Network Working Group P. Beertema
Request for Comments: 1537 CWI
Category: Informational October 1993
Common DNS Data File Configuration Errors
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.
Abstract
This memo describes errors often found in DNS data files. It points
out common mistakes system administrators tend to make and why they
often go unnoticed for long periods of time.
Introduction
Due to the lack of extensive documentation and automated tools, DNS
zone files have mostly been configured by system administrators, by
hand. Some of the rules for writing the data files are rather subtle
and a few common mistakes are seen in domains worldwide.
This document is an attempt to list "surprises" that administrators
might find hidden in their zone files. It describes the symptoms of
the malady and prescribes medicine to cure that. It also gives some
general recommendations and advice on specific nameserver and zone
file issues and on the (proper) use of the Domain Name System.
1. SOA records
A problem I've found in quite some nameservers is that the various
timers have been set (far) too low. Especially for top level domain
nameservers this causes unnecessary traffic over international and
intercontinental links.
Unfortunately the examples given in the BIND manual, in RFC's and in
some expert documents give those very short timer values, and that's
most likely what people have modeled their SOA records after.
First of all a short explanation of the timers used in the SOA
record:
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- Refresh: The SOA record of the primary server is checked
every "refresh" time by the secondary servers;
if it has changed, a zone transfer is done.
- Retry: If a secondary server cannot reach the primary
server, it tries it again every "retry" time.
- Expire: If for "expire" time the primary server cannot
be reached, all information about the zone is
invalidated on the secondary servers (i.e., they
are no longer authoritative for that zone).
- Minimum TTL: The default TTL value for all records in the
zone file; a different TTL value may be given
explicitly in a record when necessary.
(This timer is named "Minimum", and that's
what it's function should be according to
STD 13, RFC 1035, but most (all?)
implementations take it as the default value
exported with records without an explicit TTL
value).
For top level domain servers I would recommend the following values:
86400 ; Refresh 24 hours
7200 ; Retry 2 hours
2592000 ; Expire 30 days
345600 ; Minimum TTL 4 days
For other servers I would suggest:
28800 ; Refresh 8 hours
7200 ; Retry 2 hours
604800 ; Expire 7 days
86400 ; Minimum TTL 1 day
but here the frequency of changes, the required speed of propagation,
the reachability of the primary server etc. play a role in optimizing
the timer values.
2. Glue records
Quite often, people put unnecessary glue (A) records in their zone
files. Even worse is that I've even seen *wrong* glue records for an
external host in a primary zone file! Glue records need only be in a
zone file if the server host is within the zone and there is no A
record for that host elsewhere in the zone file.
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Old BIND versions ("native" 4.8.3 and older versions) showed the
problem that wrong glue records could enter secondary servers in a
zone transfer.
3. "Secondary server surprise"
I've seen it happen on various occasions that hosts got bombarded by
nameserver requests without knowing why. On investigation it turned
out then that such a host was supposed to (i.e., the information was
in the root servers) run secondary for some domain (or reverse (in-
addr.arpa)) domain, without that host's nameserver manager having
been asked or even been told so!
Newer BIND versions (4.9 and later) solved this problem. At the same
time though the fix has the disadvantage that it's far less easy to
spot this problem.
Practice has shown that most domain registrars accept registrations
of nameservers without checking if primary (!) and secondary servers
have been set up, informed, or even asked. It should also be noted
that a combination of long-lasting unreachability of primary
nameservers, (therefore) expiration of zone information, plus static
IP routing, can lead to massive network traffic that can fill up
lines completely.
4. "MX records surprise"
In a sense similar to point 3. Sometimes nameserver managers enter MX
records in their zone files that point to external hosts, without
first asking or even informing the systems managers of those external
hosts. This has to be fought out between the nameserver manager and
the systems managers involved. Only as a last resort, if really
nothing helps to get the offending records removed, can the systems
manager turn to the naming authority of the domain above the
offending domain to get the problem sorted out.
5. "Name extension surprise"
Sometimes one encounters weird names, which appear to be an external
name extended with a local domain. This is caused by forgetting to
terminate a name with a dot: names in zone files that don't end with
a dot are always expanded with the name of the current zone (the
domain that the zone file stands for or the last $ORIGIN).
Example: zone file for foo.xx:
pqr MX 100 relay.yy.
xyz MX 100 relay.yy (no trailing dot!)
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When fully written out this stands for:
pqr.foo.xx. MX 100 relay.yy.
xyz.foo.xx. MX 100 relay.yy.foo.xx. (name extension!)
6. Missing secondary servers
It is required that there be a least 2 nameservers for a domain. For
obvious reasons the nameservers for top level domains need to be very
well reachable from all over the Internet. This implies that there
must be more than just 2 of them; besides, most of the (secondary)
servers should be placed at "strategic" locations, e.g., close to a
point where international and/or intercontinental lines come
together. To keep things manageable, there shouldn't be too many
servers for a domain either.
