Internet Engineering Task Force (IETF) W. Kumari
Request for Comments: 8806 Google
Obsoletes: 7706 P. Hoffman
Category: Informational ICANN
ISSN: 2070-1721 June 2020
Running a Root Server Local to a Resolver
Abstract
Some DNS recursive resolvers have longer-than-desired round-trip
times to the closest DNS root server; those resolvers may have
difficulty getting responses from the root servers, such as during a
network attack. Some DNS recursive resolver operators want to
prevent snooping by third parties of requests sent to DNS root
servers. In both cases, resolvers can greatly decrease the round-
trip time and prevent observation of requests by serving a copy of
the full root zone on the same server, such as on a loopback address
or in the resolver software. This document shows how to start and
maintain such a copy of the root zone that does not cause problems
for other users of the DNS, at the cost of adding some operational
fragility for the operator.
This document obsoletes RFC 7706.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8806.
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Table of Contents
1. Introduction
1.1. Changes from RFC 7706
1.2. Requirements Notation
2. Requirements
3. Operation of the Root Zone on the Local Server
4. Security Considerations
5. IANA Considerations
6. References
6.1. Normative References
6.2. Informative References
Appendix A. Current Sources of the Root Zone
A.1. Root Zone Services
Appendix B. Example Configurations of Common Implementations
B.1. Example Configuration: BIND 9.12
B.2. Example Configuration: Unbound 1.8
B.3. Example Configuration: BIND 9.14
B.4. Example Configuration: Unbound 1.9
B.5. Example Configuration: Knot Resolver
B.6. Example Configuration: Microsoft Windows Server 2012
Acknowledgements
Authors' Addresses
1. Introduction
DNS recursive resolvers have to provide answers to all queries from
their clients, even those for domain names that do not exist. For
each queried name that is within a top-level domain (TLD) that is not
in the recursive resolver's cache, the resolver must send a query to
a root server to get the information for that TLD or to find out that
the TLD does not exist. Research shows that the vast majority of
queries going to the root are for names that do not exist in the root
zone.
Many of the queries from recursive resolvers to root servers get
answers that are referrals to other servers. Malicious third parties
might be able to observe that traffic on the network between the
recursive resolver and root servers.
The primary goals of this design are to provide more reliable answers
for queries to the root zone during network attacks that affect the
root servers and to prevent queries and responses from being visible
on the network. This design will probably have little effect on
getting faster responses to the stub resolver for good queries on
TLDs, because the TTL for most TLDs is usually long-lived (on the
order of a day or two) and is thus usually already in the cache of
the recursive resolver; the same is true for the TTL for negative
answers from the root servers. (Although the primary goal of the
design is for serving the root zone, the method can be used for any
zone.)
This document describes a method for the operator of a recursive
resolver to have a complete root zone locally and to hide queries for
the root zone from outsiders. The basic idea is to create an up-to-
date root zone service on the same host as the recursive server and
use that service when the recursive resolver looks up root
information. The recursive resolver validates all responses from the
root service on the same host, just as it would validate all
responses from a remote root server.
This design explicitly only allows the new root zone service to be
run on the same server as the recursive resolver in order to prevent
the server from serving authoritative answers to any other system.
Specifically, the root service on the local system MUST be configured
to only answer queries from resolvers on the same host and MUST NOT
answer queries from any other resolver.
At the time that RFC 7706 [RFC7706] was published, it was considered
controversial, because there was not consensus on whether this was a
"best practice". In fact, many people felt that it is an excessively
risky practice, because it introduced a new operational piece to
local DNS operations where there was not one before. Since then, the
DNS operational community has largely shifted to believing that local
serving of the root zone for an individual resolver is a reasonable
practice. The advantages listed above do not come free: if this new
system does not work correctly, users can get bad data, or the entire
recursive resolution system might fail in ways that are hard to
diagnose.
This design uses an authoritative service running on the same machine
as the recursive resolver. Common open source recursive resolver
software does not need to add new functionality to act as an
authoritative server for some zones, but other recursive resolver
software might need to be able to talk to an authoritative server
running on the same host. Some resolver software supports being both
an authoritative server and a resolver but separated by logical
"views", allowing a local root to be implemented within a single
process; examples of this can be seen in Appendix B.
A different approach to solving some of the problems discussed in
this document is described in [RFC8198].
Readers are expected to be familiar with [RFC8499].
1.1. Changes from RFC 7706
[RFC7706]
RFC 7706 explicitly required that a root server instance be run on
the loopback interface of the host running the validating resolver.
However, [RFC7706] RFC 7706 also had examples of how to set up common software
that did not use the loopback interface. This document loosens the
restriction on using the loopback interface and in fact allows the
use of a local service, not necessarily an authoritative server.
