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.

This document and translations of it may be copied and furnished to

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English.

The limited permissions granted above are perpetual and will not be

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Acknowledgement

Funding for the RFC Editor function is currently provided by the

Internet Society.

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