DNS Configuration for Proxying IP in HTTP
draft-ietf-masque-connect-ip-dns-01

MASQUE                                                       D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Standards Track                         18 October 2024
Expires: 21 April 2025

               DNS Configuration for Proxying IP in HTTP
                  draft-ietf-masque-connect-ip-dns-01

Abstract

   Proxying IP in HTTP allows building a VPN through HTTP load
   balancers.  However, at the time of writing, that mechanism doesn't
   offer a mechanism for communicating DNS configuration information
   inline.  In contrast, most existing VPN protocols provide a mechanism
   to exchange DNS configuration information.  This document describes
   an extension that exchanges this information using HTTP capsules.
   This mechanism supports encrypted DNS transports.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at https://ietf-wg-
   masque.github.io/draft-ietf-masque-connect-ip-dns/draft-ietf-masque-
   connect-ip-dns.html.  Status information for this document may be
   found at https://datatracker.ietf.org/doc/draft-ietf-masque-connect-
   ip-dns/.

   Discussion of this document takes place on the Multiplexed
   Application Substrate over QUIC Encryption Working Group mailing list
   (mailto:masque@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/masque/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/masque/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-masque/draft-ietf-masque-connect-ip-dns.

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/.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Domain Structure  . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Nameserver Structure  . . . . . . . . . . . . . . . . . .   4
     2.3.  DNS Configuration Structure . . . . . . . . . . . . . . .   6
     2.4.  DNS_REQUEST Capsule . . . . . . . . . . . . . . . . . . .   7
     2.5.  DNS_ASSIGN Capsule  . . . . . . . . . . . . . . . . . . .   7
   3.  Handling  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Full-Tunnel Consumer VPN  . . . . . . . . . . . . . . . .   8
     4.2.  Split-Tunnel Enterprise VPN . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

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1.  Introduction

   Proxying IP in HTTP ([CONNECT-IP]) allows building a VPN through HTTP
   load balancers.  However, at the time of writing, that mechanism
   doesn't offer a mechanism for communicating DNS configuration
   information inline.  In contrast, most existing VPN protocols provide
   a mechanism to exchange DNS configuration information (e.g.,
   [IKEv2]).  This document describes an extension that exchanges this
   information using HTTP capsules ([HTTP-DGRAM]).  This document does
   not define any new ways to convey DNS queries or responses, only a
   mechanism to exchange DNS configuration information.

   Note that this extension is meant for cases where connect-ip is used
   like a Remote Access VPN (see Section 8.1 of [CONNECT-IP]), but not
   for cases like IP Flow Forwarding (see Section 8.3 of [CONNECT-IP]).

   This specification uses Service Bindings ([SVCB]) to exchange
   information about nameservers, such as which encrypted DNS transport
   is supported.  This allows support for DNS over HTTPS ([DoH]), DNS
   over QUIC ([DoQ]), DNS over TLS ([DoT]), unencrypted DNS over UDP
   port 53 ([DNS]), and potential future DNS transports.

1.1.  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.

   This document uses terminology from [QUIC].  Where this document
   defines protocol types, the definition format uses the notation from
   Section 1.3 of [QUIC].  This specification uses the variable-length
   integer encoding from Section 16 of [QUIC].  Variable-length integer
   values do not need to be encoded in the minimum number of bytes
   necessary.

   In this document, we use the term "nameserver" to refer to a DNS
   recursive resolver as defined in Section 6 of [DNS-TERMS], and the
   term "domain name" is used as defined in Section 2 of [DNS-TERMS].

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2.  Mechanism

   Similar to how Proxying IP in HTTP exchanges IP address configuration
   information (Section 4.7 of [CONNECT-IP]), this mechanism leverages
   capsules to request DNS configuration information and to assign it.
   Similarly, this mechanism is bidirectional: either endpoint can
   request DNS configuration information by sending a DNS_REQUEST
   capsule, and either endpoint can send DNS configuration information
   in a DNS_ASSIGN capsule.  These capsules follow the format defined
   below.

2.1.  Domain Structure

   Domain {
     Domain Length (i),
     Domain Name (..),
   }

                      Figure 1: Internal Domain Format

   Each Domain contains the following fields:

   Domain Length:  Length of the following Domain field, encoded as a
      variable-length integer.