However, the document keeps the requirement that only systems running
on that single host be able to query that authoritative root server
or service.
This document changes the use cases for running a local root service
to be more consistent with the reasons operators said they had for
using [RFC7706]: RFC 7706:
* Removed the prohibition on distribution of recursive DNS servers,
including configurations for this design because some already do
and others have expressed an interest in doing so.
* Added the idea that a recursive resolver using this design might
switch to using the normal (remote) root servers if the local root
server fails.
* Refreshed the list of where one can get copies of the root zone.
* Added examples of other resolvers and updated the existing
examples.
1.2. Requirements Notation
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. Requirements
In order to implement the mechanism described in this document:
* The system MUST be able to validate every signed record in a zone
with DNSSEC [RFC4033].
* The system MUST have an up-to-date copy of the public part of the
Key Signing Key (KSK) [RFC4033] used to sign the DNS root.
* The system MUST be able to retrieve a copy of the entire root zone
(including all DNSSEC-related records).
* The system MUST be able to run an authoritative service for the
root zone on the same host. The authoritative root service MUST
only respond to queries from the same host. One way to assure ensure
that the authoritative root service does not
responding respond to queries
from other hosts is to run an authoritative server for the root
that responds only on one of the loopback addresses (that is, an
address in the range 127/8 for IPv4 or ::1 in IPv6). Another
method is to have the resolver software also act as an
authoritative server for the root zone, but only for answering
queries from itself.
A corollary of the above list is that authoritative data in the root
zone used on the local authoritative server MUST be identical to the
same data in the root zone for the DNS. It is possible to change the
unsigned data (the glue records) in the copy of the root zone, but
such changes could cause problems for the recursive server that
accesses the local root zone, and therefore any changes to the glue
records SHOULD NOT be made.
3. Operation of the Root Zone on the Local Server
The operation of an authoritative server for the root in the system
described here can be done separately from the operation of the
recursive resolver, or it might be part of the configuration of the
recursive resolver system.
The steps to set up the root zone are:
1. Retrieve a copy of the root zone. (See Appendix A for some
current locations of sources.)
2. Start the authoritative service for the root zone in a manner
that prevents any system other than a recursive resolver on the
same host from accessing it.
The contents of the root zone MUST be refreshed using the timers from
the SOA record in the root zone, as described in [RFC1035]. This
inherently means that the contents of the local root zone will likely
be a little behind those of the global root servers, because those
servers are updated when triggered by NOTIFY messages.
There is a risk that a system using a local authoritative server for
the root zone cannot refresh the contents of the root zone before the
expire time in the SOA. A system using a local authoritative server
for the root zone MUST NOT serve stale data for the root zone. To
mitigate the risk that stale data is served, the local root server
MUST immediately switch to using non-local root servers when it
detects that it would be serving state data.
In a resolver that is using an internal service for the root zone, if
the contents of the root zone cannot be refreshed before the expire
time in the SOA, the resolver MUST immediately switch to using non-
local root servers.
In the event that refreshing the contents of the root zone fails, the
results can be disastrous. For example, sometimes all the NS records
for a TLD are changed in a short period of time (such as 2 days); if
the refreshing of the local root zone is broken during that time, the
recursive resolver will have bad data for the entire TLD zone.
An administrator using the procedure in this document SHOULD have an
automated method to check that the contents of the local root zone
are being refreshed; this might be part of the resolver software.
One way to do this is to have a separate process that periodically
checks the SOA of the local root zone and makes sure that it is
changing. At the time that this document is published, the SOA for
the root zone is the digital representation of the current date with
a two-digit counter appended, and the SOA is changed every day even
if the contents of the root zone are unchanged. For example, the SOA
of the root zone on January 2, 2019 was 2019010201. A process can
use this fact to create a check for the contents of the local root
zone (using a program not specified in this document).
4. Security Considerations
A system that does not follow the DNSSEC-related requirements given
in Section 2 can be fooled into giving bad responses in the same way
as any recursive resolver that does not do DNSSEC validation on
responses from a remote root server. Anyone deploying the method
described in this document should be familiar with the operational
benefits and costs of deploying DNSSEC [RFC4033].
As stated in Section 1, this design explicitly requires the local
copy of the root zone information to be available only from resolvers
on that host. This has the security property of limiting damage to
clients of any local resolver that might try to rely on an altered
copy of the root.
5. IANA Considerations
This document has no IANA actions.
6. References
6.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[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>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.
[RFC7706] Kumari, W. and P. Hoffman, "Decreasing Access Time to Root
Servers by Running One on Loopback", RFC 7706,
DOI 10.17487/RFC7706, November 2015,
<https://www.rfc-editor.org/info/rfc7706>.
