ADD M. Boucadair, Ed. Internet-Draft Orange Intended status: Informational T. Reddy, Ed. Expires: November 18, 2021 McAfee D. Wing Citrix N. Cook Open-Xchange T. Jensen Microsoft May 17, 2021 Discovery of Encrypted DNS Resolvers: Deployment Considerations draft-boucadair-add-deployment-considerations-00 Abstract The document discusses some deployment considerations of the various options to discover encrypted DNS servers (e.g., DNS-over-HTTPS, DNS- over-TLS, DNS-over-QUIC). 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 November 18, 2021. Copyright Notice Copyright (c) 2021 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 Boucadair, et al. Expires November 18, 2021 [Page 1] Internet-Draft Discovery of Encrypted DNS Resolvers May 2021 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Sample Target Deployment Scenarios . . . . . . . . . . . . . 3 3.1. Managed CPEs . . . . . . . . . . . . . . . . . . . . . . 5 3.1.1. Direct DNS . . . . . . . . . . . . . . . . . . . . . 5 3.1.2. Proxied DNS . . . . . . . . . . . . . . . . . . . . . 6 3.2. Unmanaged CPEs . . . . . . . . . . . . . . . . . . . . . 7 3.2.1. ISP-facing Unmanaged CPEs . . . . . . . . . . . . . . 7 3.2.2. Internal Unmanaged CPEs . . . . . . . . . . . . . . . 7 4. Hosting Encrypted DNS Forwarder in Local Networks . . . . . . 8 4.1. Managed CPEs . . . . . . . . . . . . . . . . . . . . . . 8 4.1.1. DNS Forwarders . . . . . . . . . . . . . . . . . . . 8 4.1.2. ACME . . . . . . . . . . . . . . . . . . . . . . . . 9 4.2. Unmanaged CPEs . . . . . . . . . . . . . . . . . . . . . 9 5. Legacy CPEs . . . . . . . . . . . . . . . . . . . . . . . . . 10 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 9.1. Normative References . . . . . . . . . . . . . . . . . . 11 9.2. Informative References . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 1. Introduction [I-D.ietf-add-dnr] specifies how a local encrypted DNS server can be discovered by connected hosts by means of DHCP [RFC2132], DHCPv6 [RFC8415], and IPv6 Router Advertisement (RA) [RFC4861] options. These options are designed to convey the following information: the DNS Authentication Domain Name (ADN), a list of IP addresses, and a set of service parameters. This document discusses deployment considerations for the discovery of encrypted DNS servers such as DNS-over-HTTPS (DoH) [RFC8484], DNS- over-TLS (DoT) [RFC7858], or DNS-over-QUIC (DoQ) [I-D.ietf-dprive-dnsoquic] in local networks. Sample target deployment scenarios are discussed in Section 3; both managed and unmanaged Customer Premises Equipment (CPEs) are covered. Boucadair, et al. Expires November 18, 2021 [Page 2] Internet-Draft Discovery of Encrypted DNS Resolvers May 2021 It is out of the scope of this document to provide an exhaustive inventory of deployments where Encrypted DNS options can be used. Considerations related to hosting a DNS forwarder in a local network are described in Section 4. 2. Terminology This document makes use of the terms defined in [RFC8499]. The following additional terms are used: Do53: refers to unencrypted DNS. Encrypted DNS: refers to a scheme where DNS exchanges are transported over an encrypted channel. Examples of encrypted DNS are DNS-over-TLS (DoT) [RFC7858], DNS-over-HTTPS (DoH) [RFC8484], or DNS-over-QUIC (DoQ) [I-D.ietf-dprive-dnsoquic]. Encrypted DNS options: refers to the options defined in [I-D.ietf-add-dnr]. Managed CPE: refers to a CPE that is managed by an Internet Service Provider (ISP). Unmanaged CPE: refers to a CPE that is not managed by an ISP. DHCP: refers to both DHCPv4 and DHCPv6. 3. Sample Target Deployment Scenarios ISPs traditionally provide DNS resolvers to their customers. To that aim, ISPs deploy the following mechanisms to advertise a list of DNS Recursive DNS server(s) to their customers: o Protocol Configuration Options in cellular networks [TS.24008]. o DHCPv4 [RFC2132] (Domain Name Server Option) or DHCPv6 [RFC8415][RFC3646] (OPTION_DNS_SERVERS). o IPv6 Router Advertisement [RFC4861][RFC8106] (Type 25 (Recursive DNS Server Option)). The communication between a customer's device (possibly via Customer Premises Equipment (CPE)) and an ISP-supplied DNS resolver takes place by using cleartext DNS messages (Do53). Some examples are depicted in Figure 1. In the case of cellular networks, the cellular network will provide connectivity directly to a host (e.g., smartphone, tablet) or via a CPE. Do53 mechanisms used within the Boucadair, et