   Domain Name:  Fully Qualified Domain Name in DNS presentation format
      and using an Internationalized Domain Names for Applications
      (IDNA) A-label ([IDNA]).

2.2.  Nameserver Structure

   Nameserver {
     Service Priority (16),
     IPv4 Address Count (i),
     IPv4 Address (32) ...,
     IPv6 Address Count (i),
     IPv6 Address (128) ...,
     Nameserver Domain (..),
     Service Parameters Length (i),
     Service Parameters (..),
   }

                        Figure 2: Nameserver Format

   Each Nameserver structure contains the following fields:

   Service Priority:  The priority of this attribute compared to other

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      nameservers, as specified in Section 2.4.1 of [SVCB].  Since this
      specification relies on using Service Bindings in ServiceMode
      (Section 2.4.3 of [SVCB]), this field MUST NOT be set to 0.

   IPv4 Address Count:  The number of IPv4 Address fields following this
      field.  Encoded as a variable-length integer.

   IPv4 Address:  Sequence of IPv4 Addresses that can be used to reach
      this nameserver.  Encoded in network byte order.

   IPv6 Address Count:  The number of IPv6 Address fields following this
      field.  Encoded as a variable-length integer.

   IPv6 Address:  Sequence of IPv6 Addresses that can be used to reach
      this nameserver.  Encoded in network byte order.

   Nameserver Domain:  A Domain structure (see Section 2.2) representing
      the domain name of the nameserver.  This MAY be empty if the
      nameserver only supports unencrypted DNS (as traditionally sent
      over UDP port 53).

   Service Parameters Length:  Length of the following Service
      Parameters field, encoded as a variable-length integer.

   Service Parameters:  Set of service parameters that apply to this
      nameserver.  Encoded using the wire format specified in
      Section 2.2 of [SVCB].

   Service parameters allow exchanging additional information about the
   nameserver:

   *  The "port" service parameter is used to indicate which port the
      nameserver is reachable on.  If no "port" service parameter is
      included, this indicates that default port numbers should be used.

   *  The "alpn" service parameter is used to indicate which encrypted
      DNS transports are supported by this nameserver.  If the "no-
      default-alpn" service parameter is omitted, that indicates that
      the nameserver supports unencrypted DNS, as is traditionally sent
      over UDP port 53.  In that case, the sum of IPv4 Address Count and
      IPv6 Address Count MUST be nonzero.  If Nameserver Domain is
      empty, the "alpn" and "no-default-alpn" service parameter MUST be
      omitted.

   *  The "dohpath" service parameter is used to convey a relative DNS
      over HTTPS URI Template, see Section 5 of [SVCB-DNS].

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   *  The service parameters MUST NOT include "ipv4hint" or "ipv6hint"
      SvcParams, as they are superseded by the included IP addresses.

2.3.  DNS Configuration Structure

   DNS Configuration {
     Request ID (i),
     Nameserver Count (i),
     Nameserver (..) ...,
     Internal Domain Count (i),
     Internal Domain (..) ...,
     Search Domain Count (i),
     Search Domain (..) ...,
   }

                     Figure 3: Assigned Address Format

   Each DNS Configuration contains the following fields:

   Request ID:
      Request identifier, encoded as a variable-length integer.  If this
      DNS Configuration is part of a request, then this contains a
      unique request identifier.  If this DNS configuration is part of
      an assignment that is in response to a DNS configuration request
      then this field SHALL contain the value of the corresponding field
      in the request.  If this DNS configuration is part of an
      unsolicited assignment, this field SHALL be zero.

   Nameserver Count:
      The number of Nameserver structures following this field.  Encoded
      as a variable-length integer.

   Nameserver:
      A series of Nameserver structures representing nameservers.

   Internal Domain Count:
      The number of Domain structures following this field.  Encoded as
      a variable-length integer.

   Internal Domain:
      A series of Domain structures representing internal domain names.

   Search Domain Count:
      The number of Domain structures following this field.  Encoded as
      a variable-length integer.

   Search Domain:
      A series of Domain structures representing search domains.

