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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


 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.


 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

Beertema [Page 1] RFC 1537 Common DNS Data File Configuration Errors October 1993

  1. 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.
  1. Retry: If a secondary server cannot reach the primary

server, it tries it again every "retry" time.

  1. 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).
  1. 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
 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.

Beertema [Page 2] RFC 1537 Common DNS Data File Configuration Errors October 1993

 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- 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!)

Beertema [Page 3] RFC 1537 Common DNS Data File Configuration Errors October 1993

 When fully written out this stands for:  MX 100  relay.yy.  MX 100   (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
  • MX 100 mailhost

pqr MX 100 mailhost

       abc.def      MX 100  mailhost
 This makes, and valid
 domains, but the wildcard MX record covers NONE of them, nor anything
 below them.  To cover everything by MX records, the required entries

Beertema [Page 4] RFC 1537 Common DNS Data File Configuration Errors October 1993

  • 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
 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).
       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.

Beertema [Page 5] RFC 1537 Common DNS Data File Configuration Errors October 1993

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 and zones; the zone
 files need only contain a SOA and an NS record.
 Also each nameserver should run primary for;
 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:
  1. "localhost" itself is used and expected to work on some systems.
  1. translating 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 "" 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

Beertema [Page 6] RFC 1537 Common DNS Data File Configuration Errors October 1993

 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
 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
 Examples of practices that are not allowed are:
  1. imposing specific mail routing (MX records) when registering

a subdomain.

  1. making registration of a subdomain dependent on to the use of

certain networks or services.

  1. 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

Beertema [Page 7] RFC 1537 Common DNS Data File Configuration Errors October 1993

 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
  1. resolvers set up to use external hosts as nameservers
  1. 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 the user@host.cs is
 ambiguous in that in can denote both a user on the host 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).


 [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

Beertema [Page 8] RFC 1537 Common DNS Data File Configuration Errors October 1993

 [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
 Kruislaan 413
 NL-1098 SJ Amsterdam
 The Netherlands
 Phone: +31 20 592 4112
 FAX:   +31 20 592 4199

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

Beertema [Page 9]

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