al. Expires November 18, 2021 [Page 3] Internet-Draft Discovery of Encrypted DNS Resolvers May 2021 Local Area Network (LAN) are similar in both fixed and cellular CPE- based broadband service offerings. Some ISPs rely upon external resolvers (e.g., outsourced service or public resolvers); these ISPs provide their customers with the IP addresses of these resolvers. These addresses are typically configured on CPEs using dedicated management tools. Likewise, users can modify the default DNS configuration of their CPEs (e.g., supplied by their ISP) to configure their favorite DNS servers. This document permits such deployments. (a) Fixed Networks ,--,--,--. +-+ LAN +---+ ,-' `-. |H+--------------+CPE+---+ ISP ) +-+ +---+ `-. ,-' | `--'--'--' | | |<=============Do53============>| | | (b) Cellular Networks | | |<=============Do53============>| | | | ,--,--,-. +-+ LAN +---+ ,-' . |H+--------------+CPE+---+ \ +-+ +---+ ,' ISP `-. ( ) +-----+-. ,-' +-+ | `--'--'--' |H+----------------+ | +-+ | | | |<=============Do53============>| | | Legend: * H: refers to a host. Figure 1: Sample Legacy Deployments Boucadair, et al. Expires November 18, 2021 [Page 4] Internet-Draft Discovery of Encrypted DNS Resolvers May 2021 3.1. Managed CPEs This section focuses on CPEs that are managed by ISPs. 3.1.1. Direct DNS ISPs have developed an expertise in managing service-specific configuration information (e.g., CPE WAN Management Protocol [TR-069]). For example, these tools may be used to provision the DNS server's ADN to managed CPEs if an encrypted DNS is supported by a local network similar to what is depicted in Figure 2. For example, DoH-capable (or DoT) clients establish the DoH (or DoT) session with the discovered DoH (or DoT) server. The DNS client discovers whether the DNS server in the local network supports DoH/DoT/DoQ by using the service paramters (ALPN). (a) Fixed Networks ,--,--,--. +-+ LAN +---+ ,-' `-. |H+--------------+CPE+---+ ISP ) +-+ +---+ `-. ,-' | `--'--'--' | | |<========Encrypted DNS========>| | | (b) Cellular Networks | | |<========Encrypted DNS========>| | | | ,--,--,-. +-+ LAN +---+ ,-' . |H+--------------+CPE+---+ \ +-+ +---+ ,' ISP `-. ( ) +-----+-. ,-' +-+ | `--'--'--' |H+----------------+ | +-+ | | | |<========Encrypted DNS========>| | | Figure 2: Encrypted DNS in the WAN Boucadair, et al. Expires November 18, 2021 [Page 5] Internet-Draft Discovery of Encrypted DNS Resolvers May 2021 Figure 2 shows the scenario where the CPE relays the list of encrypted DNS servers it learns for the network by using mechanisms like DHCP or a specific Router Advertisement message. In such context, direct encrypted DNS sessions will be established between a host serviced by a CPE and an ISP-supplied encrypted DNS server (see the example depicted in Figure 3 for a DoH/DoT-capable host). ,--,--,--. ,--,--,--. ,-' `-. ,-' ISP `-. Host---( LAN CPE----( DNS Server ) | `-. ,-' `-. ,-' | `--'--'--' `--'--'--' | | |<=========Encrypted DNS===========>| Figure 3: Direct Encrypted DNS Sessions 3.1.2. Proxied DNS Figure 4 shows a deployment where the CPE embeds a caching DNS forwarder. The CPE advertises itself as the default DNS server to the hosts it serves. The CPE relies upon DHCP or RA to advertise itself to internal hosts as the default DoT/DoH/Do53 server. When receiving a DNS request it cannot handle locally, the CPE forwards the request to an upstream DoH/DoT/Do53 resolver. Such deployment is required for IPv4 service continuity purposes (e.g., Section 5.4.1 of [I-D.ietf-v6ops-rfc7084-bis]) or for supporting advanced services within a local network (e.g., malware filtering, parental control, Manufacturer Usage Description (MUD) [RFC8520] to only allow intended communications to and from an IoT device). When the CPE behaves as a DNS forwarder, DNS communications can be decomposed into two legs: o The leg between an internal host and the CPE. o The leg between the CPE and an upstream DNS resolver. An ISP that offers encrypted DNS to its customers may enable encrypted DNS in one or both legs as shown in Figure 4. Additional considerations related to this deployment are discussed in Section 4. Boucadair, et al. Expires November 18, 2021 [Page 6] Internet-Draft Discovery of Encrypted DNS Resolvers May 2021 (a) ,--,--,--. ,--,--,--. ,-' `-. ,-' ISP `-. Host---( LAN CPE----( DNS Server ) | `-. ,-'| `-. ,-' | `--'--'--' | `--'--'--' | | | |<=====Encrypted=====>|<=Encrypted=>| | DNS | DNS | (b) ,--,--,--. ,--,--,--. Legacy ,-' `-. ,-' ISP `-. Host---( LAN CPE----( DNS Server ) | `-. ,-'| `-. ,-' | `--'--'--' | `--'--'--' | | | |<=======Do53========>|<=Encrypted=>| | | DNS | Figure 4: Proxied Encrypted DNS Sessions 3.2. Unmanaged CPEs 3.2.1. ISP-facing Unmanaged CPEs Customers may decide to deploy unmanaged CPEs (assuming the CPE is compliant with the network access technical specification that is usually published by ISPs). Upon attachment to the network, an unmanaged CPE receives from the network its service configuration (including the DNS information) by means of, e.g., DHCP. That DNS information is shared within the LAN following the same mechanisms as those discussed in Section 3.1. A host can thus establish DoH/DoT session with a DoH/DoT server similar to what is depicted in Figure 3 or Figure 4. 3.2.2. Internal Unmanaged CPEs Customers may also decide to deploy internal routers (called hereafter, Internal CPEs) for a variety of reasons that are not detailed here. Absent any explicit configuration on the internal CPE to override the DNS configuration it receives from the ISP-supplied CPE, an Internal CPE relays the DNS information it receives via DHCP/ RA from the ISP-supplied CPE to connected hosts. Encrypted DNS sessions can be established by a host with the DNS servers of the ISP (see Figure 5). Boucadair, et al. Expires November 18, 2021 [Page 7] Internet-Draft Discovery of Encrypted DNS Resolvers May 2021 ,--,--,--. ,--,--,--. ,-' Internal ,-' ISP `-. Host--( Network#A CPE----CPE---( DNS Server ) | `-. ,-' `-. ,-' | `--'--'--' `--'--'--' | | |<==============Encrypted DNS=============>| Figure 5: Direct Encrypted DNS Sessions with the ISP DNS Resolver (Internal CPE) Similar to managed CPEs, a user may modify the default DNS configuration of an unmanaged CPE to use his/her favorite DNS servers instead. Encrypted DNS sessions can be established directly between a host and a 3rd Party DNS server (see Figure 6). ,--,--,--. ,--, ,' Internal ,-' '- 3rd Party Host--( Network#A CPE----CPE---( ISP )--- DNS Server | `. ,-' `-. -' | | `-'--'--' `--' | | | |<=================Encrypted DNS==================>| Figure 6: Direct Encrypted DNS Sessions with a Third Party DNS Resolver Section 4.2 discusses considerations related to hosting a forwarder in the Internal CPE. 4. Hosting Encrypted DNS Forwarder in Local Networks This section discusses some deployment considerations to host an encrypted DNS forwarder within a local network. 4.1. Managed CPEs The section discusses mechanisms that can be used to host an encrypted DNS forwarder in a managed CPE (Section 3.1). 4.1.1. DNS Forwarders The managed CPE should support a configuration parameter to instruct the CPE whether it has to relay the encrypted DNS server received from the ISP's network or has to announce itself as a forwarder within the local network. The default behavior of the CPE is to supply the encrypted DNS server received from the ISP's network. Boucadair, et al. Expires November 18, 2021 [Page 8] Internet-Draft Discovery of Encrypted DNS Resolvers May 2021 4.1.2. ACME The ISP can assign a unique FQDN (e.g., "cpe1.example.com") and a domain-validated public certificate to the encrypted DNS forwarder hosted on the CPE. Automatic Certificate Management Environment (ACME) [RFC8555] can be used by the ISP to automate certificate management functions such as domain validation procedure, certificate issuance and certificate revocation. 4.2. Unmanaged CPEs The approach specified in Section 4.1 does not apply for hosting a DNS forwarder in an unmanaged CPE. The unmanaged CPE administrator can host an encrypted DNS forwarder on the unmanaged CPE. This assumes the following: o The encrypted DNS server certificate is managed by the entity in- charge of hosting the encrypted DNS forwarder. Alternatively, a security service provider can assign a unique FQDN to the CPE. The encrypted DNS forwarder will act like a private encrypted DNS server only be accessible from within the local network. o The encrypted DNS forwarder will either be configured to use the ISP's or a 3rd party encrypted DNS server. o The unmanaged CPE will advertise the encrypted DNS forwarder ADN using DHCP/RA to internal hosts. Figure 7 illustrates an example of an unmanaged CPE hosting a forwarder which connects to a 3rd party encrypted DNS server. In this example, the DNS information received from the managed CPE (and therefore from the ISP) is ignored