ADD M. Boucadair
Internet-Draft Orange
Intended status: Standards Track T. Reddy
Expires: August 26, 2021 McAfee
D. Wing
Citrix
N. Cook
Open-Xchange
T. Jensen
Microsoft
February 22, 2021
DHCP and Router Advertisement Options for the Discovery of Network-
designated Resolvers (DNR)
draft-ietf-add-dnr-00
Abstract
The document specifies new DHCP and IPv6 Router Advertisement options
to discover encrypted DNS servers (e.g., DNS-over-HTTPS, DNS-over-
TLS, DNS-over-QUIC). Particularly, it allows to learn an
authentication domain name together with a list of IP addresses and a
port number to reach such encrypted DNS servers. The discovery of
DNS-over-HTTPS URI Templates is also discussed.
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 August 26, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview and Rationale . . . . . . . . . . . . . . . . . . . 4
4. DHCPv6 Encrypted DNS Options . . . . . . . . . . . . . . . . 6
4.1. Encrypted DNS ADN Option . . . . . . . . . . . . . . . . 6
4.2. Encrypted DNS Address Option . . . . . . . . . . . . . . 7
4.3. DHCPv6 Client Behavior . . . . . . . . . . . . . . . . . 9
5. DHCPv4 Encrypted DNS Option . . . . . . . . . . . . . . . . . 9
5.1. Encrypted DNS Option . . . . . . . . . . . . . . . . . . 9
5.2. DHCPv4 Client Behavior . . . . . . . . . . . . . . . . . 11
6. IPv6 RA Encrypted DNS Options . . . . . . . . . . . . . . . . 11
6.1. Encrypted DNS ADN Option . . . . . . . . . . . . . . . . 12
6.2. Encrypted DNS Address Option . . . . . . . . . . . . . . 13
7. DoH URI Templates . . . . . . . . . . . . . . . . . . . . . . 14
8. Hosting Encrypted DNS Forwarder in Local Networks . . . . . . 16
8.1. Managed CPEs . . . . . . . . . . . . . . . . . . . . . . 16
8.1.1. DNS Forwarders . . . . . . . . . . . . . . . . . . . 16
8.1.2. ACME . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1.3. Auto-Upgrade Based on Domains and their Subdomains . 16
8.2. Unmanaged CPEs . . . . . . . . . . . . . . . . . . . . . 17
9. Legacy CPEs . . . . . . . . . . . . . . . . . . . . . . . . . 18
10. Security Considerations . . . . . . . . . . . . . . . . . . . 18
10.1. Spoofing Attacks . . . . . . . . . . . . . . . . . . . . 18
10.2. Deletion Attacks . . . . . . . . . . . . . . . . . . . . 19
10.3. Passive Attacks . . . . . . . . . . . . . . . . . . . . 19
10.4. Wireless Security - Authentication Attacks . . . . . . . 20
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
11.1. Encrypted DNS Flag Bits . . . . . . . . . . . . . . . . 20
11.2. DHCPv6 Options . . . . . . . . . . . . . . . . . . . . . 21
11.3. DHCPv4 Option . . . . . . . . . . . . . . . . . . . . . 21
11.4. Neighbor Discovery Options . . . . . . . . . . . . . . . 21
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
13. Contributing Authors . . . . . . . . . . . . . . . . . . . . 22
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
14.1. Normative References . . . . . . . . . . . . . . . . . . 22
14.2. Informative References . . . . . . . . . . . . . . . . . 23
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Appendix A. Sample Target Deployment Scenarios . . . . . . . . . 26
A.1. Managed CPEs . . . . . . . . . . . . . . . . . . . . . . 28
A.1.1. Direct DNS . . . . . . . . . . . . . . . . . . . . . 28
A.1.2. Proxied DNS . . . . . . . . . . . . . . . . . . . . . 30
A.2. Unmanaged CPEs . . . . . . . . . . . . . . . . . . . . . 31
A.2.1. ISP-facing Unmanaged CPEs . . . . . . . . . . . . . . 31
A.2.2. Internal Unmanaged CPEs . . . . . . . . . . . . . . . 31
Appendix B. Make Use of Discovered Encrypted DNS Servers . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction
This document focuses on the support of encrypted DNS 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.
In particular, the document specifies how a local encrypted DNS
server can be discovered and used 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 optionally a port number. The discovery of
DoH URI Templates is discussed in Section 7.
Sample target deployment scenarios are discussed in Appendix A; both
managed and unmanaged Customer Premises Equipment (CPEs) are covered.
It is out of the scope of this document to provide an exhaustive
inventory of deployments where Encrypted DNS Options (Sections 4, 5,
and 6) can be used.
Considerations related to hosting a DNS forwarder in a local network
are described in Section 8.
2. Terminology
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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
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are DNS-over-TLS (DoT) [RFC7858], DNS-over-HTTPS (DoH) [RFC8484],
or DNS-over-QUIC (DoQ) [I-D.ietf-dprive-dnsoquic].
Managed CPE: refers to a CPE that is managed by an Internet Service
Providers (ISP).
