| Internet-Draft | Encrypted DNS Forwarders on CPEs | July 2024 |
| Reddy, et al. | Expires 9 January 2025 | [Page] |
Typical connectivity service offerings based upon on Customer Premise Equipment (CPEs) involve DNS forwarders on the CPE for various reasons (offer local services, control the scope/content of information in DNS, ensure better dependability for local service, provide control to users, etc.). Upgrading DNS to use encrypted transports introduces deployment complications as to how to sustain current offerings with local services. Solutions are needed to ease operating DNS forwarders in CPEs while allowing to make use of encrypted DNS capabilities.¶
This document describes the problem and why some existing solutions can't be used for these deployments. For example, Star certificates and name constraints extension suffer from the problem of deploying a new feature to CAs, TLS clients, and servers.¶
The scope of this document is encrypted DNS servers deployed on managed CPEs.¶
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Customer Premises Equipment (CPEs, also called Home Routers) are a critical component of the home network, and their security is essential to protecting the devices and data that are connected to them. For example, the prpl Foundation [prpl] has developed a number of initiatives to promote home router security and hardening. The prplWrt project [prplwrt] is an initiative in prpl Foundation that aims to improve the security and performance of open-source router firmware, such as OpenWrt [openwrt]. OpenWrt is an open-source operating system that is designed to run on a wide range of routers and embedded devices. It now includes support for containerization technology such as Docker, making it possible to run containerized applications on a home router. Further, DNS providers have optimized the encrypted DNS forwarder to run in a container in home routers.¶
However, upgrading DNS to use encrypted transports (e.g., DNS over HTTPS (DoH) [RFC8484], DNS over TLS (DoT) [RFC7858], and DNS over QUIC (DoQ) [RFC9250]) introduces deployment complications as to how to sustain current offerings with local services.¶
This document describes the problem encountered to host encrypted DNS resolvers in managed CPEs. It also discusses limitations of existing solutions.¶
The document does not focus on generic considerations related to deploying DNS proxies. The reader may refer to [RFC5625] for such matters.¶
This document makes use of the terms defined in [RFC9499].¶
The following additional terms are used:¶
refers to both DHCPv4 and DHCPv6.¶
refers to unencrypted DNS.¶
refers to the Discovery of Network-designated Resolvers procedure defined in [RFC9463].¶
refers to the Discovery of Designated Resolvers procedure defined in [RFC9462].¶
refers to a scheme where DNS exchanges are transported over an encrypted channel. Examples of encrypted DNS are DoH [RFC8484], DoT [RFC7858], and DoQ [RFC9250].¶
refers to a CPE that is managed by an ISP or CPE vendor or Security Service Provider.¶
Figure 1 shows various network setups where the CPE embeds a caching encrypted DNS forwarder.¶
(a)
,--,--,--. ,--,--,--.
,-' `-. ,-' ISP `-.
Host---( LAN CPE----( DNS Resolver)
| `-. ,-'| `-. ,-'
| `--'--'--' | | `--'--'--'
| |<=DNR=>| |
|<========DNR========>| | |
| | |
|<=====Encrypted=====>|<=Encrypted=>|
| DNS | DNS |
(b)
,--,--,--. ,--,--,--.
,-' `-. ,-' ISP `-. 3rd Party
Host---( LAN CPE----( )--- DNS Resolver
| `-. ,-'| `-. ,-' |
| `--'--'--' | | `--'--'--' |
| |<=DNR=>| |
|<========DNR========>| | |
| | |
|<=====Encrypted=====>|<=========Encrypted DNS======>|
| DNS | |
For all the cases shown in Figure 1, the CPE advertises itself as the default DNS server to the hosts it serves in the LAN. The CPE relies upon DHCP or RA to advertise itself to internal hosts as the default encrypted DNS forwarder using DNR. When receiving a DNS request that a CPE cannot handle locally, the CPE forwards the request to an upstream encrypted DNS. The upstream encrypted DNS can be hosted by the ISP or provided by a third party.¶
Such a forwarder presence is required for (but not limuted to):¶
IPv4 service continuity purposes (e.g., Section 3.1 of [RFC8585]).¶
Supporting advanced services within a local network such as:¶
When the CPE behaves as a DNS forwarder, DNS communications can be decomposed into two legs to resolve queries:¶
This section discusses some deployment challenges to host an encrypted DNS forwarder within a local network.¶
DDR requires proving possession of an IP address, as the DDR certificate contains the server's IPv4 and IPv6 addresses and is signed by a certificate authority. DDR is constrained to public IP addresses because (WebPKI) certificate authorities will not sign special-purpose IP addresses [RFC6890], most notably IPv4 private-use [RFC1918], IPv4 shared address [RFC6598], or IPv6 Unique-Local [RFC8190] address space.¶
A tempting solution for enabling an encrypted DNS forwarder with DDR is to use the CPE's WAN IP address for DDR and prove possession of that IP address. However, the CPE's WAN IPv4 address will not be a public IPv4 address if the CPE is behind another layer of NAT (either Carrier Grade NAT (CGN) or another on-premise NAT), reducing the success of this mechanism to CPE's WAN IPv6 address.¶
Also, if the ISP renumbers the subscriber's network suddenly (rather than slow IPv6 renumbering described in [RFC4192]), encrypted DNS service will be delayed until that new certificate is acquired.¶
DNR requires proving possession of a domain name as the encrypted resolver's certificate contains the FQDN. For example, the entity (e.g., ISP or network administrator) managing the CPE would assign a unique FQDN to the CPE. There are two mechanisms for the CPE to obtain the certificate for the FQDN: using one of its WAN IP addresses or requesting its signed certificate from an Internet-facing server used for remote CPE management (e.g., the Auto Configuration Server (ACS) in the CPE WAN Management Protocol [TR-069]).¶
