path: root/doc/rfc/rfc1537.txt

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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