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IETF DNSOPS working group T. Hardie
Internet draft Equinix, Inc
Category: Work-in-progress January, 2001
Distributing Authoritative Name Servers via Shared Unicast Addresses
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in
The list of current Internet-Drafts can be accessed at
To view the list Internet-Draft Shadow Directories, see
Copyright Notice
Copyright (C) The Internet Society 1999. All Rights Reserved.
This memo describes a set of practices intended to enable an
authoritative name server operator to provide access to a single
named server in multiple locations. The primary motivation for the
development and deployment of these practices is to increase the
distribution of DNS servers to previously under-served areas of the
network topology and to reduce the latency for DNS query responses
in those areas. This document presumes a one-to-one mapping between
named authoritative servers and administrative entities (operators).
This document contains no guidelines or recommendations for caching
name servers.
1. Architecture
1.1 Server Requirements
Operators of authoritative name servers may wish to refer to [1] and
[2] for general guidance on appropriate practice for authoritative
name servers. In addition to proper configuration as a standard
authoritative name server, each of the hosts participating in a
shared-unicast system should be configured with two network
interfaces. These interfaces may be either two physical interfaces
or one physical interface mapped to two logical interfaces. One of
the network interfaces should use the shared unicast address
associated with the authoritative name server. The other interface,
referred to as the administrative interface below, should use a
distinct address specific to that host. The host should respond to
DNS queries only on the shared-unicast interface. In order to
provide the most consistent set of responses from the mesh of
anycast hosts, it is good practice to limit responses on that
interface to zones for which the host is authoritative.
1.2 Zone file delivery
In order to minimize the risk of man-in-the-middle attacks, zone
files should be delivered to the administrative interface of the
servers participating in the mesh. Secure file transfer methods and
strong authentication should be used for all transfers. If the hosts
in the mesh make their zones available for zone transer, the administrative
interfaces should be used for those transfers as well, in order to avoid
the problems with potential routing changes for TCP traffic
noted in section 1.5 below.
1.3 Synchronization
Authoritative name servers may be loosely or tightly synchronized,
depending on the practices set by the operating organization. As
noted below in section 3.1.2, lack of synchronization among servers
using the same shared unicast address could create problems for some
users of this service. In order to minimize that risk, switch-overs
from one data set to another data set should be coordinated as much
as possible. The use of synchronized clocks on the participating
hosts and set times for switch-overs provides a basic level of
coordination. A more complete coordination process would involve:
a) receipt of zones at a distribution host
b) confirmation of the integrity of zones received
c) distribution of the zones to all of the servers in the
d) confirmation of the integrity of the zones at each server
e) coordination of the switchover times for the servers in the
f) institution of a failure process to ensure that servers that
did not receive correct data or could not switchover to the
new data ceased to respond to incoming queries until the
problem could be resolved.
Depending on the size of the mesh, the distribution host may also be
a participant; for authoritative servers, it may also be the host on
which zones are generated.
1.4 Server Placement
Though the geographic diversity of server placement helps reduce the
effects of service disruptions due to local problems, it is
diversity of placement in the network topology which is the driving
force behind these distribution practices. Server placement should
emphasize that diversity. Ideally, servers should be placed
topologically near the points at which the operator exchanges routes
and traffic with other networks.
1.5 Routing
The organization administering the mesh of servers sharing a unicast
address must have an autonomous system number and speak BGP to its
peers. To those peers, the organization announces a route to the
network containing the shared-unicast address of the name server.
The organization's border routers must then deliver the traffic
destined for the name server to the nearest instantiation. Routing
to the administrative interfaces for the servers can use the normal
routing methods for the administering organization.
One potential problem with using shared unicast addresses is that
routers forwarding traffic to them may have more than one available
route, and those routes may, in fact, reach different instances of
the shared unicast address. Because UDP is self-contained, UDP
traffic from a single source reaching different instances presents
no problem. TCP traffic, in contrast, may fail or present
unworkable performance characteristics in a limited set of
circumstances. For split-destination failures to occur, the router
forwarding the traffic must both have equal cost routes to the two
different instances and use a load sharing algorithm which does
per-packet rather than per-destination load sharing.
