Internet Engineering Task Force (IETF) W. Kumari
Request for Comments: 7706 Google
Category: Informational P. Hoffman
ISSN: 2070-1721 ICANN
November 2015
Decreasing Access Time to Root Servers by Running One on Loopback
Abstract
Some DNS recursive resolvers have longer-than-desired round-trip
times to the closest DNS root server. Some DNS recursive resolver
operators want to prevent snooping of requests sent to DNS root
servers by third parties. Such resolvers can greatly decrease the
round-trip time and prevent observation of requests by running a copy
of the full root zone on a loopback address (such as 127.0.0.1).
This document shows how to start and maintain such a copy of the root
zone that does not pose a threat to other users of the DNS, at the
cost of adding some operational fragility for the operator.
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 a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7706.
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Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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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
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 4
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Operation of the Root Zone on the Loopback Address . . . . . 5
4. Using the Root Zone Server on the Loopback Address . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Current Sources of the Root Zone . . . . . . . . . . 8
Appendix B. Example Configurations of Common Implementations . . 8
B.1. Example Configuration: BIND 9.9 . . . . . . . . . . . . . 9
B.2. Example Configuration: Unbound 1.4 and NSD 4 . . . . . . 10
B.3. Example Configuration: Microsoft Windows Server 2012 . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
DNS recursive resolvers have to provide answers to all queries from
their customers, even those for domain names that do not exist. For
each queried name that has 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. Typically, the vast majority of queries
going to the root are for names that do not exist in the root zone,
and the negative answers are cached for a much shorter period of
time. A slow path between the recursive resolver and the closest
root server has a negative effect on the resolver's customers.
Recursive resolvers currently send queries for all TLDs that are not
in their caches to root servers, even though most of those queries
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 one or more of the DNS roots.
This document describes a method for the operator of a recursive
resolver to greatly speed these queries and to hide them from
outsiders. The basic idea is to create an up-to-date root zone
server on a loopback address on the same host as the recursive
server, and use that server when the recursive resolver looks up root
information. The recursive resolver validates all responses from the
root server on the loopback address, just as it would all responses
from a remote root server.
The primary goals of this design are to provide faster negative
responses to stub resolver queries that contain junk queries, and to
prevent queries and responses from being visible on the network.
This design will probably have little effect on getting faster
positive responses to stub resolver for good queries on TLDs, because
the data for those zones is usually long-lived and already in the
cache of the recursive resolver; thus, getting faster positive
responses is a non-goal of this design.
This design explicitly only allows the new root zone server to be run
on a loopback address, in order to prevent the server from serving
authoritative answers to any system other than the recursive
resolver.
It is important to note that the design being described here is not
considered a "best practice". In fact, many people feel that it is
an excessively risky practice because it introduces a new operational
piece to local DNS operations where there was not one before. The
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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 requires the addition of authoritative name server
software running on the same machine as the recursive resolver.
Thus, recursive resolver software such as BIND will not need to add
much new functionality, but recursive resolver software such as
Unbound will need to be able to talk to an authoritative server (such
as NSD) running on the same host.
Because of the significant operational risks described in this
document, distributions of recursive DNS servers MUST NOT include
configuration for the design described here. It is acceptable to
point to this document, but not to indicate that this configuration
is something that should be considered without reading the entire
document.
A different approach to solving the problems discussed in this
document is described in [AggressiveNSEC].
1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Requirements
In order to implement the mechanism described in this document:
o The system MUST be able to validate a zone with DNSSEC [RFC4033].
o The system MUST have an up-to-date copy of the DNS root key.
o The system MUST be able to retrieve a copy of the entire root zone
(including all DNSSEC-related records).
o The system MUST be able to run an authoritative server on one of
the IPv4 loopback addresses (that is, an address in the range
127/8 for IPv4 or ::1 in IPv6).
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
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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 Loopback Address
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.
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 server with the root zone on a loopback
address that is not in use. For IPv4, this would typically be
127.0.0.1, but if that address is in use, any address in 127/8 is
acceptable. For IPv6, this would be ::1.
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. If the
contents of the zone cannot be refreshed before the expire time, the
server MUST return a SERVFAIL error response for all queries until
the zone can be successfully be set up again.
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. One way to do this is to have a separate
process that periodically checks the SOA of the root zone from 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, 2015 was 2015010201. 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).
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4. Using the Root Zone Server on the Loopback Address
A recursive resolver that wants to use a root zone server operating
as described in Section 3 simply specifies the local address as the
place to look when it is looking for information from the root. All
responses from the root server must be validated using DNSSEC.