Unmanaged CPE: refers to a CPE that is not managed by an ISP.
DHCP: refers to both DHCPv4 and DHCPv6.
3. Overview and Rationale
This document describes how a DNS client can discover a local
encrypted DNS server(s) using DHCP (Sections 4 and 5) and Neighbor
Discovery protocol (Section 6).
As reported in 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.
In order to allow for PKIX-based authentication between a DNS client
and an encrypted DNS server while accommodating the current best
practices for issuing certificates, this document allows for
configuring an authentication domain name to be presented as a
reference identifier for DNS authentication purposes.
To avoid adding a dependency on another server to resolve the ADN,
the options return a list of IP addresses to locate the encrypted DNS
server. In the various scenarios sketched in Appendix A, encrypted
DNS servers may terminate on the same IP address or distinct IP
addresses. Terminating encrypted DNS servers on the same or distinct
IP addresses is deployment specific. It is RECOMMENDED to return
both the ADN and a list of IP addresses to a requesting host.
Note that in order to optimize the size of discovery messages when
all servers terminate on the same IP address, a host may rely 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 to probe that list of IP addresses. To avoid such
probing, the options defined in the following sections associate an
IP address with an encrypted DNS type. No probing is required in
such design.
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A list of IP addresses to reach an encrypted DNS server can be
returned in the option to accommodate current deployments relying
upon primary and backup servers. Whether one IP address or more are
returned in an option is deployment specific. For example, a router
embedding a recursive server or forwarder has to include one single
IP address pointing to one of its LAN-facing interfaces. This
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
server option, these addresses are ordered in the preference for use
by the client.
Because DoT and DoQ may make use of customized port numbers instead
of default ones, the Encrypted DNS server options are designed to
return alternate port numbers.
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.
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.
The DNS client establishes an encrypted DNS session with the
discovered DNS IP address(es) and port number, and uses 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.
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.
DHCP/RA options to discover encrypted DNS servers (including, DoH URI
Templates should the WG pursue that approach pending feedback) takes
precedence over DEER [I-D.pauly-add-deer] since DEER uses unencrypted
DNS to an external DNS resolver, which is susceptible to both
internal and external attacks whereas DHCP/RA is only vulnerable to
internal attacks.
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4. DHCPv6 Encrypted DNS Options
This section defines two DHCPv6 options: DHCPv6 Encrypted DNS ADN
option (Section 4.1) and DHCPv6 Encrypted DNS Address option
(Section 4.2).
4.1. Encrypted DNS ADN Option
The DHCPv6 Encrypted DNS ADN option is used to configure an
authentication domain name of the encrypted DNS server. The format
of this option is shown in Figure 1.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_V6_DNR_ADN | Option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enc DNS Flags | |
+---------------+ +
| |
~ authentication-domain-name ~
| |
+---------------------------------------------------------------+
Figure 1: DHCPv6 Encrypted DNS ADN Option
The fields of the option shown in Figure 1 are as follows:
Option-code: OPTION_V6_DNR_ADN (TBA1, see Section 11.2)
Option-length: Length of the enclosed data in octets.
Enc DNS Flags (Encrypted DNS Flags): Indicates the type(s) of the
encrypted DNS server conveyed in this attribute. The format of
this 8-bit field is shown in Figure 2.
+-+-+-+-+-+-+-+-+
|U|U|U|U|U|Q|H|T|
+-+-+-+-+-+-+-+-+
Figure 2: Encrypted DNS Flags Field
T: If set, this bit indicates that the server supports DoT
[RFC7858].
H: If set, this bit indicates that the server supports DoH
[RFC8484].
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Q: If set, this bit indicates that the server supports DoQ
[I-D.ietf-dprive-dnsoquic].
U: Unassigned bits. These bits MUST be unset by the sender.
Associating a meaning with an unassigned bit can be done as per
Section 11.1.
In a request, these bits are assigned to indicate the requested
encrypted DNS server type(s) by the client. In a response, these
bits are set as a function of the encrypted DNS supported by the
server and the requested encrypted DNS server type(s).
To keep the packet small, if more than one encrypted DNS type
(e.g., both DoH and DoT) are to be returned to a requesting client
and the same ADN is used for these types, the corresponding bits
must be set in the 'Encrypted DNS Types' field of the same option
instance in a response. For example, if the client requested DoH
and DoT and the server supports both with the same ADN, then both
T and H bits must be set.
authentication-domain-name: A fully qualified domain name of the
encrypted DNS server. This field is formatted as specified in
Section 10 of [RFC8415].
An example of the authentication-domain-name encoding is shown in
Figure 3. This example conveys the FQDN "doh1.example.com.", and
the resulting Option-length field is 18.
+------+------+------+------+------+------+------+------+------+
| 0x04 | d | o | h | 1 | 0x07 | e | x | a |
+------+------+------+------+------+------+------+------+------+
| m | p | l | e | 0x03 | c | o | m | 0x00 |
+------+------+------+------+------+------+------+------+------+
Figure 3: An Example of the DNS authentication-domain-name Encoding
4.2. Encrypted DNS Address Option
The DHCPv6 Encrypted DNS Address option is used to configure a list
of IP addresses and a port number of the encrypted DNS server. The
format of this option is shown in Figure 4.
