Network Working Group T. Peterson
Internet-Draft July 23, 2019
Intended status: Standards Track
Expires: January 24, 2020

DNS over Transport Layer Security announcements using DHCP or Router Advertisements


This specification describes a DHCP option and Router Advertisement (RA) extension to inform clients of the presence of DNS resolvers with Transport Layer Security (TLS).

Status of This Memo

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Table of Contents

1. Introduction

DHCPv4 [RFC2131], DHCPv6 [RFC3646], and IPv6 Router Announcements [RFC8106] all provide means to inform clients of available resolvers using the incumbent DNS protocol for querying, however there is no means of specifying alternate protocols to perform DNS queries.

2. Conventions and Definitions

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.

“DoT”, “Do53” and other related abbreviations follow the definitions as defined in [I-D.draft-hoffman-dns-terminology-ter-01].

3. The DNS over TLS Option

The DoT DHCP/RA option informs the client that a DoT service is available for use for answering DNS queries using the same IP address(es) returned in Do53 DNS Server DHCP/RA options. Thus networks which announce DoT services MUST announce DNS resolver availability via their respective options, and provide a TLS certificate on the DoT service which passes verification ([RFC6125]) against the DNS Host Name provided in the DoT DHCP/RA option.

The maximum length of the DNS Host Name that can be carried in IPv4 DHCP is 255 bytes, so DNS Host Names longer than 255 bytes SHOULD NOT be used in IPv6 DHCP or IPv6 RA.

3.1. IPv4 DHCP Option

The format of the IPv4 DoT DHCP option is shown below.

 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
|      Code     |      Len      |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
|                                                               |
|                          DNS Host Name                        |
|                                                               |

The DoT DHCPv4 option (one octet)
Length of the DNS Host Name
DNS Host Name:
The DNS Host Name

3.2. IPv6 DHCP Option

The format of the IPv6 Captive-Portal DHCP option is shown below.

 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-code          |           option-len          |
|                                                               |
|                           DNS Host Name                       |
|                                                               |

TODO (two octets)
Length of the DNS Host Name, in octects
DNS Servers:
The DNS Host Name

3.3. The DoT IPv6 RA Option

The format of the DoT Router Advertisement option is shown below.

 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
|      Type     |      Len      |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
|                                                               |
|                          DNS Host Name                        |
|                                                               |

TODO (one octet)
The length of the DNS Host Name, in octets
DNS Server:
The DNS Host Name

3.4. Trust Anchoring

TODO: Put in considerations for using DANE and/or existing certificate authorities for trust anchoring.

4. Security Considerations

An attacker with the ability to inject DHCP messages could include this option and present a malicious resolver.

TODO: Further risk and threat assessments.

5. IANA Considerations

TODO: This section must be updated after assignments have been issued.

This document requires the assignment of an option code assigned under the “BOOTP Vendor Extensions and DHCP Options” [bootp-registry], in addition to an option code assigned under the “Option Codes” registry under DHCPv6 parameters [dhcpv6-registry].

Also, an assignment for an IPv6 RA Option Type from the “IPv6 Neighbor Discovery Option Formats” registry under ICMPv6 paramters [icmpv6-registry].

6. References

6.1. Normative References

[I-D.draft-hoffman-dns-terminology-ter-01] Hoffman, P., "Terminology for DNS Transports and Location", Internet-Draft draft-hoffman-dns-terminology-ter-01, April 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997.
[RFC3646] Droms, R., "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, DOI 10.17487/RFC3646, December 2003.
[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.
[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.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

6.2. Informative References

[bootp-registry] "Dynamic Host Configuration Protocol (DHCP) and Bootstrap Protocol (BOOTP) Parameters", n.d..
[dhcpv6-registry] "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", n.d..
[icmpv6-registry] "Internet Control Message Protocol version 6 (ICMPv6) Parameters", n.d..
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, DOI 10.17487/RFC2131, March 1997.


The author would like to acknowledge the extensive feedback from Martin Thomson.

Author's Address

Thomas Peterson EMail: