DHCP and Router Advertisement Options for the Discovery of Network-designated Resolvers (DNR)

3.1. Configuration Data for Encrypted DNS

In order to allow for PKIX-based authentication between a DNS client and an encrypted DNS server, the Encrypted DNS options are designed to include an authentication domain name. This ADN is presented as a reference identifier for DNS authentication purposes. This design accommodates the current best practices for issuing certificates as per Section 1.7.2 of [RFC6125]:¶

   |  Some certification authorities issue server certificates based on
   |  IP addresses, but preliminary evidence indicates that such
   |  certificates are a very small percentage (less than 1%) of issued
   |  certificates.

To avoid adding a dependency on another server to resolve the ADN, the Encrypted DNS options return the IP address(es) to locate the encrypted DNS server. These encrypted DNS servers may be hosted on the same or distinct IP addresses. Such a decision is deployment specific.¶

In order to optimize the size of discovery messages when all DNS servers terminate on the same IP address, early versions of this document considered relying upon the discovery mechanisms specified in [RFC2132][RFC3646][RFC8106] to retrieve a list of IP addresses to reach their DNS servers. Nevertheless, this approach requires a client that supports more than one encrypted DNS protocol (e.g., DoH and DoT) to probe that list of IP addresses. To avoid such a probing, the options defined in Sections 4, 5, and 6 associate an IP address with an encrypted DNS protocol. No probing is required in such a design.¶

A list of IP addresses to reach an encrypted DNS server may be returned in an Encrypted DNS option to accommodate current deployments relying upon primary and backup servers. Whether one or more IP addresses are returned in an Encrypted DNS option is deployment specific. For example, a router embedding a recursive server or a forwarder has to include one single IP address pointing to one of its LAN-facing interfaces. This IP address can be a private IPv4 address, a link-local address, a Unique Local IPv6 unicast Address (ULA), or a Global Unicast Address (GUA).¶

If more than one IP address are to be returned in an Encrypted DNS option, these addresses are ordered in the preference for use by the client.¶

Because distinct encrypted DNS protocols may be provisioned by a network (e.g., DoT, DoH, and DoQ) and that some of these protocols may make use of customized port numbers instead of default ones, the Encrypted DNS options are designed to return a set of service parameters. These parameters are encoded following the same rules for encoding SvcParams in Section 2.1 of [I-D.ietf-dnsop-svcb-https]. This encoding approach may increase the size of the options but it has the merit to rely upon an existing IANA registry and, thus, to accommodate new encrypted DNS protocols and service parameters that may be defined in the future. For example, "dohpath" service parameter (Section 5.1 of [I-D.ietf-add-svcb-dns]) supplies a relative DoH URI Template.¶

A single option is used to convey both the ADN and IP addresses because otherwise means to correlate an IP address with an ADN will be required if, for example, more than one ADN is supported by the network.¶

The DHCP options defined in Sections 4 and 5 follow the option ordering guidelines in Section 17 of [RFC7227].¶

3.2. Handling Configuration Data Conflicts

If the encrypted DNS is discovered by a host using both RA and DHCP, the rules discussed in Section 5.3.1 of [RFC8106] MUST be followed.¶

DHCP/RA options to discover encrypted DNS servers (including, DoH URI Templates) takes precedence over DDR [I-D.ietf-add-ddr] since DDR uses unencrypted DNS to an external DNS resolver, which is susceptible to both internal and external attacks whereas DHCP/RA is typically protected using the mechanisms discussed in Section 7.1.¶

3.3. Connection Establishment

If the local DNS client supports one of the discovered Encrypted DNS protocols identified by Application Layer Protocol Negotiation (ALPN) protocol identifiers, the DNS client establishes an encrypted DNS session following the order of the discovered servers. The client follows the mechanism discussed in Section 8 of [RFC8310] to authenticate the DNS server certificate using the authentication domain name conveyed in the Encrypted DNS options. ALPN-related considerations can be found in Section 6.1 of [I-D.ietf-dnsop-svcb-https].¶

3.4. Multihoming Considerations

Devices may be connected to multiple networks; each providing their own DNS configuration using the discovery mechanisms specified in this document. Nevertheless, it is out of the scope of this specification to discuss DNS selection of multi-interface devices. The reader may refer to [RFC6731] for a discussion of issues and an example of DNS server selection for multi-interfaced devices.¶

4.2. DHCPv6 Client Behavior

To discover an encrypted DNS server, the DHCPv6 client MUST include OPTION_V6_DNR in an Option Request Option (ORO), as in Sections 18.2.1, 18.2.2, 18.2.4, 18.2.5, 18.2.6, and 21.7 of [RFC8415].¶

The DHCP client MUST be prepared to receive multiple instances of the OPTION_V6_DNR option; each option is to be treated as a separate encrypted DNS server. These instances SHOULD be processed following their service priority (i.e., smaller service priority indicates a higher preference).¶

The DHCPv6 client MUST silently discard multicast and host loopback addresses conveyed in OPTION_V6_DNR.¶

5.1. Option Format

The format of the DHCPv4 Encrypted DNS option is illustrated in Figure 4.¶

 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     TBA2      |     Length    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Service Priority        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Addr Length  |               |
+-+-+-+-+-+-+-+-+               +
~        IPv4 Address(es)       ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   ADN Length  |               |
+-+-+-+-+-+-+-+-+               +
~  authentication-domain-name   ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~Service Parameters (SvcParams) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The fields of the option shown in Figure 4 are as follows:¶