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_V6_DNR_ADD | Option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enc DNS Flags | Unassigned | Port Number |
+---------------+---------------+-------------------------------+
| |
| ipv6-address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ipv6-address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: DHCPv6 Encrypted DNS Address Option
The fields of the option shown in Figure 4 are as follows:
Option-code: OPTION_V6_DNR_ADD (TBA2, see Section 11.2)
Option-length: Length of the enclosed data in octets.
Enc DNS Flags (Encrypted DNS Flags): Indicates the type(s) of the
encrypted DNS server conveyed in this attribute. The format of
this 8-bit field is shown in Figure 2. In a request, these bits
are set to indicate the requested encrypted DNS server type(s) by
the client. In a response, these bits are set as a function of
the encrypted DNS supported by the server and the requested
encrypted DNS server type(s).
Unassigned: These bits MUST be unset by the sender. Associating a
meaning with an unassigned bit can be done via Standards Action
[RFC8126].
Port Number: If not null, it indicates the port number to be used
for the encrypted DNS. If this field is set to zero, this
indicates that default port numbers should be used. As a
reminder, the default port number is 853 for DoT and 443 for DoH.
ipv6-address(es): Indicates one or more IPv6 addresses to reach the
encrypted DNS server. An address can be link-local, ULA, or GUA.
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Multiple instances of OPTION_V6_DNR_ADN (or OPTION_V6_DNR_ADD) may be
returned to a DHCPv6 client; each pointing to a distinct encrypted
DNS server type.
If more than one encrypted DNS server types is supported on the same
IP address and default port numbers are used, one instance of
OPTION_V6_DNR_ADD option with the appropriate bits set in "Encr DNS
Types" field should be returned by the DHCP server.
4.3. DHCPv6 Client Behavior
To discover an encrypted DNS server, the DHCPv6 client MUST include
OPTION_V6_DNR_ADN and OPTION_V6_DNR_ADD 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 DHCPv6 client sets the Encrypted DNS Types
field to the requested encrypted DNS server type(s).
If the DHCPv6 client requested more than one encrypted DNS server
type, the DHCP client MUST be prepared to receive multiple
OPTION_V6_DNR_ADN (or OPTION_V6_DNR_ADD) options; each option is to
be treated as a separate encrypted DNS server.
The DHCPv6 client MUST silently discard multicast and host loopback
addresses conveyed in OPTION_V6_DNR_ADD.
5. DHCPv4 Encrypted DNS Option
5.1. Encrypted DNS Option
The DHCPv4 Encrypted DNS option is used to configure an
authentication domain name, a list of IP addresses, and a port number
of the encrypted DNS server. The format of this option is
illustrated in Figure 5.
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBA3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enc DNS Flags | Num Addresses |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ IPv4 Address(es) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ authentication-domain-name ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: DHCPv4 Encrypted DNS Option
The fields of the option shown in Figure 5 are as follows:
Code: OPTION_V4_DNR_DNS (TBA3, see Section 11.3).
Length: Length of the enclosed data in octets.
Enc DNS Flags (Encrypted DNS Flags): Indicates the type(s) of the
encrypted DNS server conveyed in this attribute. The format of
this field is shown in Figure 2.
Num Addresses: Indicates the number of included IPv4 addresses.
Port Number: If not null, it indicates the port number to be used
for the encrypted DNS. A null value indicates that default port
numbers are used. As a reminder, the default port number is 853
for DoT and 443 for DoH.
IPv4 Address(es): 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 6. This format assumes that an IPv4 address is encoded as
a1.a2.a3.a4.
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0 8 16 24 32 40 48
+-----+-----+-----+-----+-----+-----+--
| a1 | a2 | a3 | a4 | a1 | a2 | ...
+-----+-----+-----+-----+-----+-----+--
IPv4 Address 1 IPv4 Address 2 ...
Figure 6: Format of the IPv4 Addresses Field
authentication-domain-name: The 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 7. The
values s1, s2, s3, etc. represent the domain name labels in the
domain name encoding.
+-----+-----+-----+-----+-----+--
| s1 | s2 | s3 | s4 | s5 | ...
+-----+-----+-----+-----+-----+--
authentication-domain-name
Figure 7: Format of the Authentication Domain Name Field
OPTION_V4_DNR_DNS is a concatenation-requiring option. As such, the
mechanism specified in [RFC3396] MUST be used if OPTION_V4_DNR_DNS
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_DNS in a Parameter
Request List option [RFC2132]. The DHCPv4 client sets the Encrypted
DNS Types field to the requested encrypted DNS server.
If the DHCPv4 client requested more than one encrypted DNS server
type, the DHCPv4 client MUST be prepared to receive multiple DHCP
OPTION_V4_DNR_DNS options; each option is to be treated as a separate
encrypted DNS server.
The DHCPv4 client MUST silently discard multicast and host loopback
addresses conveyed in OPTION_V4_DNR_DNS.