[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>.
[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>.
6.2. Informative References
[Manning2013]
Manning, W., "Client Based Naming", May 2013,
<http://www.sfc.wide.ad.jp/dissertation/bill_e.html>.
[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<https://www.rfc-editor.org/info/rfc5936>.
[RFC8198] Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
July 2017, <https://www.rfc-editor.org/info/rfc8198>.
Appendix A. Current Sources of the Root Zone
The root zone can be retrieved from anywhere as long as it comes with
all the DNSSEC records needed for validation. Currently, one can get
the root zone from ICANN by zone transfer AXFR [RFC5936] over TCP
from DNS servers at xfr.lax.dns.icann.org and xfr.cjr.dns.icann.org.
The root zone file can be obtained using methods described at
<https://www.iana.org/domains/root/files>.
Currently, the root can also be retrieved by AXFR over TCP from the
following root server operators:
* b.root-servers.net
* c.root-servers.net
* d.root-servers.net
* f.root-servers.net
* g.root-servers.net
* k.root-servers.net
It is crucial to note that none of the above services are guaranteed
to be available. It is possible that ICANN or some of the root
server operators will turn off the AXFR capability on the servers
listed above. Using AXFR over TCP to addresses that are likely to be
anycast (as the ones above are) may conceivably have transfer
problems due to anycast, but current practice shows that to be
unlikely.
A.1. Root Zone Services
At the time that this document is published, there is one root zone
service that is active and one that has been announced as in the
planning stages. This section describes all known active services.
LocalRoot (<https://localroot.isi.edu/>) is an experimental service
that embodies many of the ideas in this document. It distributes the
root zone by AXFR and also offers DNS NOTIFY messages when the
LocalRoot system sees that the root zone has changed.
Appendix B. Example Configurations of Common Implementations
This section shows fragments of configurations for some popular
recursive server software that is believed to correctly implement the
requirements given in this document. The examples have been updated
since the publication of [RFC7706].
The IPv4 and IPv6 addresses in this section were checked in March
2020 by testing for AXFR over TCP from each address for the known
single-letter names in the root-servers.net zone.
B.1. Example Configuration: BIND 9.12
BIND 9.12 acts both as a recursive resolver and an authoritative
server. Because of this, there is "fate-sharing" between the two
servers in the following configuration. That is, if the root server
dies, it is likely that all of BIND is dead.
Note that a future version of BIND will support a much more robust
method for creating a local mirror of the root or other zones; see
Appendix B.3.
Using this configuration, queries for information in the root zone
are returned with the Authoritative Answer (AA) bit not set.
When slaving a zone, BIND 9.12 will treat zone data differently if
the zone is slaved into a separate view (or a separate instance of
the software) versus slaved into the same view or instance that is
also performing the recursion.
Validation: When using separate views or separate instances, the DS
records in the slaved zone will be validated as the zone data is
accessed by the recursive server. When using the same view, this
validation does not occur for the slaved zone.
Caching: When using separate views or instances, the recursive
server will cache all of the queries for the slaved zone, just as
it would using the traditional "root hints" method. Thus, as the
zone in the other view or instance is refreshed or updated,
changed information will not appear in the recursive server until
the TTL of the old record times out. Currently, the TTL for DS
and delegation NS records is two days. When using the same view,
all zone data in the recursive server will be updated as soon as
it receives its copy of the zone.
view root {
match-destinations { 127.12.12.12; };
zone "." {
type slave;
file "rootzone.db";
notify no;
masters {
199.9.14.201; # b.root-servers.net
192.33.4.12; # c.root-servers.net
199.7.91.13; # d.root-servers.net
192.5.5.241; # f.root-servers.net
192.112.36.4; # g.root-servers.net
193.0.14.129; # k.root-servers.net
192.0.47.132; # xfr.cjr.dns.icann.org
192.0.32.132; # xfr.lax.dns.icann.org
2001:500:200::b; # b.root-servers.net
2001:500:2::c; # c.root-servers.net
2001:500:2d::d; # d.root-servers.net
2001:500:2f::f; # f.root-servers.net
2001:500:12::d0d; # g.root-servers.net
2001:7fd::1; # k.root-servers.net
2620:0:2830:202::132; # xfr.cjr.dns.icann.org
2620:0:2d0:202::132; # xfr.lax.dns.icann.org
};
};
};
view recursive {
dnssec-validation auto;
allow-recursion { any; };
recursion yes;
zone "." {
type static-stub;
server-addresses { 127.12.12.12; };
};
};
B.2. Example Configuration: Unbound 1.8
Similar to BIND, Unbound, starting with version 1.8, can act both as
a recursive resolver and an authoritative server.
