Internet DRAFT - draft-jt-add-dns-server-redirection
draft-jt-add-dns-server-redirection
ADD J. Todd
Internet-Draft Quad9
Intended status: Standards Track T. Jensen
Expires: 13 September 2023 Microsoft
C. Mosher
Quad9
12 March 2023
Handling Encrypted DNS Server Redirection
draft-jt-add-dns-server-redirection-01
Abstract
This document defines Encrypted DNS Server Redirection (EDSR), a
mechanism for encrypted DNS servers to redirect clients to other
encrypted DNS servers. This enables dynamic routing to geo-located
or otherwise more desirable encrypted DNS servers without modifying
DNS client endpoint configurations or the use of anycast by the DNS
server.
About This Document
This note is to be removed before publishing as an RFC.
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redirection/.
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Table of Contents
1. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DNS client behavior . . . . . . . . . . . . . . . . . . . . . 3
3.1. Discovering redirections . . . . . . . . . . . . . . . . 3
3.2. Refreshing redirections . . . . . . . . . . . . . . . . . 4
3.3. Multiple redirections . . . . . . . . . . . . . . . . . . 4
3.4. Network changes . . . . . . . . . . . . . . . . . . . . . 5
4. DNS server behavior . . . . . . . . . . . . . . . . . . . . . 5
5. Deployment Considerations . . . . . . . . . . . . . . . . . . 5
5.1. Large trees of redirections . . . . . . . . . . . . . . . 6
5.2. Redirection TTLs . . . . . . . . . . . . . . . . . . . . 6
5.3. Including IP addresses in EDSR responses . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6.1. Trusting the redirected connection . . . . . . . . . . . 6
6.2. Use with unencrypted DNS . . . . . . . . . . . . . . . . 7
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 7
8. Data Flow Considerations . . . . . . . . . . . . . . . . . . 7
8.1. Data Scope . . . . . . . . . . . . . . . . . . . . . . . 7
8.2. Data Visibility . . . . . . . . . . . . . . . . . . . . . 8
8.3. Data centralization . . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 9
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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.
2. Introduction
Encrypted DNS Server Redirection (EDSR) is a protocol that allows an
encrypted DNS resolver whose configuration is well known to clients
to redirect them to other, more desirable resolvers without having to
support anycast and without having to configure clients with these
other resolvers ahead of time. It uses the mechanism defined by DDR
[I-D.ietf-add-ddr] to redirect an encrypted DNS client from one
encrypted DNS resolver to another encrypted DNS resolver. Where DDR
uses a threat model that presumes the initial DNS traffic is
unencrypted, EDSR applies when the initial DNS traffic is already
encrypted.
One example of what makes redirection to another resolver desirable
is geolocation. A DNS service may document one or a few well known
resolver configurations even though it routes traffic to hundreds or
thousands of resolvers that are closer to the client, reducing
latency and making DNS resolutions more applicable to the client.
3. DNS client behavior
3.1. Discovering redirections
When a DNS client first opens a connection to an encrypted DNS
server, it MUST send a SVCB query for the name of the resolver to
discover its encrypted DNS configuration. The DNS client SHOULD open
a connection to the server returned in the SVCB query using the
TargetName and one of the IP addresses returned in additional A/AAAA
records for the same name. Once a connection has been successfully
opened, as subsequently described by reaching a suitable server at
the end of the redirection chain, the client SHOULD close the first
connection.
If the returned SVCB record indicates a server with the same domain
name as the current encrypted DNS connection, even if it contains
different values in additional A or AAAA records, or different values
in the ipv4hint or ipv6hint fields, then the redirection is
considered to be from the server to itself. Clients SHOULD NOT
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follow these redirections generally. However, clients receiving
preferable encryption parameters as part of the SVCB response MAY
choose to reconnect to negotiate to upgrade to the preferred
encryption method. When doing so, there is no need for the client to
repeat EDSR as the redirection from the server to itself has
terminated the redirection chain.
The client does not need to wait for the results of the redirection
discovery query before sending other DNS queries on the connection,
though they SHOULD gracefully close the connection as soon as it has
successfully established a connection to the server it was redirected
to and received or timed out the outstanding queries on the original
connection.
See the considerations section for reasons a client MAY choose to
decline a redirection.