6. IPv6 RA Encrypted DNS Options
This section defines two Neighbor Discovery [RFC4861]: IPv6 Router
Advertisement (RA) Encrypted DNS ADN option (Section 6.1) and IPv6 RA
Encrypted DNS Address option (Section 6.2). These options are useful
in contexts similar to those discussed in Section 1.1 of [RFC8106].
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6.1. Encrypted DNS ADN Option
The IPv6 RA Encrypted DNS ADN option is used to configure an
authentication domain name of the encrypted DNS server. The format
of this option is illustrated in Figure 8.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBA4 | Length | Enc DNS Flags | Unassigned |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ authentication-domain-name ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: RA Encrypted DNS ADN Option
The fields of the option shown in Figure 8 are as follows:
Type: 8-bit identifier of the Encrypted DNS Option as assigned by
IANA (TBA4, see Section 11.4).
Length: 8-bit unsigned integer. The length of the option (including
the Type and Length fields) is in units of 8 octets.
Enc DNS Flags (Encrypted DNS Flags): Indicates the type(s) of the
encrypted DNS server conveyed in this attribute. The format of
this field is shown in Figure 2.
Unassigned: This field is unused. It MUST be initialized to zero by
the sender and MUST be ignored by the receiver.
Lifetime: 32-bit unsigned integer. The maximum time in seconds
(relative to the time the packet is received) over which the
discovered Authentication Domain Name is valid.
The value of Lifetime SHOULD by default be at least 3 *
MaxRtrAdvInterval, where MaxRtrAdvInterval is the maximum RA
interval as defined in [RFC4861].
A value of all one bits (0xffffffff) represents infinity.
A value of zero means that this Authentication Domain Name MUST no
longer be used.
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authentication Domain Name: The domain name of the encrypted DNS
server. This field is formatted as specified in Section 10 of
[RFC8415].
This field MUST be padded with zeros so that its size is a
multiple of 8 octets.
6.2. Encrypted DNS Address Option
The IPv6 RA Encrypted DNS Address option is used to configure a port
number and a list of IPv6 addresses of the encrypted DNS server. The
format of this option is illustrated in Figure 9. All of the
addresses share the same Lifetime value. Similar to [RFC8106], if it
is desirable to have different Lifetime values per IP address,
multiple Encrypted DNS Address options may be used.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBA5 | Length | Unassigned |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enc DNS Flags | Unassigned | Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ipv6-address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ipv6-address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: RA Encrypted DNS Address Option
The fields of the RA Encrypted DNS Address option shown in Figure 9
are as follows:
Type: 8-bit identifier of the Encrypted DNS Address Option as
assigned by IANA (TBA5, see Section 11.4).
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Length: 8-bit unsigned integer. The length of the option (including
the Type and Length fields) is in units of 8 octets.
Unassigned: This field is unused. It MUST be initialized to zero by
the sender and MUST be ignored by the receiver.
Lifetime: 32-bit unsigned integer. The maximum time in seconds
(relative to the time the packet is received) over which the
discovered encrypted DNS IPv6 addresses are valid.
The value of Lifetime SHOULD by default be at least 3 *
MaxRtrAdvInterval, where MaxRtrAdvInterval is the maximum RA
interval as defined in [RFC4861].
A value of all one bits (0xffffffff) represents infinity.
A value of zero means that these IPv6 addresses MUST no longer be
used.
Enc DNS Flags (Encrypted DNS Flags): Indicates the type(s) of the
encrypted DNS server conveyed in this attribute. The format of
this field is shown in Figure 2.
Port Number: If not null, it indicates the port number to be used
for the encrypted DNS. A null value indicates that default port
numbers must be used. As a reminder, the default port number is
853 for DoT and 443 for DoH.
ipv6-address(es): One or more IPv6 addresses of the encrypted DNS
server. An address can be link-local, ULA, or GUA.
7. DoH URI Templates
DoH servers may support more than one URI Template [RFC8484]. Also,
if the resolver hosts several DoH services (e.g., no-filtering,
blocking adult content, blocking malware), these services can be
discovered as templates. The following discusses a mechanism for a
DoH client to retrieve the list of supported templates by a DoH
server.
Upon discovery of a DoH resolver (Sections 4, 5, and 6), the DoH
client may contact that DoH resolver to retrieve the list of
supported DoH services using DEER [I-D.pauly-add-deer]. This will
allow the client to discover the resolver's supported DoH templates
or DoH resolvers that the discovered resolver designates using DNS
SVCB queries [I-D.schwartz-svcb-dns]. The designated DoH resolvers
and DoH resolver discovered using DHCP/RA may be hosted on the same
or distinct IP addresses.
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Let's suppose that a host has discovered an encrypted DNS server that
is DoH-capable. The host has also discovered the following
information:
o ADN: doh.example.com
o Locator: 2001:db8:1::1
The client will use DEER [I-D.pauly-add-deer] to discover the DoH
templates supported by the DNS server at the Locator (2001:db8:1::1).