by the Internal CPE hosting the forwarder. Boucadair, et al. Expires November 18, 2021 [Page 9] Internet-Draft Discovery of Encrypted DNS Resolvers May 2021 ,--,--,--. ,--, ,' Internal Managed ,-' '- 3rd Party Host--( Network#A CPE--------CPE------( ISP )--- DNS Server | `. ,-'| | `-. -' | | `-'--'--' | |<==DHCP==>|`--' | | |<==DHCP==>| | | |<======DHCP=======>| | | | {RI, @i} | | |<==Encrypted DNS==>|<==========Encrypted DNS==========>| Legend: * @i: IP address of the DNS forwarder hosted in the Internal CPE. Figure 7: Example of an Internal CPE Hosting a Forwarder 5. Legacy CPEs Hosts serviced by legacy CPEs that can't be upgraded to support the options defined in Sections 4, 5, and 6 of [I-D.ietf-add-dnr] won't be able to learn the encrypted DNS server hosted by the ISP, in particular. If the ADN is not discovered using DHCP/RA, such hosts will have to fallback to use discovery using the resolver IP address as defined in Section 4 of [I-D.ietf-add-ddr] to discover the designated resolvers. The guidance in Sections 4.1 and 4.2 of [I-D.ietf-add-ddr] related to the designated resolver verification has to be followed in such case. 6. Security Considerations DNR-related security considerations are discussed in Section 7 of [I-D.ietf-add-dnr]. 7. IANA Considerations This document does not require any IANA action. 8. Acknowledgements This text was initially part of [I-D.ietf-add-dnr]. 9. References Boucadair, et al. Expires November 18, 2021 [Page 10] Internet-Draft Discovery of Encrypted DNS Resolvers May 2021 9.1. Normative References [I-D.ietf-add-dnr] Boucadair, M., Reddy, T., Wing, D., Cook, N., and T. Jensen, "DHCP and Router Advertisement Options for the Discovery of Network-designated Resolvers (DNR)", draft- ietf-add-dnr-00 (work in progress), February 2021. 9.2. Informative References [I-D.ietf-add-ddr] Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T. Jensen, "Discovery of Designated Resolvers", draft-ietf- add-ddr-00 (work in progress), February 2021. [I-D.ietf-dprive-dnsoquic] Huitema, C., Mankin, A., and S. Dickinson, "Specification of DNS over Dedicated QUIC Connections", draft-ietf- dprive-dnsoquic-02 (work in progress), February 2021. [I-D.ietf-v6ops-rfc7084-bis] Martinez, J. P., "Basic Requirements for IPv6 Customer Edge Routers", draft-ietf-v6ops-rfc7084-bis-04 (work in progress), June 2017. [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997, . [RFC3646] Droms, R., Ed., "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, DOI 10.17487/RFC3646, December 2003, . [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, . [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., and P. Hoffman, "Specification for DNS over Transport Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 2016, . [RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, "IPv6 Router Advertisement Options for DNS Configuration", RFC 8106, DOI 10.17487/RFC8106, March 2017, . Boucadair, et al. Expires November 18, 2021 [Page 11] Internet-Draft Discovery of Encrypted DNS Resolvers May 2021 [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., Richardson, M., Jiang, S., Lemon, T., and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 8415, DOI 10.17487/RFC8415, November 2018, . [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, . [RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, January 2019, . [RFC8520] Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage Description Specification", RFC 8520, DOI 10.17487/RFC8520, March 2019, . [RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Kasten, "Automatic Certificate Management Environment (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, . [TR-069] The Broadband Forum, "CPE WAN Management Protocol", December 2018, . [TS.24008] 3GPP, "Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 (Release 16)", December 2019, . Authors' Addresses Mohamed Boucadair (editor) Orange Rennes 35000 France Email: mohamed.boucadair@orange.com Boucadair, et al. Expires November 18, 2021 [Page 12] Internet-Draft Discovery of Encrypted DNS Resolvers May 2021 Tirumaleswar Reddy (editor) McAfee, Inc. Embassy Golf Link Business Park Bangalore, Karnataka 560071 India Email: TirumaleswarReddy_Konda@McAfee.com Dan Wing Citrix Systems, Inc. USA Email: dwing-ietf@fuggles.com Neil Cook Open-Xchange UK Email: neil.cook@noware.co.uk Tommy Jensen Microsoft USA Email: tojens@microsoft.com Boucadair, et al. Expires November 18, 2021 [Page 13]