3.2. Refreshing redirections
EDSR allows a client to be redirected from an encrypted DNS resolver
it was somehow configured to use. When the redirection TTL expires,
the client SHOULD return to using its originally configured server
unless it can refresh the redirection beforehand. This allows the
client to honor the intention of whatever configuration method was
used to instruct it to use the original encrypted DNS resolver.
If a chain of redirections was followed, the effective TTL of the
redirection is the minimum of the TTLs encountered along the chain.
Clients SHOULD however cap this value to some minimum value at their
discretion to avoid frequent redirection checking when latency plus
an incidentally low TTL along the chain results in near-zero
effective TTLs.
3.3. Multiple redirections
When clients receive more than one valid SVCB response, they SHOULD
prefer using the redirections that match their configuration (such as
supported IP address family or desired encrypted DNS protocol) in
ascending order of the SVCB priority. Once a successful connection
is made to a redirected destination, clients MAY choose to discard
other results in favor of restarting EDSR with the originally
configured resolver.
Redirections are considered to be a one-to-one relationship (starting
with one recursive resolver and following its redirections should
result in one replacement recursive resolver). It is not expected
that a stub resolver ends up using more recursive resolvers than it
was originally configured with when using EDSR.
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3.4. Network changes
When a client device changes what network it is connected to, it
SHOULD forget pre-existing redirections and start EDSR over with the
originally configured resolvers. This ensures that a resolver which
redirects clients based on their source network can behave
accordingly.
Note that this is unrelated to what resolvers a client is originally
configured with. For example, a client which is configured to always
used the resolvers advertised by DHCP will likely start with
different original resolvers when changing networks. How a client is
configured with DNS resolvers is out of scope for this document.
EDSR only provides a mechanism for clients to discover redirections
from resolvers they were previously configured to use.
4. DNS server behavior
DNS resolvers who want to redirect clients to other resolvers MUST
respond to SVCB [I-D.ietf-add-svcb-dns] queries for their own domain
names with records that describe the configuration of the destination
server. Servers SHOULD be prepared for clients to not follow the
redirection immediately as connection failures or other issues may
lead to clients being unable to follow the redirection. Servers who
are redirecting due to being overloaded MAY respond as they normally
would to overwhelming traffic.
Guidance in Section 5 of [I-D.ietf-dnsop-svcb-https] to improve
performance by including additional A/AAAA records with the SVCB
response SHOULD be followed.
Redirections MUST only redirect to resolvers which support at least
the same protocol, address family, port, and TLS minimum versions as
the referring resolver. This ensures that redirections do not lead
clients to resolvers that are not compatible with the client. In
addition, servers SHOULD avoid redirecting to servers which will also
redirect clients unless they are actively coordinating to ensure a
positive client experience. See the Deployment Considerations
section for more details.
5. Deployment Considerations
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5.1. Large trees of redirections
It is possible for DNS servers to redirect clients to DNS servers
which also redirect clients. Clients which are presented with long
chains of redirections MAY choose to stop following redirections to
reduce connection thrashing. DNS server operators SHOULD deploy
redirection behavior mindfully to avoid long chains of redirection.
Servers SHOULD ensure their redirections do not create loops, where
clients are redirected to a server it already encountered earlier in
the process. Clients MAY stop following redirections when they
detect this, but may also take a simpler approach, following only a
maximum number of redirections.
5.2. Redirection TTLs
Servers SHOULD provide sufficiently long TTLs for clients to avoid
the need to constantly repeat EDSR queries. Server operators should
be mindful of redirection chains because unless they collaboratively
control the TTLs of one another's redirections, redirection chains
will end up with greatly reduced effective TTLs because the client
will always use the lowest.
5.3. Including IP addresses in EDSR responses
If a recursive resolver does not include additional A/AAAA records
per Section 5 of [I-D.ietf-dnsop-svcb-https], stub resolvers might
end up failing the redirection if the redirection destination name
cannot be resolved. Additionally, the recursive resolver SHOULD
ensure records conntaining the same IP version as the existing
connection are returned (if the stub is currently connected over
IPv4, one or more A records SHOULD be included, and if the stub is
currently connected over IPv6, one or more AAAA records SHOULD be
included).
6. Security Considerations
6.1. Trusting the redirected connection
EDSR does not provide novel authentication or security mechanisms.
Redirection is trusted by virtue of the server authentication via PKI
through TLS [RFC5280]. The DNS stub resolver implementing EDSR
SHOULD use whatever policies it uses for other TLS connections for
encrypted DNS traffic to determine if a given TLS cert chain is
trustworthy before proceeding with EDSR.
