INTERNET-DRAFT
SOA Values
June 1999
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Peter Koch
Universitaet Bielefeld
June 1999
Recommendations for DNS SOA Values
draft-koch-dns-soa-values-01.txt
Abstract
The configuration and maintainance of DNS zones offer many degrees of
freedom and thus several opportunities for making mistakes. Most DNS
zones today are small and have to be set up and maintained by non-
experts. This document gives recommendations on which values to use
for the SOA resource record of small, stable DNS zones to aid novice
administrators and to contribute to DNS stability end efficiency.
1. Conventions used in this document
Domain names used in this document are for explanatory purposes only
and should not be expected to lead to useful information in real life
[RFC2606].
2. Background
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Various DNS surveying activities show that the vast majority of
today's DNS zones are populated by very few hosts. In most cases
there is only an HTTP server announced under the common name "www",
sometimes accompanied by distinct mail or DNS servers or a bastion
host. For many of these zones the configuration is touched once when
it is set up and then left alone without modification for a long
time.
These recommendations are aimed at small and stable DNS zones. There
are many legitimate reasons to use different values, e.g. proposed
changes or special purpose applications. Administrators of those
zones should consult one of the various more detailed DNS guidelines
or books. Several other recommendations for SOA values exist
[RFC1535, RFC1912], which are not obsoleted by this document but
which have a different focus. At the time of their writing DNS zones
were usually more densely populated and their target audience was
supposed to have more interest in DNS.
ISPs and DNS server vendors are encouraged to use this information
for their customers, in configuration tools or as default values.
Additional hints for initial name server setup and configuration of
this type of zone can be found in [DNSGUIDE1], [DNSGUIDE2].
3. Recommended SOA Values
example.com. 3600 SOA dns.example.com. hostmaster.example.com. (
1999022301 ; serial YYYYMMDDnn
86400 ; refresh ( 24 hours)
7200 ; retry ( 2 hours)
3600000 ; expire (1000 hours)
172800 ) ; minimum ( 2 days)
4. Remarks and Explanation
The values presented in the example.com SOA RR are discussed in
detail. One main goal was to provide for fixed cut-and-paste values
wherever possible instead of intervals to reduce the chance of
operational problems caused by unfortunate combinations. Other values
or sets of values will work as well, this is one set of values which
reflects successful current practice with respect to scalability and
stability.
4.1. The MNAME Value
The DNS specification explicitly states that the primary master
server be named here. The value must be determined and used.
Especially it is a mistake to repeat the zone name here, unless this
also leads to a valid address of the primary master.
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4.2. The RNAME Value
The RNAME is to publish a mail address of a person or role account
dealing with this zone with the "@" converted to a ".".
The best practice is to define (and maintain) a dedicated mail alias
"hostmaster" [RFC2142] for DNS operations.
4.3. The Serial Number
The most important issue is that this value be incremented after any
modification to the zone data. For debugging purposes it has shown to
be helpful to encode the modification date into the serial number.
The value "1999022301" so is an example of the YYYYMMDDnn scheme and
must be replaced by proper values for the year (YYYY, four digits),
month (MM, two digits), day of month (DD, two digits) and version per
day (nn, two digits). The first version of the day should have the
value "01". It is important to preserve the order year - month -
day. People using this as a debugging aid must, however, not rely on
the date informnation, since experience shows that after initial
setup maintainance of this value is often left to the auto-increment
feature the software sometimes provides.
Other schemes exist - documentation of which is out of the scope of
this document.
4.4. The Refresh and Retry Values
The refresh and retry values primarily affect the zone maintainer and
the secondary service providers and may be negotiated between them.
The values chosen here are aimed at scalability. Modern DNS software
implements NOTIFY [RFC1996] and reduces the need for frequent SOA
checks, as does the assumption of stability of the zone. While lower
values would only slightly increase the bandwidth usage, they would
increase the load on servers which are slaves for thousands of zones.
4.5. The Expire Value
The primary goal is to ensure stability of the zone data, even if a
mistake invalidating (non-authorizing) the zone or a network outage
last for several days. A value of a week or two has proven to be way
too short, so a longer time must be used. The specific value was
chosen for aesthetic and historic reasons and to disambiguate between
the different proposed values of "long".
4.6. The Minimum TTL Value
There are two meanings for this value with practical relevance.
First, it serves as a default value for the TTL of all RRs without a
given value. To be cache-friendly this value was chosen to be two
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days, which also follows the stability assumption. The second meaning
is the default negative TTL [RFC2308], which would call for a lower
value. We are in a transition phase now with software implementing
either of both meanings, so the TTL of one hour is recommended for
the SOA itself, which will lead to nearly the same effect.
5. Security Considerations
Filling in the recommended values will not directly influence
security of the name servers for the particular zone, any system with
a name in that zone or any other system in the Internet. However,
following these guidelines will likely contribute to DNS stability
and thus to reachability.
Maintaining proper contact information in the SOA RNAME field helps
people in reporting problems, although the address distributed there
is not recommended as a primary security contact.
6. Acknowledgements
This work is a product of the RIPE DNS working group.
7. References
[RFC1034] Mockapetris,P., "Domain Names - Concepts and Facilities",
RFC 1034, STD 13, November 1987
[RFC1035] Mockapetris,P., "Domain Names - Implementation and
Specification", RFC 1035, STD 13, November 1987
[RFC1123] Braden,R., "Requirements for Internet Hosts -- Application
and Support", RFC 1123, STD 3, October 1989
[RFC1537] Beertema,P., "Common DNS Data File Configuration Errors",
RFC 1537, October 1993
[RFC1912] Barr,D., "Common DNS Operational and Configuration
Errors", RFC 1912, February 1996
[RFC1996] Vixie,P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, August 1996
[RFC2142] Crocker,D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
FUNCTIONS", RFC 2142, May 1997
[RFC2308] Andrews,M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998
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[RFC2606] Eastlake,D., Panitz,A., "Reserved Top Level DNS Names",
RFC 2606, BCP 32, June 1999
[DNSGUIDE1] Liman,L., "SIMPLE DNS CONFIGURATION EXAMPLE", work in
progress
[DNSGUIDE2] Koch,P., "RIPE Guide To Setting Up a DNS Server", work in
progress
8. Author's Address
Peter Koch
Universitaet Bielefeld
Technische Fakultaet
Postfach 10 01 31
D-33501 Bielefeld
Germany
+49 521 106 2902
<pk@TechFak.Uni-Bielefeld.DE>
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