Operational Requirements Area Bill Manning (ISI)
INTERNET-DRAFT Paul Vixie (ISC)
Expires December 1996 June 1996
Operational Criteria for Root Name Servers
<draft-manning-dnssvr-criteria-04.txt>
Status of this Memo
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Abstract
This document specifies the operational requirements of root name
servers, including host hardware capacities, name server software
revisions, network connectivity, and physical environment.
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1 - Rationale and Scope
1.1. Historically, the name servers responsible for the root (``.'')
zone have also been responsible for all international top-level domains
(iTLD's, for example: COM, EDU, INT, ARPA). These name servers have
been operated by a cadre of highly capable volunteers, and their
administration has been loosely coordinated by the NIC (first SRI-NIC
and now InterNIC). Ultimate responsibility for the correct operation of
these servers and for the content of the DNS zones they served has
always rested with the IANA.
1.2. As described in [Postel96], many new TLD's may be created
shortly. Servers for all new and existing iTLD's will be subject to the
operational requirements given in [Postel96]. The set of servers for
the root (``.'') zone is likely to become disjoint from the set of
servers for any TLD or group of TLD's, including those maintained by
the InterNIC.
1.3. In spite of the similarities in operational requirements between
the servers for the iTLD's and the servers for the root (``.'') zone,
they are in fact different server sets with different administrators and
slightly different operational requirements. It is likely that many
contry code tld servers will have even more divergent operational
requirements. That said, the requirements set down in this document
could be successfully applied to any name server (whether root, top
level, or any other level), but may be more draconian than necessary
for servers other than those of the root (``.'') zone.
Disclaimer: The selection of name server locations and administrators,
and the procedures for addressing noncompliance with these
stated operational requirements, are outside the scope of this
document.
Definition: For the purpose of this document, the term ``zone master''
shall be used to designate the administrative owner of the
content of a zone. This person is expected to have final
responsibility for the selection and correct operation of
all of the zone's servers. For the root (``.'') zone, this
is the IANA.
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2 - Operational Requirements
2.1. Name server software. The zone master shall initially and
periodically choose a name server package to run on all of the zone's
servers. It is expected that the BIND server will be used, at least
initially, and that new versions or other servers will be specified from
time to time.
Rationale: This requirement is based on the wide and free
availability of BIND's source code, and the active
analysis and development it constantly receives from
several members of the IETF.
Name server software upgrades will be specified and scheduled by the
zone master, and must occur on all of a zone's servers within a
specified 96 hour window.
Rationale: In some cases it has proven necessary to ``cold start'' a
zone's servers in order to clear out oscillating bad
data. By forcing all software upgrades to happen at
about the same time, it will be possible to coordinate a
software change with a zone content change.
2.2. UDP checksums. UDP checksums must be generated when sending
datagrams, and verified when receiving them.
Rationale: Some vendors turn off UDP checksums for performance
reasons, citing the presence of MAC-level frame checks
(CRC, for example) as ``strong enough.'' This has been a
disaster in actual practice.
2.3. Dedicated host. A name server host should have no other function,
and no login accounts other than for system or network administrators.
No other network protocols should be served by a name server host (e.g.,
SMTP, NNTP, FTP, et al). If login is permitted from other than the
system console, then the login service must be by encrypted channel
(e.g., Kerberized and encrypted rlogin/telnet, the secure shell (SSH),
or an equivilent).
Rationale: Each additional service performed by a host makes it less
reliable and potentially less secure, as well as
complicating fault isolation procedures. While name
service does not consume very much in the way of system
resources, it is thought best that a host do a few things
well rather than many things poorly.
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2.4. Clock synchronization. A name server host should synchronize its
clock using the NTP protocol (currnet version) with authentication. At
least two NTP servers should be used. As an exception to section 2.3
above, a name server host can be an NTP server as well.
Rationale: For distributed fault isolation reasons, synchronized
time stamps in system event logs are quite helpful. NTP
is easily spoofed by UDP blast attacks, thus the
requirement for authentication between the name server
host and its NTP servers. A name server host is allowed
to be an NTP server because it has been observed that a
single host running both name service and stratum 1 NTP
is still quite reliable and secure.
2.5. Network interfaces. Name servers must send UDP responses with an
IP source address (and UDP source port number) equal to the IP
destination address (and UDP destination port number) of the request.
Also, a name server might have multiple real interfaces, but only one
will be advertised in the zone's NS RRset and associated glue A RRs.
The advertised address should be that of the ``best'' interface on the
host, in terms of network performance and reliability to the largest
number of destinations.
Rationale: While not required by [RFC1035], many extant DNS
implementations require the source address and port of a
reply to match the destination address and port to which
the request was sent. The number of advertised addresses
is limited to one (1) so that DNS delegation responses
containing this name server can be as short as possible.