Important aspects in selecting the location of primary and secondary
servers are reliability (network, host) and expedient contacts: in
case of problems, changes/fixes must be carried out quickly. It
should be considered logical that primary servers for European top
level domains should run on a host in Europe, preferably (if
possible) in the country itself. For each top level domain there
should be 2 secondary servers in Europe and 2 in the USA, but there
may of course be more on either side. An excessive number of
nameservers is not a good idea though; a recommended maximum is 7
nameservers. In Europe, EUnet has offered to run secondary server
for each European top level domain.
7. Wildcard MX records
Wildcard MX records should be avoided where possible. They often
cause confusion and errors: especially beginning nameserver managers
tend to overlook the fact that a host/domain listed with ANY type of
record in a zone file is NOT covered by an overall wildcard MX record
in that zone; this goes not only for simple domain/host names, but
also for names that cover one or more domains. Take the following
example in zone foo.bar:
* MX 100 mailhost
pqr MX 100 mailhost
abc.def MX 100 mailhost
This makes pqr.foo.bar, def.foo.bar and abd.def.foo.bar valid
domains, but the wildcard MX record covers NONE of them, nor anything
below them. To cover everything by MX records, the required entries
are:
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* MX 100 mailhost
pqr MX 100 mailhost
*.pqr MX 100 mailhost
abc.def MX 100 mailhost
*.def MX 100 mailhost
*.abc.def MX 100 mailhost
An overall wildcard MX record is almost never useful.
In particular the zone file of a top level domain should NEVER
contain only an overall wildcard MX record (*.XX). The effect of such
a wildcard MX record can be that mail is unnecessarily sent across
possibly expensive links, only to fail at the destination or gateway
that the record points to. Top level domain zone files should
explicitly list at least all the officially registered primary
subdomains.
Whereas overall wildcard MX records should be avoided, wildcard MX
records are acceptable as an explicit part of subdomain entries,
provided they are allowed under a given subdomain (to be determined
by the naming authority for that domain).
Example:
foo.xx. MX 100 gateway.xx.
MX 200 fallback.yy.
*.foo.xx. MX 100 gateway.xx.
MX 200 fallback.yy.
8. Hostnames
People appear to sometimes look only at STD 11, RFC 822 to determine
whether a particular hostname is correct or not. Hostnames should
strictly conform to the syntax given in STD 13, RFC 1034 (page 11),
with *addresses* in addition conforming to RFC 822. As an example
take "c&w.blues" which is perfectly legal according to RFC 822, but
which can have quite surprising effects on particular systems, e.g.,
"telnet c&w.blues" on a Unix system.
9. HINFO records
There appears to be a common misunderstanding that one of the data
fields (usually the second field) in HINFO records is optional. A
recent scan of all reachable nameservers in only one country revealed
some 300 incomplete HINFO records. Specifying two data fields in a
HINFO record is mandatory (RFC 1033), but note that this does *not*
mean that HINFO records themselves are mandatory.
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10. Safety measures and specialties
Nameservers and resolvers aren't flawless. Bogus queries should be
kept from being forwarded to the root servers, since they'll only
lead to unnecessary intercontinental traffic. Known bogus queries
that can easily be dealt with locally are queries for 0 and broadcast
addresses. To catch such queries, every nameserver should run
primary for the 0.in-addr.arpa and 255.in-addr.arpa zones; the zone
files need only contain a SOA and an NS record.
Also each nameserver should run primary for 0.0.127.in-addr.arpa;
that zone file should contain a SOA and NS record and an entry:
1 PTR localhost.
There has been extensive discussion about whether or not to append
the local domain to it. The conclusion was that "localhost." would be
the best solution; reasons given were:
- "localhost" itself is used and expected to work on some systems.
- translating 127.0.0.1 into "localhost.my_domain" can cause some
software to connect to itself using the loopback interface when
it didn't want to.
Note that all domains that contain hosts should have a "localhost" A
record in them.
People maintaining zone files with the Serial number given in dotted
decimal notation (e.g., when SCCS is used to maintain the files)
should beware of a bug in all BIND versions: if the serial number is
in Release.Version (dotted decimal) notation, then it is virtually
impossible to change to a higher release: because of the wrong way
that notation is turned into an integer, it results in a serial
number that is LOWER than that of the former release.
For this reason and because the Serial is an (unsigned) integer
according to STD 13, RFC 1035, it is recommended not to use the
dotted decimal notation. A recommended notation is to use the date
(yyyymmdd), if necessary with an extra digit (yyyymmddn) if there is
or can be more than one change per day in a zone file.