Four things mitigate the severity of this problem. The first is
that UDP is a fairly high proportion of the query traffic to name
servers. The second is that the aim of this proposal is to
diversify topological placement; for most users, this means that the
coordination of placement will ensure that new instances of a name
server will be at a significantly different cost metric from
existing instances. Some set of users may end up in the middle, but
that should be relatively rare. The third is that per packet load
sharing is only one of the possible load sharing mechanisms, and
other mechanisms are increasing in popularity.
Lastly, in the case where the traffic is TCP, per packet load
sharing is used, and equal cost routes to different instances of a
name server are available, any implementation which measures the
performance of servers to select a preferred server will quickly
prefer a server for which this problem does not occur. For
authoritative servers, care must be taken that all of the servers
for a specific zone are not participants in the same shared-unicast
mesh. To guard even against the case where multiple meshes have a
set of users affected by per packet load sharing along equal cost
routes, organizations implementing these practices should always
provide at least one authoritative server which is not a participant
in any shared unicast mesh. Those deploying shared-unicast meshes
should note that any specific host may become unreachable to a
client should a server fail, a path fail, or the route to that host
be withdrawn. These error conditions are, however, not specific to
shared-unicast distributions, but would occur for standard unicast
Appendix A. contains an ASCII diagram of a simple implementation of
this system. In it, the odd numbered routers deliver traffic to the
shared-unicast interface network and filter traffic from the
administrative network; the even numbered routers deliver traffic to
the administrative network and filter traffic from the shared-unicast
network. These are depicted as separate routers for the ease this
gives in explanation, but they could easily be separate interfaces
on the same router. Similarly, a local NTP source is depicted for
synchronization, but the level of synchronization needed would not
require that source to be either local or a stratum one NTP server.
2. Administration
2.1 Points of Contact
A single point of contact for reporting problems is crucial to the
correct administration of this system. If an external user of the
system needs to report a problem related to the service, there must
be no ambiguity about whom to contact. If internal monitoring does
not indicate a problem, the contact may, of course, need to work
with the external user to identify which server generated the
3. Security Considerations
As a core piece of internet infrastructure, authoritative name
servers are common targets of attack. The practices outlined here
increase the risk of certain kinds of attack and reduce the risk of
3.1 Increased Risks
3.1.1 Increase in physical servers
The architecture outlined in this document increases the number of
physical servers, which could increase the possibility that a
server mis-configuration will occur which allows for a security
breach. In general, the entity administering a mesh should ensure
that patches and security mechanisms applied to a single member of
the mesh are appropriate for and applied to all of the members of a
mesh. "Genetic diversity" (code from different code bases) can be
a useful security measure in avoiding attacks based on
vulnerabilities in a specific code base; in order to ensure
consistency of responses from a single named server, however, that
diversity should be applied to different shared-unicast meshes or
between a mesh and a related unicast authoritative server.
3.1.2 Data synchronization problems
The level of systemic synchronization described above should be
augmented by synchronization of the data present at each of the
servers. While the DNS itself is a loosely coupled system,
debugging problems with data in specific zones would be far more
difficult if two different servers sharing a single unicast address
might return different responses to the same query. For example,
if the data associated with has changed and the
administrators of the domain are testing for the changes at the authoritative name servers, they should not need to
check each instance of a named root server. The use of ntp to
provide a synchronized time for switch-over eliminates some aspects
of this problem, but mechanisms to handle failure during the
switchover are required. In particular, a server which cannot make
the switchover must not roll-back to a previous version; it must
cease to respond to queries so that other servers are queried.
3.1.3 Distribution risks
If the mechanism used to distribute zone files among the servers is
not well secured, a man-in-the-middle attack could result in the
injection of false information. Digital signatures will alleviate
this risk, but encrypted transport and tight access lists are a
necessary adjunct to them. Since zone files will be distributed to
the administrative interfaces of meshed servers, the access control
list for distribution of the zone files should include the
administrative interface of the server or servers, rather than
their shared unicast addresses.
3.2 Decreased Risks
The increase in number of physical servers reduces the likelihood
that a denial-of-service attack will take out a significant portion
of the DNS infrastructure. The increase in servers also reduces
the effect of machine crashes, fiber cuts, and localized disasters
by reducing the number of users dependent on a specific machine.
4. Full copyright statement
Copyright (C) The Internet Society 1999. All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain
it or assist in its implementation may be prepared, copied,
published and distributed, in whole or in part, without restriction
of any kind, provided that the above copyright notice and this
paragraph are included on all such copies and derivative works.