2.6. Physical environment. A name server host must be located in a
secure space such as a locked computer room or a data center with
restricted access. The power supply should be redundant, using
batteries, generators or some other means to protect against utility
power failures. Network connectivity should be redundant, so that a
single wide area line failure cannot completely isolate the name server
host from the rest of the network.
2.7. Network security. The system and network administrators should
educate themselves about potential threats, and stay current on CERT
bulletins regarding network breakins. The system staff should
periodically audit the name server host's activity logs and be able to
detect breakins during or after the fact.
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2.8. Host performance. As of the time of this writing, a name server
must be able to answer 1,200 UDP transactions per second with less than
5 milliseconds of average latency. Because the network is still growing
at a high rate, the ability to grow to 2,000 transactions per second and
still support a 5 millisecond latency is highly desirable. Note that
this requirement affects both the host and the network infrastructure to
which that host is attached.
2.9. Response time. The administrators responsible for a name server
will respond to e-mail trouble reports within 24 hours. Personnel
issues such as vacations and illness will cause responsibilities to be
delegated and/or reassigned rather than ignored. After hours telephone
numbers must be made available to the zone master for nonpublished use
in emergencies. An escalation contact name, e-mail address, and
telephone number will also be made available to the zone master in the
event of nonresponse through the normal channel.
2.10. Zone transfer access control. The name server shall be configured
so that outbound zone transfers are permitted only to destinations on
the server's local networks, and to whichever networks the zone master
designates for remote debugging purposes.
Rationale: Zone transfers can present a significant load on a name
server, especially if several transfers are started
simultaneously against the same server. There is no
operational reason to allow anyone outside the name
server's and zone's administrators to transfer the entire
zone.
2.11. Zone transfer protocol. DNS AXFR shall be used in preference to
FTP or any other non-DNS transfer protocol. DNS NOTIFY (see [NOTIFY])
and DNS IXFR (see [IXFR]) shall be supported and enabled when available.
Rationale: Historically, the common implementations of DNS (a.k.a.,
BIND) did not support zone transfer of the root (``.'')
zone due to programming errors. Thus, FTP was used. In
the future, DNS implementations which do not support zone
transfer of all zones will not be considered suitable for
use as root name servers. The benefits of [IXFR] and
[NOTIFY] should be obvious.
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2.12. Recursion shall be disabled for queries.
Rationale: Recursion is a major source of cache pollution, and can
be a major drain on name server performance. An
organization's recursive DNS needs should be served by
some other host than its root name server(s). An
exception is made for missing glue since it's possible
that glue needed for some delegations will not be within
or beneath any zone for which the server is
authoritative. Such glue must be fetched via recursive
lookups to other servers.
2.13. Outages shall be reported. All outages, scheduled or not, shall
be reported to the zone master via e-mail. If an outage is unscheduled
or if an outage is scheduled less than 24 hours in advance, then an
additional notification of the zone master shall be made via telephone.
Extended or repeated outages may beget special handling by the zone
master.
2.14. Inverse name lookups. The PTR RR associated with a server's
primary interface address (that is, the address shown in in the zone's
delegation) shall have its target specified by the zone master.
Rationale: Since each organization has local control of their
network's PTR RRs, and since it is necessary for the
correct operation of some software that the forward and
reverse lookups have symmetrical results, it is left up
to the zone master to select the name for each authority
server's primary address.
3 - Possible Selection Criteria
3.1. Host population. A server's location on the network should be such
that it has a low IP hop count to a high number of end hosts.
Duplication of service should be avoided, such that any given set of end
hosts needs to have a low IP hop count to at most one authority server
for any given zone.
3.2. Infrastructure diversity. A server's location on the network
should be such that most failures capable of isolating it from a large
number of end hosts are diverse from the failures capable of similarly
isolating other authority servers for the same zone(s).
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4 - Security Considerations
See section 2.7.
5 - References
[RFC1035]
P. Mockapetris, ``Domain Names - Implementation and Specification,''
RFC 1035, USC/Information Sciences Institute, November 1987.
[Postel96]
J. Postel, "New Registries and the Delegation of International Top
Level Domains", <draft-postel-iana-itld-admin-00.txt>, May 3, 1996.
[IXFR]
M. Ohta, ``Incremental Zone Transfer,'' Internet Draft, February
1996, <draft-ietf-dnsind-ixfr-06.txt>.
[NOTIFY]
P. Vixie, ``A Mechanism for Prompt Notification of Zone Changes,''
Internet Draft, March 1996, <draft-ietf-dnsind-notify-07.txt>.
6 - Acknowledgements
Constructive comments have been received from: Jon Postel, Michael
Patton, Andrew Partan, Michael Dillon, Don Mitchell Steven Doyle, Owen
DeLong and other members of the internet community.
7 - Author's Address
Bill Manning
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292
+1 310 822 1511
<bmanning@isi.edu>
Paul Vixie
Internet Software Consortium
Star Route Box 159A
Woodside, CA 94062
+1 415 747 0204
<paul@vix.com>
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