If the CPE's WAN IP address is used, the CPE needs a public IPv4 or a global unicast IPv6 address together with DNS A or AAAA records pointing to that CPE's WAN address to prove possession of the DNS name to obtain a (WebPKI) CA-signed certificate. That is, the CPE fulfills the DNS or HTTP challenge discussed in Automatic Certificate Management Environment (ACME) [RFC8555]. However, a CPE's WAN address will not be a public IPv4 address if the CPE is behind another layer of NAT (either a CGN or another on-premise NAT), reducing the success of this mechanism to a CPE's WAN IPv6 address. If the ISP renumbers the subscriber's network, the DNS record will also need to expire and changed to reflect the new IP address.¶
The following lists some limitations for certificate issuance:¶
In case of large scale of CPEs (e.g., millions of devices), issuing certificate request for a large number of subdomains could be treated as an attack by the certificate authorities to overwhelm it.¶
Dependency on the CA to issue a large number of certificates.¶
If the CPE uses one of its WAN IP addresses to obtain the certificate for the FQDN, the Internet-facing HTTP server or a DNS authoritative server on the CPE to complete the HTTP or DNS challenge can be subjected to DDoS attacks.¶
Let's consider that the encrypted DNS forwarder is hosted on a CPE and provisioned by a service (e.g., ACS) in the operator's network. Also, let's assume that each CPE is assigned a unique FQDN (e.g., "cpe-12345.example.com" where 12345 is a unique number).¶
It is best to ensure that such an FQDN does not carry any Personally Identifiable Information (PII) or device identification details like the customer number or device's serial number.¶
The CPE generates a public and private key-pair, builds a certificate signing request (CSR), and sends the CSR to a service in the operator managing the CPE. Upon receipt of the CSR, the operator's service can utilize certificate management protocols like ACME [RFC8555] to automate certificate management functions such as domain validation procedure, certificate issuance, and certificate revocation.¶
The challenge with this technique is that the service will have to communicate with the CA to issue certificates for millions of CPEs. If an external CA is unable to issue a certificate in time or replace an expired certificate, the service would no longer be able to present a valid certificate to a CPE. When the service requests certificate issuance for a large number of subdomains (e.g., millions of CPEs), it may be treated as an attacker by the CA to overwhelm it. Furthermore, the short-lived certificates (e.g., certificates that expire after 90 days) issued by the CA will have to be renewed frequently. With short-lived certificates, there is a smaller time window to renew a certificate and, therefore, a higher risk that a CA outage will negatively affect the uptime of the encrypted DNS forwarders on CPEs (and the services offered via these CPEs).¶
These challenges can be addressed by using protocols like ACME to automate the certificate renewal process, ensuring certificates are renewed well before expiration. Additionally, incorporating another CA as a backup can provide redundancy and further mitigate the risk of outages. By having a secondary CA, the service can switch to the backup CA in case the primary CA is unavailable, thus maintaining continuous service availability and reducing the risk of service disruption.¶
It offers the additional advantage of improving the security of Browser and CPE interactions. This ensures that HTTPS access to the CPE is possible, allowing the device administrator to securely communicate with and manage the CPE.¶
A service managing the CPEs could get a CA certificate with name constraints extension (Section 4.2.1.10 of [RFC5280]) and the service would in-turn act as an ACME server to provision end-entity certificates on CPEs.¶
[RFC9115] defines a profile of the ACME protocol for generating Delegated certificates. It allows the CPEs to request from a service managing the CPEs, acting as a profiled ACME server, a certificate for a delegated identity, i.e., one belonging to the service. The service then uses the ACME protocol (with the extensions described in [RFC8739]) to request issuance of a short-term, Automatically Renewed (STAR) certificate for the same delegated identity. The generated short-term certificate is automatically renewed by the ACME CA, periodically fetched by the CPEs, and used to act as encrypted DNS forwarders.¶
The service can end the delegation at any time by instructing the CA to stop the automatic renewal and letting the certificate expire shortly thereafter. Star certificates requires support by CAs but does not require changes to the deployed TLS ecosystem.¶
Cons:¶
Star certificates require support by CAs.¶
A primary use case of Star certificates is that of a Content Delivery Network (CDN), the third party, terminating TLS sessions on behalf of a content provider (the holder of a domain name). The number of star certificates required for a CDN use case will be very much lower than the use case discussed in this draft. It is yet to be seen if CAs will agree to support star certificates at a scale of millions of CPEs.¶
Pro:¶
Avoids changing TLS client and server.¶
DNR-related security considerations are discussed in Section 7 of [RFC9463]. Likewise, DDR-related security considerations are discussed in Section 7 of [RFC9462].¶
The communication between the CPE and endpoints is encrypted using mechanisms such as WPA2/3, and any communication with the DNS server co-located on the CPE is also protected. However, the client does not know whether the DNS server is co-located on the CPE or not. If the client uses clear text DNS (i.e., Do53), it will assume that the DNS messages are susceptible to pervasive monitoring. For instance, in an Enterprise deployment, multiple network devices could exist between an endpoint and the CPE; hosting an encrypted DNS server on the CPE minimizes the impact of a breach, which is an essential zero trust principle. Furthermore, the client and user would be able to identify the entity hosting the encrypted DNS server using the ADN assigned to it.¶
This document has no IANA actions.¶
This draft is triggered by the discussion that occured at IETF#119 (Brisbane) about the need to frame the problem to be solved. Thanks to all the participants to that discussion.¶
Thanks to Neil Cook, Martin Thomson, Tommy Pauly, Benjamin Schwartz, and Michael Richardson for the discussion and comments.¶