Code:

OPTION_V4_DNR (TBA2, see Section 8.2).¶

Length:

Indicates the length of the enclosed data in octets.¶

Service Priority:

The priority of this OPTION_V4_DNR instance compared to other instances. This field is encoded following the rules specified in Section 2.4.1 of [I-D.ietf-dnsop-svcb-https].¶

Addr Length:

Indicates the length of included IPv4 addresses in octets. It MUST be a multiple of 4.¶

IPv4 Address(es) (variable length):

Indicates one or more IPv4 addresses to reach the encrypted DNS server. Both private and public IPv4 addresses can be included in this field. The format of this field is shown in Figure 5. This format assumes that an IPv4 address is encoded as a1.a2.a3.a4.¶

0     8     16    24    32    40    48
+-----+-----+-----+-----+-----+-----+--
|  a1 |  a2 |  a3 |  a4 |  a1 |  a2 | ...
+-----+-----+-----+-----+-----+-----+--
  IPv4 Address 1          IPv4 Address 2 ...

ADN Length:

Indicates the length of the authentication-domain-name in octets.¶

authentication-domain-name (variable length):

Includes the authentication domain name of the encrypted DNS server. This field is formatted as specified in Section 10 of [RFC8415]. The format of this field is shown in Figure 6. The values s1, s2, s3, etc. represent the domain name labels in the domain name encoding.¶

+-----+-----+-----+-----+-----+--
|  s1 |  s2 |  s3 |  s4 | s5  |  ...
+-----+-----+-----+-----+-----+--
  authentication-domain-name

Service Paramters (SvcParams) (variable length):

Specifies a set of service parameters that are encoded following the rules in Section 2.1 of [I-D.ietf-dnsop-svcb-https]. Service parameters may include, for example, a list of ALPN protocol identifiers or alternate port numbers. The service parameters MUST NOT include "ipv4hint" or "ipv6hint" SvcParams as they are superseded by the included IP addresses.¶

If no port service parameter is included, this indicates that default port numbers should be used.¶

The length of this field is ('Option-length' - 4 - 'ADN Length' - 'Addr Length').¶

OPTION_V4_DNR is a concatenation-requiring option. As such, the mechanism specified in [RFC3396] MUST be used if OPTION_V4_DNR exceeds the maximum DHCPv4 option size of 255 octets.¶

5.2. DHCPv4 Client Behavior

To discover an encrypted DNS server, the DHCPv4 client requests the Encrypted DNS server by including OPTION_V4_DNR in a Parameter Request List option [RFC2132].¶

The DHCPv4 client MUST be prepared to receive multiple instances of the OPTION_V4_DNR option; each option is to be treated as a separate encrypted DNS server. These instances SHOULD be processed following their service priority (i.e., smaller service priority indicates a higher preference).¶

The DHCPv4 client MUST silently discard multicast and host loopback addresses conveyed in OPTION_V4_DNR.¶

6.2. IPv6 Host Behavior

The procedure for DNS configuration is the same as it is with any other Neighbor Discovery option [RFC4861]. In addition, the host follows the procedure described in Section 5.3.1 of [RFC8106].¶

The host MUST silently discard multicast and host loopback addresses conveyed in the Encrypted DNS options.¶

7.2. Deletion Attacks

If the DHCP responses or RAs are dropped by the attacker, the client can fallback to use a preconfigured encrypted DNS server. However, the use of policies to select servers is out of the scope of this document.¶

Note that deletion attack is not specific to DHCP/RA.¶

7.3. Passive Attacks

A passive attacker (Section 3.2 of [RFC3552]) can identify a host is using DHCP/RA to discover an encrypted DNS server and can infer that host is capable of using DoH/DoT/DoQ to encrypt DNS messages. However, a passive attacker cannot spoof or modify DHCP/RA messages.¶

7.4. Wireless Security - Authentication Attacks

Wireless LAN (WLAN) as frequently deployed in local networks (e.g., home networks) is vulnerable to various attacks (e.g., [Evil-Twin], [Krack], [Dragonblood]). Because of these attacks, only cryptographically authenticated communications are trusted on WLANs. This means that an information (e.g., NTP server, DNS server, default domain) provided by such networks via DHCP, DHCPv6, or RA are untrusted because DHCP and RA messages are not authenticated.¶

If the pre-shared key is the same for all clients that connect to the same WLAN, the shared key will be available to all nodes, including attackers. As such, it is possible to mount an active on-path attack. Man-in-the-middle attacks are possible within local networks because such WLAN authentication lacks peer entity authentication.¶

This leads to the need for provisioning unique credentials for different clients. Endpoints can be provisioned with unique credentials (username and password, typically) provided by the local network administrator to mutually authenticate to the local WLAN Access Point (e.g., 802.1x Wireless User Authentication on OpenWRT [dot1x], EAP-pwd [RFC8146]). Not all endpoint devices (e.g., IoT devices) support 802.1x supplicant and need an alternate mechanism to connect to the local network. To address this limitation, unique pre-shared keys can be created for each such device and WPA-PSK is used (e.g., [PSK]).¶