Note that using this configuration will cause the recursive resolver
to fail if the local root zone server fails. See Appendix B for more
discussion of this for specific software.
To test the proper operation of the recursive resolver with the local
root server, use a DNS client to send a query for the SOA of the root
to the recursive server. Make sure the response that comes back has
the AA bit in the message header set to 0.
5. 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 only allows the new
root zone server to be run on a loopback address, in order to prevent
the server from serving authoritative answers to any system other
than the recursive resolver. This has the security property of
limiting damage to any other system that might try to rely on an
altered copy of the root.
6. References
6.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://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,
<http://www.rfc-editor.org/info/rfc2119>.
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[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,
<http://www.rfc-editor.org/info/rfc4033>.
6.2. Informative References
[AggressiveNSEC]
Fujiwara, K. and A. Kato, "Aggressive use of NSEC/NSEC3",
Work in Progress, draft-fujiwara-dnsop-nsec-
aggressiveuse-02, October 2015.
[Manning2013]
Manning, W., "Client Based Naming", 2013,
<http://www.sfc.wide.ad.jp/dissertation/bill_e.html>.
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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) over TCP from DNS
servers at xfr.lax.dns.icann.org and xfr.cjr.dns.icann.org.
Currently, the root can also be retrieved by AXFR over TCP from the
following root server operators:
o b.root-servers.net
o c.root-servers.net
o f.root-servers.net
o g.root-servers.net
o 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.
To repeat the requirement from earlier in this document: if the
contents of the zone cannot be refreshed before the expire time, the
server MUST return a SERVFAIL error response for all queries until
the zone can be successfully be set up again.
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 IPv4 and IPv6 addresses in this section were checked recently by
testing for AXFR over TCP from each address for the known single-
letter names in the root-servers.net zone.
The examples here use a loopback address of 127.12.12.12, but typical
installations will use 127.0.0.1. The different address is used in
order to emphasize that the root server does not need to be on the
device at "localhost".
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B.1. Example Configuration: BIND 9.9
BIND 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.
Using this configuration, queries for information in the root zone
are returned with the AA bit not set.
When slaving a zone, BIND 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.
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view root {
match-destinations { 127.12.12.12; };
zone "." {
type slave;
file "rootzone.db";
notify no;
masters {
192.228.79.201; # b.root-servers.net
192.33.4.12; # c.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
2001:500:84::b; # b.root-servers.net
2001:500:2f::f; # f.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 1.4 and NSD 4
Unbound and NSD are separate software packages. Because of this,
there is no "fate-sharing" between the two servers in the following
configurations. That is, if the root server instance (NSD) dies, the
recursive resolver instance (Unbound) will probably keep running but
will not be able to resolve any queries for the root zone.
Therefore, the administrator of this configuration might want to
carefully monitor the NSD instance and restart it immediately if it
dies.
Using this configuration, queries for information in the root zone
are returned with the AA bit not set.
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# Configuration for Unbound
server:
do-not-query-localhost: no
stub-zone:
name: "."
stub-prime: no
stub-addr: 127.12.12.12
# Configuration for NSD
server:
ip-address: 127.12.12.12
zone:
name: "."
request-xfr: 192.228.79.201 NOKEY # b.root-servers.net
request-xfr: 192.33.4.12 NOKEY # c.root-servers.net
request-xfr: 192.5.5.241 NOKEY # f.root-servers.net
request-xfr: 192.112.36.4 NOKEY # g.root-servers.net
request-xfr: 193.0.14.129 NOKEY # k.root-servers.net
request-xfr: 192.0.47.132 NOKEY # xfr.cjr.dns.icann.org
request-xfr: 192.0.32.132 NOKEY # xfr.lax.dns.icann.org
request-xfr: 2001:500:84::b NOKEY # b.root-servers.net
request-xfr: 2001:500:2f::f NOKEY # f.root-servers.net
request-xfr: 2001:7fd::1 NOKEY # k.root-servers.net
request-xfr: 2620:0:2830:202::132 NOKEY # xfr.cjr.dns.icann.org
request-xfr: 2620:0:2d0:202::132 NOKEY # xfr.lax.dns.icann.org
B.3. 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. 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 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".
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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 but to a very different problem
(intermittent connectivity, instead of constant but slow
connectivity) 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 localhost, and
not a recommendation to do so.
Authors' Addresses
Warren Kumari
Google
Email: Warren@kumari.net
Paul Hoffman
ICANN
Email: paul.hoffman@icann.org
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