auth-zone:
name: "."
master: 199.9.14.201 # b.root-servers.net
master: 192.33.4.12 # c.root-servers.net
master: 199.7.91.13 # d.root-servers.net
master: 192.5.5.241 # f.root-servers.net
master: 192.112.36.4 # g.root-servers.net
master: 193.0.14.129 # k.root-servers.net
master: 192.0.47.132 # xfr.cjr.dns.icann.org
master: 192.0.32.132 # xfr.lax.dns.icann.org
master: 2001:500:200::b # b.root-servers.net
master: 2001:500:2::c # c.root-servers.net
master: 2001:500:2d::d # d.root-servers.net
master: 2001:500:2f::f # f.root-servers.net
master: 2001:500:12::d0d # g.root-servers.net
master: 2001:7fd::1 # k.root-servers.net
master: 2620:0:2830:202::132 # xfr.cjr.dns.icann.org
master: 2620:0:2d0:202::132 # xfr.lax.dns.icann.org
fallback-enabled: yes
for-downstream: no
for-upstream: yes
B.3. Example Configuration: BIND 9.14
BIND 9.14 can set up a local mirror of the root zone with a small
configuration option:
zone "." {
type mirror;
};
The simple "type mirror" configuration for the root zone works for
the root zone because a default list of primary servers for the IANA
root zone is built into BIND 9.14. In order to set up mirroring of
any other zone, an explicit list of primary servers needs to be
provided.
See the documentation for BIND 9.14 for more detail about how to use
this simplified configuration.
B.4. Example Configuration: Unbound 1.9
Recent versions of Unbound have an "auth-zone" feature that allows
local mirroring of the root zone. Configuration looks as follows:
auth-zone:
name: "."
master: "b.root-servers.net"
master: "c.root-servers.net"
master: "d.root-servers.net"
master: "f.root-servers.net"
master: "g.root-servers.net"
master: "k.root-servers.net"
fallback-enabled: yes
for-downstream: no
for-upstream: yes
zonefile: "root.zone"
B.5. Example Configuration: Knot Resolver
Knot Resolver uses its "prefill" module to load the root zone
information. This is described at <https://knot-
resolver.readthedocs.io/en/v5.0.1/modules-rfc7706.html>.
B.6. Example Configuration: Microsoft Windows Server 2012
Windows Server 2012 contains a DNS server in the "DNS Manager"
component. When activated, that component acts as a recursive
server. The DNS Manager can also act as an authoritative server.
Using this configuration, queries for information in the root zone
are returned with the AA bit set.
The steps to configure the DNS Manager to implement the requirements
in this document are:
1. Launch the DNS Manager GUI. This can be done from the command
line ("dnsmgmt.msc") or from the Service Manager (the "DNS"
command in the "Tools" menu).
2. In the hierarchy under the server on which the service is
running, right-click on the "Forward Lookup Zones", and select
"New Zone". This brings up a succession of dialog boxes.
3. In the "Zone Type" dialog box, select "Secondary zone".
4. In the "Zone Name" dialog box, enter ".".
5. In the "Master DNS Servers" dialog box, enter
"b.root-servers.net". The system validates that it can do a zone
transfer from that server. (After this configuration is
completed, the DNS Manager will attempt to transfer from all of
the root zone servers.)
6. In the "Completing the New Zone Wizard" dialog box, click
"Finish".
7. Verify that the DNS Manager is acting as a recursive resolver.
Right-click on the server name in the hierarchy, choosing the
"Advanced" tab in the dialog box. See that "Disable recursion
(also disables forwarders)" is not selected and that "Enable
DNSSEC validation for remote responses" is selected.
Acknowledgements
The authors fully acknowledge that running a copy of the root zone on
the loopback address is not a new concept and that we have chatted
with many people about that idea over time. For example, Bill
Manning described a similar solution to the problems in his doctoral
dissertation in 2013 [Manning2013].
Evan Hunt contributed greatly to the logic in the requirements.
Other significant contributors include Wouter Wijngaards, Tony Hain,
Doug Barton, Greg Lindsay, and Akira Kato. The authors also received
many offline comments about making the document clear that this is
just a description of a way to operate a root zone on the same host
and not a recommendation to do so.
People who contributed to this update to [RFC7706] include Florian
Obser, nusenu, Wouter Wijngaards, Mukund Sivaraman, Bob Harold, and
Leo Vegoda.
Authors' Addresses
Warren Kumari
Google
Email: Warren@kumari.net
Paul Hoffman
ICANN
Email: paul.hoffman@icann.org