EDSR MUST NOT be used with encrypted DNS protocols that are not based
on TLS. This scenario will require future standards work.
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EDSR should not introduce any additional security considerations
beyond use of the original encrypted resolver prior to redirection.
Because the original connection was trusted, information sent over it
about a new connection to use should be as trusted. This is
analogous to the use of 3xx codes in HTTP to redirect HTTP clients to
other servers. However, clients that wish to time bound
vulnerabilities to attackers who compromise the original resolver MAY
choose to implement a maximum TTL to honor on SVCB records that
redirect to other servers.
6.2. Use with unencrypted DNS
EDSR MUST NOT be used to redirect unencrypted DNS traffic to any
other resolver. This use case is called designation and is covered
by Discovery of Designated Resolvers (DDR) as defined in
[I-D.ietf-add-ddr]. Not following DDR opens up a DNS client to
malicious redirection to an attacker-controlled DNS server. For more
information, see Section 7 of [I-D.ietf-add-ddr].
EDSR also MUST NOT be used to redirect encrypted DNS traffic to a
resolver that advertises support for unencrypted DNS. This would
reduce the security posture of the client. Clients MUST NOT follow
an encrypted DNS redirection and then send unencrypted DNS traffic to
the new resolver.
7. Privacy Considerations
A client MAY choose to not send other name queries until redirection
is complete, but there should be no issue with sending queries to
intermediate resolvers before redirection takes place. This is
because the intermediate resolvers are considered to be appropriate
destinations by the client even if the resolver wants to substitute
another resolver for reasons other than name resolution results such
as latency optimization or load balancing.
8. Data Flow Considerations
8.1. Data Scope
EDSR does not result in any additional data being shared by the end
user, as the DNS queries going to the new resolver were already going
to go to the original resolver.
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8.2. Data Visibility
EDSR results in a 1:1 replacement of DNS resolvers used (future
queries sent to the new resolver will not be sent to the original
resolver anymore). This means the number of servers which see any
given query remain the same.
This is only true if clients only use one redirected DNS server per
original DNS server. If the DNS server offers more than one
redirection, and the client validates and uses two or more of those
redirections, then there will be greater data visibility (more
destinations). This is however entirely within the client's choice
following their own policy as a redundancy versus volume of exhausted
data trade-off.
EDSR requires the redirection to another server to also use encrypted
DNS, so no third-party will be introduced to the data flow unless the
encryption is broken.
8.3. Data centralization
EDSR can only increase data centralization if multiple resolver
operators choose to redirect DNS clients to the same, other DNS
resolver. To prevent the reduction of their resolution redundancy,
DNS clients MAY choose to ignore redirections if they find that they
point to resolvers they are already configured to use, by a previous
redirection or some other configuration.
9. IANA Considerations
This document has no IANA actions.
10. References
10.1. Normative References
[I-D.ietf-add-ddr]
Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
Jensen, "Discovery of Designated Resolvers", Work in
Progress, Internet-Draft, draft-ietf-add-ddr-10, 5 August
2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
add-ddr-10>.
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[I-D.ietf-add-svcb-dns]
Schwartz, B. M., "Service Binding Mapping for DNS
Servers", Work in Progress, Internet-Draft, draft-ietf-
add-svcb-dns-07, 11 August 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-add-
svcb-dns-07>.
[I-D.ietf-dnsop-svcb-https]
Schwartz, B. M., Bishop, M., and E. Nygren, "Service
binding and parameter specification via the DNS (DNS SVCB
and HTTPS RRs)", Work in Progress, Internet-Draft, draft-
ietf-dnsop-svcb-https-12, 11 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
svcb-https-12>.
[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>.
10.2. Informative References
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/rfc/rfc5280>.
Appendix A. Acknowledgments
The authors would like to thank the following individuals for their
invaluable feedback to this document: Ben Schwartz, Eric Orth, Erik
Nygren, Ralph Weber, Ted Hardie, Tommy Pauly, Viktor Dukhovni, and
Vittorio Bertola.
Authors' Addresses
J. Todd
Quad9
Email: jtodd@quad9.net
T. Jensen
Microsoft
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Email: tojens@microsoft.com
C. Mosher
Quad9
Email: cmosher@quad9.net
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