In addition to the checks included in DEER, clients should verify the
ADN (doh.example.com) is valid for the certificate provided by the
DoH resolver. However, the IP address of the DEER-discovered
resolver may differ from the Locator field value. This will allow
the ISP to offer different DoH services to the endpoints attached to
local networks.
Alternatively, dedicated DHCP/RA options may be defined to convey an
URI template in order to avoid additional network traffic to
bootstrap DoH configuration. An example of the format of such an
option is depicted in Figure 10.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_V6_DOH_TEMPLATE | Option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ uri-template-data ~
| . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each instance of the uri-template-data is formatted as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
| uri-template-len | URI Template |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
Figure 10: Example of a DHCPv6 URI Template Option
Note: More feedback from the WG is needed to decide which
approach(es) to follow.
How a DoH client makes use of the configured DoH services is out of
the scope of this document.
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8. Hosting Encrypted DNS Forwarder in Local Networks
This section discusses some deployment considerations (not
recommendations) to host an encrypted DNS forwarder within a local
network.
8.1. Managed CPEs
The section discusses mechanisms that can be used to host an
encrypted DNS forwarder in a managed CPE (Appendix A.1).
8.1.1. 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.
8.1.2. 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.
8.1.3. Auto-Upgrade Based on Domains and their Subdomains
If the ADN conveyed in DHCP/RA (Sections 4, 5, and 6) is
preconfigured in popular OSes or browsers as a verified resolver and
the auto-upgrade (Appendix B) is allowed for both the preconfigured
ADN and its sub-domains, the encrypted DNS client will learn the
local encrypted DNS forwarder using DHCP/RA and auto-upgrade because
the preconfigured ADN would match the subjectAltName value in the
server certificate. For example, if the preconfigured ADN is
"*.example.com" and the discovered encrypted DNS forwarder is
"cpe1.example.com", auto-upgrade will take place.
In this case, the CPE can communicate the ADN of the local DoH
forwarder (Section 8.1.2) to internal hosts using DHCP/RA (Sections
4, 5, and 6).
Let's suppose that "*.example.net" is preconfigured as a verified
resolved in the browser or OS. If the encrypted DNS client discovers
a local forwarder "cpe1-internal.example.net", the encrypted DNS
client will auto-upgrade because the preconfigured ADN would match
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subjectAltName value "cpe1-internal.example.net" of type dNSName. As
shown in Figure 11, the auto-upgrade to a rogue server advertising
"rs.example.org" will fail because it does not match "*.example.net".
Encrypted DNS CPE
capable client (@i)
| |
|<=================DHCP===============|
| {ADN=cpe1-internal.example.net, @i} |
| |
| Rogue Server |
| (@rs) |
| | |
X<===========DHCP=========| |
|{ADN=rs.example.org, @rs}| |
| | |
| |
|<=================DoH===============>|
| |
Legend:
* @i: internal IP address of the CPE
* @rs: IP address of a rogue server
Figure 11: A Simplified Example of Auto-upgrade based on Subdomains
8.2. Unmanaged CPEs
The approach specified in Section 8.1 does not apply for hosting a
DNS forwarder in an unmanaged CPE.
The unmanaged CPE administrator (referred to as 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
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.
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Figure 12 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.
,--,--,--. ,--,
,' 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 12: Example of an Internal CPE Hosting a Forwarder
9. Legacy CPEs
Hosts serviced by legacy CPEs that can't be upgraded to support the
options defined in Sections 4, 5, and 6 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
DEER as defined in [I-D.pauly-add-deer] to discover the encrypted DNS
server and to retrieve the list of supported DoH services using the
SVCB RRtype [I-D.schwartz-svcb-dns] without verifying the hostname of
discovered templates with the ADN. Other guidance in DEER relating
to resolver verification must be followed in this case. This will
prevent an unencrypted resolver on a local address from referring to
an encrypted resolver at a different address without an out-of-band
configuration in the client beyond the scope of this document or
DEER.
10. Security Considerations
10.1. Spoofing Attacks
DHCP/RA messages are not encrypted or protected against modification
within the LAN. Unless mitigated (described below), the content of
DHCP and RA messages can be spoofed or modified by active attackers,
such as compromised devices within the local network. An active
attacker (Section 3.3 of [RFC3552]) can spoof the DHCP/RA response to
provide the attacker's Encrypted DNS server. Note that such an
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attacker can launch other attacks as discussed in Section 22 of
[RFC8415]. The attacker can get a domain name with a domain-
validated public certificate from a CA and host an Encrypted DNS
server. Also, an attacker can use a public IP address and get an 'IP
address'-validated public certificate from a CA to host an Encrypted
DNS server.
Attacks of spoofed or modified DHCP responses and RA messages by
attackers within the local network may be mitigated by making use of
the following mechanisms:
o DHCPv6-Shield described in [RFC7610], the CPEs discards DHCP
response messages received from any local endpoint.
o RA-Guard described in [RFC7113], the CPE discards RAs messages
received from any local endpoint.
o Source Address Validation Improvement (SAVI) solution for DHCP
described in [RFC7513], the CPE filters packets with forged source
IP addresses.
Encrypted DNS sessions with rogue servers that spoof the IP address
of a DNS server will fail because the DNS client will fail to
authenticate that rogue server based upon PKIX authentication
[RFC6125], particularly the authentication domain name in the
Encrypted DNS Option. DNS clients that ignore authentication
failures and accept spoofed certificates will be subject to attacks
(e.g., redirect to malicious servers, intercept sensitive data).
Encrypted DNS connections received from outside the local network
MUST be discarded by the encrypted DNS forwarder in the CPE. This
behavior adheres to REQ#8 in [RFC6092]; it MUST apply for both IPv4
and IPv6.
10.2. 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.
10.3. 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
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host is capable of using DoH/DoT/DoQ to encrypt DNS messages.
However, a passive attacker cannot spoof or modify DHCP/RA messages.
10.4. 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 information provided by such networks via DHCP, DHCPv6, or
RA (e.g., NTP server, DNS server, default domain) are untrusted
because DHCP and RA 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, so 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 of 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]).
11. IANA Considerations
11.1. Encrypted DNS Flag Bits
1 2 3 4 5 6 7 8
+-+-+-+-+-+-+-+-+
Encrypted DNS Types is a set of 8 flags: |U|U|U|U|U|Q|H|T|
+-+-+-+-+-+-+-+-+
where flag bits in positions 1-5 are for future assignment as
additional flag bits.
This document requests IANA to create a new registry called
"Encrypted DNS Types". The initial values of the registry are as
follows:
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+--------------+-------+---------------------+----------------+
| Bit Position | Label | Description | Reference |
+--------------+-------+---------------------+----------------+
| 1 | U | Unassigned | |
| 2 | U | Unassigned | |
| 3 | U | Unassigned | |
| 4 | U | Unassigned | |
| 5 | U | Unassigned | |
| 6 | Q | DNS-over-QUIC (DoQ) | [ThisDocument] |
| 7 | H | DNS-over-HTTP (DoH) | [ThisDocument] |
| 8 | T | DNS-over-TLS (DoT) | [ThisDocument] |
+--------------+-------+---------------------+----------------+
New flag bits are assigned via Standards Action [RFC8126].
11.2. DHCPv6 Options
IANA is requested to assign the following new DHCPv6 Option Code in
the registry maintained in [DHCPV6].
+-------+-------------------+---------+------------+----------------+
| Value | Description | Client | Singleton | Reference |
| | | ORO | Option | |
+-------+-------------------+---------+------------+----------------+
| TBA1 | OPTION_V6_DNR_ADN | Yes | No | [ThisDocument] |
| TBA2 | OPTION_V6_DNR_ADD | Yes | No | [ThisDocument] |
+-------+-------------------+---------+------------+----------------+
11.3. DHCPv4 Option
IANA is requested to assign the following new DHCP Option Code in the
registry maintained in [BOOTP].
+------+------------------+-------+----------------+----------------+
| Tag | Name | Data | Meaning | Reference |
| | | Length| | |
+------+------------------+-------+----------------+----------------+
| TBA3 | OPTION_V4_DNR_DNS| N | Encrypted DNS | [ThisDocument] |
| | | | Server | |
+------+------------------+-------+----------------+----------------+
11.4. Neighbor Discovery Options
IANA is requested to assign the following new IPv6 Neighbor Discovery
Option type in the "IPv6 Neighbor Discovery Option Formats" sub-
registry under the "Internet Control Message Protocol version 6
(ICMPv6) Parameters" registry maintained in [ND].
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+------+----------------------------------+----------------+
| Type | Description | Reference |
+------+----------------------------------+----------------+
| TBA4 | DNS Encrypted DNS ADN Option | [ThisDocument] |
| TBA5 | DNS Encrypted DNS Address Option | [ThisDocument] |
+------+----------------------------------+----------------+
12. Acknowledgements
Many thanks to Christian Jacquenet and Michael Richardson for the
review.
Thanks to Stephen Farrell, Martin Thomson, Vittorio Bertola, Stephane
Bortzmeyer, Ben Schwartz, and Iain Sharp for the comments.
Thanks to Mark Nottingham for the feedback on HTTP redirection.
The use of DHCP to retrieve an authentication domain name was
discussed in Section 7.3.1 of [RFC8310] and
[I-D.pusateri-dhc-dns-driu].
13. Contributing Authors
Nicolai Leymann
Deutsche Telekom
Germany
Email: n.leymann@telekom.de
Zhiwei Yan
CNNIC
No.4 South 4th Street, Zhongguancun
Beijing 100190
China
EMail: yan@cnnic.cn
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<https://www.rfc-editor.org/info/rfc2132>.
[RFC3396] Lemon, T. and S. Cheshire, "Encoding Long Options in the
Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
DOI 10.17487/RFC3396, November 2002,
<https://www.rfc-editor.org/info/rfc3396>.
[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,
<https://www.rfc-editor.org/info/rfc4861>.
[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,
<https://www.rfc-editor.org/info/rfc8106>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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,
<https://www.rfc-editor.org/info/rfc8415>.