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2.4.  DNS_REQUEST Capsule

   The DNS_REQUEST capsule (see Section 6 for the value of the capsule
   type) allows an endpoint to request DNS configuration from its peer.
   The capsule allows the endpoint to optionally indicate a preference
   for which DNS configuration it would get assigned.  The sender can
   indicate that it has no preference by not sending any nameservers or
   domain names in its request DNS Configuration.

   DNS_REQUEST Capsule {
     Type (i) = DNS_REQUEST,
     Length (i),
     DNS Configuration (..),
   }

                    Figure 4: DNS_REQUEST Capsule Format

   When sending a DNS_REQUEST capsule, the sender MUST generate and send
   a new non-zero request ID that was not previously used on this IP
   Proxying stream.  Note that this request ID namespace is distinct
   from the one used by ADDRESS_ASSIGN capsules (see Section 4.7.1 of
   [CONNECT-IP]).

   An endpoint that receives a DNS_REQUEST capsule SHALL reply by
   sending a DNS_ASSIGN capsule with the corresponding request ID.  That
   DNS_ASSIGN capsule MAY be empty, that indicates that its sender has
   no DNS configuration to share with its peer.

2.5.  DNS_ASSIGN Capsule

   The DNS_ASSIGN capsule (see Section 6 for the value of the capsule
   type) allows an endpoint to send DNS configuration to its peer.

   DNS_ASSIGN Capsule {
     Type (i) = DNS_ASSIGN,
     Length (i),
     DNS Configuration (..),
   }

                    Figure 5: DNS_ASSIGN Capsule Format

   When sending a DNS_ASSIGN capsule in response to a received
   DNS_REQUEST capsule, the Request ID field in the DNS_ASSIGN capsule
   SHALL be set to the value in the received DNS_REQUEST capsule.
   Otherwise the request ID MUST be set to zero.

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3.  Handling

   Note that internal domains include subdomains.  In other words, if
   the DNS configuration contains a domain, that indicates that the
   corresponding domain and all of its subdomains can be resolved by the
   nameservers exchanged in the same DNS configuration.  Sending an
   empty string as an internal domain indicates the DNS root; i.e., that
   the corresponding nameserver can resolve all domain names.

   As with other IP Proxying capsules, the receiver can decide whether
   to use or ignore the configuration information.  For example, in the
   consumer VPN scenario, clients will trust the IP proxy and apply
   received DNS configuration, whereas IP proxies will ignore any DNS
   configuration sent by the client.

   If the IP proxy sends a DNS_ASSIGN capsule containing a DNS over
   HTTPS nameserver, then the client can validate whether the IP proxy
   is authoritative for the origin of the URI template.  If this
   validation succeeds, the client SHOULD send its DNS queries to that
   nameserver directly as independent HTTPS requests.  When possible,
   those requests SHOULD be coalesced over the same HTTPS connection.

4.  Examples

4.1.  Full-Tunnel Consumer VPN

   A full-tunnel consumer VPN hosted at masque.example could configure
   the client to use DNS over HTTPS to the IP proxy itself by sending
   the following configuration.

   DNS Configuration = {
     Nameservers = [{
       Service Priority = 1,
       IPv4 Address = [],
       IPv6 Address = [],
       Nameserver Domain = "masque.example",
       Service Parameters = {
         alpn=h2,h3
         dohpath=/dns-query{?dns}
       },
     }],
     Internal Domains = [""],
     Search Domains = [],
   }

                       Figure 6: Full Tunnel Example

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4.2.  Split-Tunnel Enterprise VPN

   An enterprise switching their preexisting IKEv2/IPsec split-tunnel
   VPN to connect-ip could use the following configuration.

   DNS Configuration = {
     Nameservers = [{
       Service Priority = 1,
       IPv4 Address = [192.0.2.33],
       IPv6 Address = [2001:db8::1],
       Nameserver Domain = "",
       Service Parameters = {},
     }],
     Internal Domains = ["internal.corp.example"],
     Search Domains = [
       "internal.corp.example",
       "corp.example",
     ],
   }

                       Figure 7: Split Tunnel Example

5.  Security Considerations

   Acting on received DNS_ASSIGN capsules can have significant impact on
   endpoint security.  Endpoints MUST ignore DNS_ASSIGN capsules unless
   it has reason to trust its peer and is expecting DNS configuration
   from it.

