Running a Root Server Local to a ResolverGoogleWarren@kumari.netICANNpaul.hoffman@icann.orgDNSlocal-rootSome 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
.
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Table of Contents
. Introduction
. Changes from RFC 7706
. Requirements Notation
. Requirements
. Operation of the Root Zone on the Local Server
. Security Considerations
. IANA Considerations
. References
. Normative References
. Informative References
. Current Sources of the Root Zone
. Root Zone Services
. Example Configurations of Common Implementations
. Example Configuration: BIND 9.12
. Example Configuration: Unbound 1.8
. Example Configuration: BIND 9.14
. Example Configuration: Unbound 1.9
. Example Configuration: Knot Resolver
. Example Configuration: Microsoft Windows Server 2012
Acknowledgements
Authors' Addresses
IntroductionDNS 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 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 .A different approach to solving some of the problems discussed in
this document is described in .Readers are expected to be familiar with .Changes from RFC 7706RFC 7706 explicitly required that
a root server instance be run
on the loopback interface of the host running the validating resolver.
However, 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 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.
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
when, and only when, they appear in all capitals, as shown here.
RequirementsIn order to implement the mechanism described in this document:
The system MUST be able to validate every signed
record in a zone with DNSSEC .
The system MUST have an up-to-date copy of the
public part of the Key Signing Key (KSK) 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 ensure that the
authoritative root service does not 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.Operation of the Root Zone on the Local ServerThe 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:
Retrieve a copy of the root zone. (See
for some current locations of sources.)
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
. 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).Security ConsiderationsA system that does not follow the DNSSEC-related requirements given
in 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 .As stated in , 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.IANA ConsiderationsThis document has no IANA actions.ReferencesNormative ReferencesDomain names - implementation and specificationThis RFC is the revised specification of the protocol and format used in the implementation of the Domain Name System. It obsoletes RFC-883. This memo documents the details of the domain name client - server communication.Key words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.DNS Security Introduction and RequirementsThe Domain Name System Security Extensions (DNSSEC) add data origin authentication and data integrity to the Domain Name System. This document introduces these extensions and describes their capabilities and limitations. This document also discusses the services that the DNS security extensions do and do not provide. Last, this document describes the interrelationships between the documents that collectively describe DNSSEC. [STANDARDS-TRACK]Decreasing Access Time to Root Servers by Running One on LoopbackSome 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.Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.DNS TerminologyThe Domain Name System (DNS) is defined in literally dozens of different RFCs. The terminology used by implementers and developers of DNS protocols, and by operators of DNS systems, has sometimes changed in the decades since the DNS was first defined. This document gives current definitions for many of the terms used in the DNS in a single document.This document obsoletes RFC 7719 and updates RFC 2308.Informative ReferencesClient Based NamingDNS Zone Transfer Protocol (AXFR)The standard means within the Domain Name System protocol for maintaining coherence among a zone's authoritative name servers consists of three mechanisms. Authoritative Transfer (AXFR) is one of the mechanisms and is defined in RFC 1034 and RFC 1035.The definition of AXFR has proven insufficient in detail, thereby forcing implementations intended to be compliant to make assumptions, impeding interoperability. Yet today we have a satisfactory set of implementations that do interoperate. This document is a new definition of AXFR -- new in the sense that it records an accurate definition of an interoperable AXFR mechanism. [STANDARDS-TRACK]Aggressive Use of DNSSEC-Validated CacheThe DNS relies upon caching to scale; however, the cache lookup generally requires an exact match. This document specifies the use of NSEC/NSEC3 resource records to allow DNSSEC-validating resolvers to generate negative answers within a range and positive answers from wildcards. This increases performance, decreases latency, decreases resource utilization on both authoritative and recursive servers, and increases privacy. Also, it may help increase resilience to certain DoS attacks in some circumstances.This document updates RFC 4035 by allowing validating resolvers to generate negative answers based upon NSEC/NSEC3 records and positive answers in the presence of wildcards.Current Sources of the Root ZoneThe 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.
The root zone file can be obtained using methods described at
<>.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.Root Zone ServicesAt 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 (<>) 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.Example Configurations of Common ImplementationsThis 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 .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.Example Configuration: BIND 9.12BIND 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
.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; };
};
};
Example Configuration: Unbound 1.8Similar 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
Example Configuration: BIND 9.14BIND 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.Example Configuration: Unbound 1.9Recent 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"
Example Configuration: Knot ResolverKnot Resolver uses its "prefill" module to load the root zone
information. This is described at
<>.Example Configuration: Microsoft Windows Server 2012Windows 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:
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).
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.
In the "Zone Type" dialog box, select "Secondary zone".
In the "Zone Name" dialog box, enter ".".
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.)
In the "Completing the New Zone Wizard" dialog box, click
"Finish".
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.
AcknowledgementsThe 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,
described a similar solution to the problems in his doctoral
dissertation in 2013 . contributed greatly to the logic in
the requirements. Other
significant contributors include , , , , and . 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 include ,
, ,
, , and .Authors' AddressesGoogleWarren@kumari.netICANNpaul.hoffman@icann.org