14.2. Informative References
[Auto-upgrade]
The Unicode Consortium, "DoH providers: criteria, process
for Chrome", <docs.google.com/document/
d/128i2YTV2C7T6Gr3I-81zlQ-_Lprnsp24qzy_20Z1Psw/edit>.
[BOOTP] "BOOTP Vendor Extensions and DHCP Options",
<https://www.iana.org/assignments/bootp-dhcp-parameters/
bootp-dhcp-parameters.xhtml#options>.
[DHCPV6] "DHCPv6 Option Codes", <https://www.iana.org/assignments/
dhcpv6-parameters/dhcpv6-parameters.xhtml#dhcpv6-
parameters-2>.
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[dot1x] Cisco, "Basic 802.1x Wireless User Authentication",
<https://openwrt.org/docs/guide-user/network/wifi/
wireless.security.8021x>.
[Dragonblood]
The Unicode Consortium, "Dragonblood: Analyzing the
Dragonfly Handshake of WPA3 and EAP-pwd",
<https://papers.mathyvanhoef.com/dragonblood.pdf>.
[Evil-Twin]
The Unicode Consortium, "Evil twin (wireless networks)",
<https://en.wikipedia.org/wiki/
Evil_twin_(wireless_networks)>.
[I-D.ietf-dprive-dnsoquic]
Huitema, C., Mankin, A., and S. Dickinson, "Specification
of DNS over Dedicated QUIC Connections", draft-ietf-
dprive-dnsoquic-01 (work in progress), October 2020.
[I-D.ietf-v6ops-rfc7084-bis]
Palet, J., "Basic Requirements for IPv6 Customer Edge
Routers", draft-ietf-v6ops-rfc7084-bis-04 (work in
progress), June 2017.
[I-D.pauly-add-deer]
Pauly, T., Kinnear, E., Wood, C., McManus, P., and T.
Jensen, "Discovery of Equivalent Encrypted Resolvers",
draft-pauly-add-deer-00 (work in progress), November 2020.
[I-D.pusateri-dhc-dns-driu]
Pusateri, T. and W. Toorop, "DHCPv6 Options for private
DNS Discovery", draft-pusateri-dhc-dns-driu-00 (work in
progress), July 2018.
[I-D.schwartz-svcb-dns]
Schwartz, B., "Service Binding Mapping for DNS Servers",
draft-schwartz-svcb-dns-01 (work in progress), August
2020.
[Krack] The Unicode Consortium, "Key Reinstallation Attacks",
2017, <https://www.krackattacks.com/>.
[ND] "IPv6 Neighbor Discovery Option Formats",
<http://www.iana.org/assignments/icmpv6-parameters/
icmpv6-parameters.xhtml#icmpv6-parameters-5>.
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[PSK] Cisco, "Identity PSK Feature Deployment Guide",
<https://www.cisco.com/c/en/us/td/docs/wireless/
controller/technotes/8-5/
b_Identity_PSK_Feature_Deployment_Guide.html>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[RFC3646] Droms, R., Ed., "DNS Configuration options for Dynamic
Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
DOI 10.17487/RFC3646, December 2003,
<https://www.rfc-editor.org/info/rfc3646>.
[RFC6092] Woodyatt, J., Ed., "Recommended Simple Security
Capabilities in Customer Premises Equipment (CPE) for
Providing Residential IPv6 Internet Service", RFC 6092,
DOI 10.17487/RFC6092, January 2011,
<https://www.rfc-editor.org/info/rfc6092>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC6731] Savolainen, T., Kato, J., and T. Lemon, "Improved
Recursive DNS Server Selection for Multi-Interfaced
Nodes", RFC 6731, DOI 10.17487/RFC6731, December 2012,
<https://www.rfc-editor.org/info/rfc6731>.
[RFC7113] Gont, F., "Implementation Advice for IPv6 Router
Advertisement Guard (RA-Guard)", RFC 7113,
DOI 10.17487/RFC7113, February 2014,
<https://www.rfc-editor.org/info/rfc7113>.
[RFC7513] Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address
Validation Improvement (SAVI) Solution for DHCP",
RFC 7513, DOI 10.17487/RFC7513, May 2015,
<https://www.rfc-editor.org/info/rfc7513>.
[RFC7610] Gont, F., Liu, W., and G. Van de Velde, "DHCPv6-Shield:
Protecting against Rogue DHCPv6 Servers", BCP 199,
RFC 7610, DOI 10.17487/RFC7610, August 2015,
<https://www.rfc-editor.org/info/rfc7610>.
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[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, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8146] Harkins, D., "Adding Support for Salted Password Databases
to EAP-pwd", RFC 8146, DOI 10.17487/RFC8146, April 2017,
<https://www.rfc-editor.org/info/rfc8146>.
[RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018,
<https://www.rfc-editor.org/info/rfc8310>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[RFC8520] Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
Description Specification", RFC 8520,
DOI 10.17487/RFC8520, March 2019,
<https://www.rfc-editor.org/info/rfc8520>.
[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/info/rfc8555>.
[TR-069] The Broadband Forum, "CPE WAN Management Protocol",
December 2018, <https://www.broadband-
forum.org/technical/download/TR-069.pdf>.