   This mechanism can cause an endpoint to use a nameserver that is
   outside of the connect-ip tunnel.  While this is acceptable in some
   scenarios, in others it could break the privacy properties provided
   by the tunnel.  To avoid this, implementations need to ensure that
   DNS_ASSIGN capsules are not sent before the corresponding
   ROUTE_ADVERTISEMENT capsule.

   The requirement for an endpoint to always send DNS_ASSIGN capsules in
   response to DNS_REQUEST capsules could lead it to buffer unbounded
   amounts of memory if the underlying stream is blocked by flow or
   congestion control.  Implementations MUST place an upper bound on
   that buffering and abort the stream if that limit is reached.

6.  IANA Considerations

   This document, if approved, will request IANA add the following
   values to the "HTTP Capsule Types" registry maintained at
   <https://www.iana.org/assignments/masque>.

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                       +===========+==============+
                       | Value     | Capsule Type |
                       +===========+==============+
                       | 0x818F79E | DNS_ASSIGN   |
                       +-----------+--------------+
                       | 0x818F79F | DNS_REQUEST  |
                       +-----------+--------------+

                          Table 1: New Capsules

   Note that, if this document is approved, the values defined above
   will be replaced by smaller ones before publication.

   All of these new entries use the following values for these fields:

   Status:  provisional (permanent if this document is approved)
   Reference:  This document
   Change Controller:  IETF
   Contact:  masque@ietf.org
   Notes:  None

7.  References

7.1.  Normative References

   [CONNECT-IP]
              Pauly, T., Ed., Schinazi, D., Chernyakhovsky, A.,
              Kühlewind, M., and M. Westerlund, "Proxying IP in HTTP",
              RFC 9484, DOI 10.17487/RFC9484, October 2023,
              <https://www.rfc-editor.org/rfc/rfc9484>.

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/rfc/rfc1035>.

   [DNS-TERMS]
              Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 8499, DOI 10.17487/RFC8499, January
              2019, <https://www.rfc-editor.org/rfc/rfc8499>.

   [DoH]      Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/rfc/rfc8484>.

   [DoQ]      Huitema, C., Dickinson, S., and A. Mankin, "DNS over
              Dedicated QUIC Connections", RFC 9250,
              DOI 10.17487/RFC9250, May 2022,
              <https://www.rfc-editor.org/rfc/rfc9250>.

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   [DoT]      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/rfc/rfc7858>.

   [HTTP-DGRAM]
              Schinazi, D. and L. Pardue, "HTTP Datagrams and the
              Capsule Protocol", RFC 9297, DOI 10.17487/RFC9297, August
              2022, <https://www.rfc-editor.org/rfc/rfc9297>.

   [IDNA]     Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/rfc/rfc5890>.

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.

   [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/rfc/rfc2119>.

   [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/rfc/rfc8174>.

   [SVCB]     Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <https://www.rfc-editor.org/rfc/rfc9460>.

   [SVCB-DNS] Schwartz, B., "Service Binding Mapping for DNS Servers",
              RFC 9461, DOI 10.17487/RFC9461, November 2023,
              <https://www.rfc-editor.org/rfc/rfc9461>.

7.2.  Informative References

   [IKEv2]    Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/rfc/rfc7296>.

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   [IKEv2-DNS]
              Pauly, T. and P. Wouters, "Split DNS Configuration for the
              Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 8598, DOI 10.17487/RFC8598, May 2019,
              <https://www.rfc-editor.org/rfc/rfc8598>.

   [IKEv2-SVCB]
              Boucadair, M., Reddy.K, T., Wing, D., and V. Smyslov,
              "Internet Key Exchange Protocol Version 2 (IKEv2)
              Configuration for Encrypted DNS", RFC 9464,
              DOI 10.17487/RFC9464, November 2023,
              <https://www.rfc-editor.org/rfc/rfc9464>.

Acknowledgments

   The mechanism in this document was inspired by [IKEv2], [IKEv2-DNS],
   and [IKEv2-SVCB].  The author would like to thank Alex
   Chernyakhovsky, Tommy Pauly, and other enthusiasts in the MASQUE
   Working Group for their contributions.

Author's Address

   David Schinazi
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States of America
   Email: dschinazi.ietf@gmail.com

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