Very old versions of DNS resolver code have a bug that causes queries
for A records with domain names like "192.16.184.3" to go out. This
happens when users type in IP addresses and the resolver code does
not catch this case before sending out a DNS query. This problem has
been fixed in all resolver implementations known to us but if it
still pops up it is very serious because all those queries will go to
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the root servers looking for top level domains like "3" etc. It is
strongly recommended to install the latest (publicly) available BIND
version plus all available patches to get rid of these and other
problems.
Running secondary nameserver off another secondary nameserver is
possible, but not recommended unless really necessary: there are
known cases where it has led to problems like bogus TTL values. This
can be caused by older or flawed implementations, but secondary
nameservers in principle should always transfer their zones from the
official primary nameserver.
11. Some general points
The Domain Name System and nameserver are purely technical tools, not
meant in any way to exert control or impose politics. The function of
a naming authority is that of a clearing house. Anyone registering a
subdomain under a particular (top level) domain becomes naming
authority and therewith the sole responsible for that subdomain.
Requests to enter MX or NS records concerning such a subdomain
therefore always MUST be honored by the registrar of the next higher
domain.
Examples of practices that are not allowed are:
- imposing specific mail routing (MX records) when registering
a subdomain.
- making registration of a subdomain dependent on to the use of
certain networks or services.
- using TXT records as a means of (free) commercial advertising.
In the latter case a network service provider could decide to cut off
a particular site until the offending TXT records have been removed
from the site's zone file.
Of course there are obvious cases where a naming authority can refuse
to register a particular subdomain and can require a proposed name to
be changed in order to get it registered (think of DEC trying to
register a domain IBM.XX).
There are also cases were one has to probe the authority of the
person: sending in the application - not every systems manager should
be able to register a domain name for a whole university. The naming
authority can impose certain extra rules as long as they don't
violate or conflict with the rights and interest of the registrars of
subdomains; a top level domain registrar may e.g., require that there
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be primary subdomain "ac" and "co" only and that subdomains be
registered under those primary subdomains.
The naming authority can also interfere in exceptional cases like the
one mentioned in point 4, e.g., by temporarily removing a domain's
entry from the nameserver zone files; this of course should be done
only with extreme care and only as a last resort.
When adding NS records for subdomains, top level domain nameserver
managers should realize that the people setting up the nameserver for
a subdomain often are rather inexperienced and can make mistakes that
can easily lead to the subdomain becoming completely unreachable or
that can cause unnecessary DNS traffic (see point 1). It is therefore
highly recommended that, prior to entering such an NS record, the
(top level) nameserver manager does a couple of sanity checks on the
new nameserver (SOA record and timers OK?, MX records present where
needed? No obvious errors made? Listed secondary servers
operational?). Things that cannot be caught though by such checks
are:
- resolvers set up to use external hosts as nameservers
- nameservers set up to use external hosts as forwarders
without permission from those hosts.
Care should also be taken when registering 2-letter subdomains.
Although this is allowed, an implication is that abbreviated
addressing (see STD 11, RFC 822, paragraph 6.2.2) is not possible in
and under that subdomain. When requested to register such a domain,
one should always notify the people of this consequence. As an
example take the name "cs", which is commonly used for Computer
Science departments: it is also the name of the top level domain for
Czecho-Slovakia, so within the domain cs.foo.bar the user@host.cs is
ambiguous in that in can denote both a user on the host
host.cs.foo.bar and a user on the host "host" in Czecho-Slovakia.
(This example does not take into account the recent political changes
in the mentioned country).
References
[1] Mockapetris, P., "Domain Names Concepts and Facilities", STD 13,
RFC 1034, USC/Information Sciences Institute, November 1987.
[2] Mockapetris, P., "Domain Names Implementation and Specification",
STD 13, RFC 1035, USC/Information Sciences Institute, November
1987.
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[3] Partridge, C., "Mail Routing and the Domain System", STD 14, RFC
974, CSNET CIC BBN, January 1986.
[4] Gavron, E., "A Security Problem and Proposed Correction With
Widely Deployed DNS Software", RFC 1535, ACES Research Inc.,
October 1993.
[5] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S. Miller,
"Common DNS Implementation Errors and Suggested Fixes", RFC 1536,
USC/Information Sciences Institute, USC, October 1993.
Security Considerations
Security issues are not discussed in this memo.
Author's Address
Piet Beertema
CWI
Kruislaan 413
NL-1098 SJ Amsterdam
The Netherlands
Phone: +31 20 592 4112
FAX: +31 20 592 4199
EMail: Piet.Beertema@cwi.nl
Editor's Address
Anant Kumar
USC Information Sciences Institute
4676 Admiralty Way
Marina Del Rey CA 90292-6695
Phone:(310) 822-1511
FAX: (310) 823-6741
EMail: anant@isi.edu
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