Network Working Group T. Hardie
Request for Comments: 3258 Nominum, Inc.
Category: Informational April 2002
Distributing Authoritative Name Servers via Shared Unicast Addresses
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
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 Domain Name System (DNS) servers to previously
under-served areas of the network topology and to reduce the latency
for DNS query responses in those areas.
1. Introduction
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. The shared unicast system described here is specific
to IPv4; applicability to IPv6 is an area for further study. It
should also be noted that the system described here is related to
that described in [ANYCAST], but it does not require dedicated
address space, routing changes, or the other elements of a full
anycast infrastructure which that document describes.
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2. Architecture
2.1 Server Requirements
Operators of authoritative name servers may wish to refer to
[SECONDARY] and [ROOT] 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 IPv4 shared
unicast address associated with the authoritative name server. The
other interface, referred to as the administrative interface below,
should use a distinct IPv4 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.
2.2 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 transfer, 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 2.5 below.
2.3 Synchronization
Authoritative name servers may be loosely or tightly synchronized,
depending on the practices set by the operating organization. As
noted below in section 4.1.2, 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 mesh
d) confirmation of the integrity of the zones at each server
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e) coordination of the switchover times for the servers in the
mesh
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.
This document presumes that the usual DNS failover methods are the
only ones used to ensure reachability of the data for clients. It
does not advise that the routes be withdrawn in the case of failure;
it advises instead that the DNS process shutdown so that servers on
other addresses are queried. This recommendation reflects a choice
between performance and operational complexity. While it would be
possible to have some process withdraw the route for a specific
server instance when it is not available, there is considerable
operational complexity involved in ensuring that this occurs
reliably. Given the existing DNS failover methods, the marginal
improvement in performance will not be sufficient to justify the
additional complexity for most uses.
2.4 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.
2.5 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
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the shared unicast address. Applications like the DNS, whose
communication typically consists of independent request-response
messages each fitting in a single UDP packet present no problem.
Other applications, in which multiple packets must reach the same
endpoint (e.g., TCP) may fail or present unworkable performance
characteristics in some circumstances. Split-destination failures
may occur when a router does per-packet (or round-robin) load
sharing, a topology change occurs that changes the relative metrics
of two paths to the same anycast destination, etc.
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 DNS 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 the DNS
failover mechanisms to reliably avoid this problem, however, those
using shared unicast distribution mechanisms must take care 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 hosts.
Since ICMP response packets might go to a different member of the
mesh than that sending a packet, packets sent with a shared unicast
source address should also avoid using path MTU discovery.
Appendix A. contains an ASCII diagram of an example 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
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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.
3. Administration
3.1 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 error.
4. 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 attacks and reduce the risk of
others.
4.1 Increased Risks
4.1.1 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.
4.1.2 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
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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 www.example.com has changed and the administrators of
the domain are testing for the changes at the example.com
authoritative name servers, they should not need to check each
instance of a named authoritative 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.
4.1.3 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.
4.2 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.
5. Acknowledgments
Masataka Ohta, Bill Manning, Randy Bush, Chris Yarnell, Ray Plzak,
Mark Andrews, Robert Elz, Geoff Huston, Bill Norton, Akira Kato,
Suzanne Woolf, Bernard Aboba, Casey Ajalat, and Gunnar Lindberg all
provided input and commentary on this work. The editor wishes to
remember in particular the contribution of the late Scott Tucker,
whose extensive systems experience and plain common sense both
contributed greatly to the editor's own deployment experience and are
missed by all who knew him.
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6. References
[SECONDARY] Elz, R., Bush, R., Bradner, S. and M. Patton, "Selection
and Operation of Secondary DNS Servers", BCP 16, RFC
2182, July 1997.
[ROOT] Bush, R., Karrenberg, D., Kosters, M. and R. Plzak, "Root
Name Server Operational Requirements", BCP 40, RFC 2870,
June 2000.
[ANYCAST] Patridge, C., Mendez, T. and W. Milliken, "Host
Anycasting Service", RFC 1546, November 1993.
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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] |
|
|
|
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|
__________________ |
Peer 1-| | |
Peer 2-| | |
Peer 3-| Switch | |
Transit| | _________ _________ |
etc | |--|Router7|---|----|----------|Router8|---WAN-|
| | --------- | | ---------
| | | |
| | | |
------------------ [NTP] [DNS]
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7. Editor's Address
Ted Hardie
Nominum, Inc.
2385 Bay Road.
Redwood City, CA 94063
Phone: 1.650.381.6226
EMail: Ted.Hardie@nominum.com
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8. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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