their encrypted DNS resolver away from the local network, for other reasons. The use of an upstream resolver can mean the loss of local knowledge, such as the ability to respond to queries for locally-relevant names. Solutions should consider how to guide clients when to direct their queries to the local Do53.  For example this could be through pre- emptive communication ("if you ever need to query *.example.com, use your local Do53"), or reactively ("I don't know the answer to that, but your local Do53 should know"). 3.5.3. In cases where the local network has designated a Do53 resolver on the public Internet, this resolver may designate its own or another public encrypted DNS service.  Since public IP addresses may appear in TLS certificates, solutions may use this as one way to validate that the designated encrypted resolver is legitimately associated with the original Do53. 3.6. In cases where the designation is delivered over an authenticated and encrypted channel, such as when one encrypted DNS resolver designates another, one form of attack is removed.  Specifically, clients may be more confident that the received designation was actually sent by the designator.  Clients may take this into account when deciding whether to follow the designation. 4. </ins> Encrypted (and authenticated) DNS improves the privacy and security of DNS queries and answers in the presence of malicious attackers. administrator. Cause clients to use a discovered resolver which has no  <ins> designation from a client-known entity. </ins> When discovering DNS resolvers on a local network, clients have no mechanism to distinguish between cases where an active attacker with discovery solutions designed around this threat model may have limited value.  <ins> 5. </ins> This section lists requirements that flow from the above sections.  <ins> 6. </ins> See priv-sec.  <ins> 7. </ins> This document has no IANA actions.