[TS.24008]
3GPP, "Mobile radio interface Layer 3 specification; Core
network protocols; Stage 3 (Release 16)", December 2019,
<http://www.3gpp.org/DynaReport/24008.htm>.
Appendix A. Sample Target Deployment Scenarios
Internet Service Providers (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:
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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 13. 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
Local Area Network (LAN) are similar in both fixed and cellular CPE-
based broadband service offerings.
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(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 13: Sample Legacy Deployments
A.1. Managed CPEs
This section focuses on CPEs that are managed by ISPs.
A.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 14.
For example, DoH-capable (or DoT) clients establish the DoH (or DoT)
session with the discovered DoH (or DoT) server.
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The DNS client discovers whether the DNS server in the local network
supports DoH/DoT/DoQ by using a dedicated field in the discovery
message: Encrypted DNS Types (Sections 4, 5, and 6) .
(a) Fixed Networks
,--,--,--.
+-+ LAN +---+ ,-' `-.
|H+--------------+CPE+---+ ISP )
+-+ +---+ `-. ,-'
| `--'--'--'
| |
|<========Encrypted DNS========>|
| |
(b) Cellular Networks
| |
|<========Encrypted DNS========>|
| |
| ,--,--,-.
+-+ LAN +---+ ,-' .
|H+--------------+CPE+---+ \
+-+ +---+ ,' ISP `-.
( )
+-----+-. ,-'
+-+ | `--'--'--'
|H+----------------+ |
+-+ |
| |
|<========Encrypted DNS========>|
| |
Figure 14: Encrypted DNS in the WAN
Figure 14 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 15 for a DoH/DoT-capable host).
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,--,--,--. ,--,--,--.
,-' `-. ,-' ISP `-.
Host---( LAN CPE----( DNS Server )
| `-. ,-' `-. ,-'
| `--'--'--' `--'--'--'
| |
|<=========Encrypted DNS===========>|
Figure 15: Direct Encrypted DNS Sessions
A.1.2. Proxied DNS
Figure 16 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 16. Additional
considerations related to this deployment are discussed in Section 8.
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(a)
,--,--,--. ,--,--,--.
,-' `-. ,-' ISP `-.
Host---( LAN CPE----( DNS Server )
| `-. ,-'| `-. ,-'
| `--'--'--' | `--'--'--'
| | |
|<=====Encrypted=====>|<=Encrypted=>|
| DNS | DNS |
(b)
,--,--,--. ,--,--,--.
Legacy ,-' `-. ,-' ISP `-.
Host---( LAN CPE----( DNS Server )
| `-. ,-'| `-. ,-'
| `--'--'--' | `--'--'--'
| | |
|<=======Do53========>|<=Encrypted=>|
| | DNS |
Figure 16: Proxied Encrypted DNS Sessions
A.2. Unmanaged CPEs
A.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 Appendix A.1. A host can thus establish DoH/DoT
session with a DoH/DoT server similar to what is depicted in
Figure 15 or Figure 16.
A.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 17).
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,--,--,--. ,--,--,--.
,-' Internal ,-' ISP `-.
Host--( Network#A CPE----CPE---( DNS Server )
| `-. ,-' `-. ,-'
| `--'--'--' `--'--'--'
| |
|<==============Encrypted DNS=============>|
Figure 17: 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 18).
,--,--,--. ,--,
,' Internal ,-' '- 3rd Party
Host--( Network#A CPE----CPE---( ISP )--- DNS Server
| `. ,-' `-. -' |
| `-'--'--' `--' |
| |
|<=================Encrypted DNS==================>|
Figure 18: Direct Encrypted DNS Sessions with a Third Party DNS
Resolver
Section 8.2 discusses considerations related to hosting a forwarder
in the Internal CPE.
Appendix B. Make Use of Discovered Encrypted DNS Servers
Even if the use of a discovered encrypted DNS server is beyond the
discovery process and falls under encrypted server selection, the
following discusses typical conditions under which discovered
encrypted DNS server can be used.
o If the DNS server's IP address discovered by using DHCP/RA is
preconfigured in the OS or Browser as a verified resolver (e.g.,
part of an auto-upgrade program such as [Auto-upgrade]), the DNS
client auto-upgrades to use the preconfigured encrypted DNS server
tied to the discovered DNS server IP address. In such a case the
DNS client will perform additional checks out of band, such as
confirming that the Do53 IP address and the encrypted DNS server
are owned and operated by the same organisation.
o Similarly, if the ADN conveyed in DHCP/RA (Sections 4, 5, and 6)
is preconfigured in the OS or browser as a verified resolver, the
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DNS client auto-upgrades to establish an encrypted a DoH/DoT/DoQ
session with the ADN.
In such case, the DNS client matches the domain name in the
Encrypted DNS DHCP/RA option with the 'DNS-ID' identifier type
within subjectAltName entry in the server certificate conveyed in
the TLS handshake.
Authors' Addresses
Mohamed Boucadair
Orange
Rennes 35000
France
Email: mohamed.boucadair@orange.com
Tirumaleswar Reddy
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
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