However, this document itself may not be modified in any way, such
as by removing the copyright notice or references to the Internet
Society or other Internet organizations, except as needed for the
purpose of developing Internet standards in which case the
procedures for copyrights defined in the Internet Standards process
must be followed, or as required to translate it into languages
other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on
5. Acknowledgements
Masataka Ohta, Bill Manning, Randy Bush, Chris Yarnell, Ray Plzak,
Mark Andrews, Robert Elz, Geoff Houston, Bill Norton, Akira Kato,
Suzanne Woolf, Scott Tucker, and Gunnar Lindberg all provided input
and commentary on this work.
6. References
[1] "Selection and Operation of Secondary Name Servers". R. Elz, R. Bush,
S Bradner, M. Patton, BCP0016.
[2] "Root Name Server Operational Requirements". R. Bush,
D. Karrenberg, M. Kosters, R. Plzak, BCP0040.
7. Editor's address
Ted Hardie
Equinix, Inc.
2450 Bayshore Parkway
Mountain View, CA 94043-1107
Tel: 1.650.316.6226
Fax: 1.650.315.6903
Appendix A.
Peer 1-| |
Peer 2-| |
Peer 3-| Switch |
Transit| | _________ _________
etc | |--|Router1|---|----|--------------|Router2|---WAN-|
| | --------- | | --------- |
| | | | |
| | | | |
------------------ [NTP] [DNS] |
__________________ |
Peer 1-| | |
Peer 2-| | |
Peer 3-| Switch | |
Transit| | _________ _________ |
etc | |--|Router3|---|----|--------------|Router4|---WAN-|
| | --------- | | --------- |
| | | | |
| | | | |
------------------ [NTP] [DNS] |
__________________ |
Peer 1-| | |
Peer 2-| | |
Peer 3-| Switch | |
Transit| | _________ _________ |
etc | |--|Router5|---|----|--------------|Router6|---WAN-|
| | --------- | | --------- |
| | | | |
| | | | |
------------------ [NTP] [DNS] |
__________________ |
Peer 1-| | |
Peer 2-| | |
Peer 3-| Switch | |
Transit| | _________ _________ |
etc | |--|Router7|---|----|--------------|Router8|---WAN-|
| | --------- | | ---------
| | | |
| | | |
------------------ [NTP] [DNS]
Category: BCP Amaranth Networks Inc.
Expires in six months February 2001
Requiring DNS IN-ADDR Mapping
Status of this Memo
This draft, file name draft-ietf-dnsop-inaddr-required-00.txt, is
intended to be become a Best Current Practice RFC. Distribution of
this document is unlimited. Comments should be sent to the Domain
Name Server Operations working group mailing list <> or
to the author.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
To view the list Internet-Draft Shadow Directories, see
Copyright Notice
Copyright (C) The Internet Society (2000,2001). All Rights Reserved.
1. Introduction
The Domain Name Service has provision for providing mapping of IP
addresses to host names. It is common practice to ensure both name to
address, and address to name mappings are provided for networks. This
practice, while documented, has never been documented as a
requirement placed upon those who control address blocks. This
document fills this gap.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in RFC 2119.
2. Discussion
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From the early days of the Domain Name Service [RFC 883] a special
domain has been set aside for resolving mappings of IP addresses to
domain names. This was refined in [RFC1035], describing the .IN-
ADDR.ARPA in use today.
The assignment of blocks of IP Address space was delegated to three
regional registries. Guidelines for the registries are specified in
[RFC2050], which requires regional registries to maintain IN-ADDR
records on the large blocks of space issued to ISPs and others.
ARIN's policy only requires ISPs to maintain IN-ADDR if they have a
/16 or larger allocation [ARIN]. APNIC indicates in their policy
document [APNIC] that those to whom they allocate blocks, and those
further downstream SHOULD maintain IN-ADDR records. RIPE appears to
have the strongest policy in this area [ripe-185] indicating Local
Internet Registries are required to perform IN-ADDR services, and
delegate those as appropriate when address blocks are delegated.
As we can see, the regional registries have their own policies for
requirements for IN-ADDR maintenance. It should be noted, however,
that many address blocks were allocated before the creation of the
regional registries, and thus it is unclear whether any of the
policies of the registries are binding on those who hold blocks from
that era.
Registries allocate address blocks on CIDR [RFC1519] boundaries.
Unfortunately the IN-ADDR zones are based on classful allocations.
Guidelines [RFC2317] for delegating on non-octet-aligned boundaries
exist, but are not always implemented. Providers SHOULD follow these
guidelines and ensure their clients set up zone files to answer the
3. Effects of missing IN-ADDR
Many applications use DNS lookups for security checks. To ensure
validity of claimed names, some applications will look up IN-ADDR
records to get names, and then look up the resultant name to see if
it maps back to the address originally known. Failure to resolve
matching names is seen as a potential security concern.
Some popular FTP sites will flat-out reject users, even for anonymous
FTP, if the IN-ADDR lookup fails or if the result of the IN-ADDR
lookup when itself resolved, does not match. Some Telnet servers also
implement this check.
Web sites are in some cases using IN-ADDR checks to verify whether
the client is located within a certain geopolitical entity. This is
being employed for downloads of crypto software, for example, where
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export of that software is prohibited to some locales. Credit card
anti-fraud systems also use these methods for geographic placement
The popular TCP Wrappers program found on most Unix and Linux systems
has options to enforce IN-ADDR checks and to reject any client which
does not resolve.
Wider-scale implementation of IN-ADDR on dialup, CDPD and other such
client-oriented portions of the Internet would result in lower
latency for queries (due to lack of negative caching), and lower name
server load and DNS traffic.
Some anti-spam (anti junk email) systems use IN-ADDR to verify return
addresses before accepting email.
Many web servers look up the IN-ADDR of visitors to be used in log
analysis. This adds to the server load, but in the case of IN-ADDR
unavailability, it can lead to delayed web page accesses for users.
Traceroutes with descriptive IN-ADDR naming proves useful when
debugging problems spanning large areas. When this information is
missing, the traceroutes take longer, and it takes additional steps
to determine who's network is the cause of problems.
4. Requirements
All IP address space which is assigned and in use SHOULD be resolved
by IN-ADDR records. Internet providers and other users to whom a
block of addresses are delegated SHOULD provide for lookup of host
names from IP addresses. This may be provided directly or by
delegation to the user of the address block. The ISP is responsible
for one or the other. In the event of delegation, the user is
responsible for resolution.
Only IP addresses not presently in use within a block, or which are
not valid for use (zeros or ones broadcast addresses) are permitted
to have no mapping. It should be noted that due to CIDR, many
addresses which appear to be otherwise valid host addresses may
actually be zeroes or ones broadcast addresses. As such, attempting
to audit a site's degree of compliance can only be done with a
knowledge of the internal routing structure of the site. However, any
host which originates an IP packet necessarily will have a valid host
address, and must therefore have an IN-ADDR mapping.
Regional Registries and any Local Registries to whom they delegate
SHOULD establish and convey a policy to those to whom they delegate
blocks that IN-ADDR mappings are required. Internet providers and end
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users with address blocks must verify their own internal networks are
properly represented in IN-ADDR records, either by providing that
service themselves, or delegating it to others.
Those to whom blocks have been delegated SHOULD convey a policy to
degegatees requiring that they too provide IN-ADDR records and
require and delegations below to do the same. ISPs may wish to
provide IN-ADDR records for their clients if the customers are unable
to provide this for themselves.
5. Security Considerations
This document has no negative impact on security. While it could be
argued that lack of PTR record capabilities provides a degree of
anonimity, this is really not valid. Trace routes, whois lookups and
other sources will still provide methods for discovering identity.
6. References
[RFC883] P.V. Mockapetris, "Domain names: Implementation
specification," RFC883, November 1983.
[RFC1035] P.V. Mockapetris, "Domain Names: Implementation
Specification," RFC 1035, November 1987.
[RFC1519] V. Fuller, et. al., "Classless Inter-Domain Routing (CIDR):
an Address Assignment and Aggregation Strategy," RFC 1519, September
[RFC2317] H. Eidnes, et. al., "Classless IN-ADDR.ARPA delegation,"
RFC 2317, March 1998.
[RFC2050] K. Hubbard, et. al., "Internet Registry IP Allocation
Guidelines", RFC2050, BCP 12, Novebmer 1996.
[ARIN] "ISP Guidelines for Requesting Initial IP Address Space," date
[APNIC] "Policies for address space management in the Asia Pacific
Region," Approved October 1999, effective January 2000,
[RIPE185] "European Internet Registry Policies and Procedures,"
ripe-185, October 26, 1998.
7. Acknowledgements