rfc1912.txt

Network Working Group                                            D. Barr
Request for Comments: 1912             The Pennsylvania State University
Obsoletes: 1537                                            February 1996
Category: Informational


            Common DNS Operational and Configuration Errors

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This memo describes errors often found in both the operation of
   Domain Name System (DNS) servers, and in the data that these DNS
   servers contain.  This memo tries to summarize current Internet
   requirements as well as common practice in the operation and
   configuration of the DNS.  This memo also tries to summarize or
   expand upon issues raised in [RFC 1537].

1. Introduction

   Running a nameserver is not a trivial task.  There are many things
   that can go wrong, and many decisions have to be made about what data
   to put in the DNS and how to set up servers.  This memo attempts to
   address many of the common mistakes and pitfalls that are made in DNS
   data as well as in the operation of nameservers.  Discussions are
   also made regarding some other relevant issues such as server or
   resolver bugs, and a few political issues with respect to the
   operation of DNS on the Internet.

2. DNS Data

   This section discusses problems people typically have with the DNS
   data in their nameserver, as found in the zone data files that the
   nameserver loads into memory.

2.1 Inconsistent, Missing, or Bad Data

   Every Internet-reachable host should have a name.  The consequences
   of this are becoming more and more obvious.  Many services available
   on the Internet will not talk to you if you aren't correctly
   registered in the DNS.





Barr                         Informational                      [Page 1]

RFC 1912                   Common DNS Errors               February 1996


   Make sure your PTR and A records match.  For every IP address, there
   should be a matching PTR record in the in-addr.arpa domain.  If a
   host is multi-homed, (more than one IP address) make sure that all IP
   addresses have a corresponding PTR record (not just the first one).
   Failure to have matching PTR and A records can cause loss of Internet
   services similar to not being registered in the DNS at all.  Also,
   PTR records must point back to a valid A record, not a alias defined
   by a CNAME.  It is highly recommended that you use some software
   which automates this checking, or generate your DNS data from a
   database which automatically creates consistent data.

   DNS domain names consist of "labels" separated by single dots.  The
   DNS is very liberal in its rules for the allowable characters in a
   domain name.  However, if a domain name is used to name a host, it
   should follow rules restricting host names.  Further if a name is
   used for mail, it must follow the naming rules for names in mail
   addresses.

   Allowable characters in a label for a host name are only ASCII
   letters, digits, and the `-' character.  Labels may not be all
   numbers, but may have a leading digit  (e.g., 3com.com).  Labels must
   end and begin only with a letter or digit.  See [RFC 1035] and [RFC
   1123].  (Labels were initially restricted in [RFC 1035] to start with
   a letter, and some older hosts still reportedly have problems with
   the relaxation in [RFC 1123].)  Note there are some Internet
   hostnames which violate this rule (411.org, 1776.com).  The presence
   of underscores in a label is allowed in [RFC 1033], except [RFC 1033]
   is informational only and was not defining a standard.  There is at
   least one popular TCP/IP implementation which currently refuses to
   talk to hosts named with underscores in them.  It must be noted that
   the language in [1035] is such that these rules are voluntary -- they
   are there for those who wish to minimize problems.  Note that the
   rules for Internet host names also apply to hosts and addresses used
   in SMTP (See RFC 821).

   If a domain name is to be used for mail (not involving SMTP), it must
   follow the rules for mail in [RFC 822], which is actually more
   liberal than the above rules.  Labels for mail can be any ASCII
   character except "specials", control characters, and whitespace
   characters.  "Specials" are specific symbols used in the parsing of
   addresses.  They are the characters "()<>@,;:\".[]".  (The "!"
   character wasn't in [RFC 822], however it also shouldn't be used due
   to the conflict with UUCP mail as defined in RFC 976)  However, since
   today almost all names which are used for mail on the Internet are
   also names used for hostnames, one rarely sees addresses using these
   relaxed standard, but mail software should be made liberal and robust
   enough to accept them.




Barr                         Informational                      [Page 2]

RFC 1912                   Common DNS Errors               February 1996


   You should also be careful to not have addresses which are valid
   alternate syntaxes to the inet_ntoa() library call.  For example 0xe
   is a valid name, but if you were to type "telnet 0xe", it would try
   to connect to IP address 0.0.0.14.  It is also rumored that there
   exists some broken inet_ntoa() routines that treat an address like
   x400 as an IP address.

   Certain operating systems have limitations on the length of their own
   hostname.  While not strictly of issue to the DNS, you should be
   aware of your operating system's length limits before choosing the
   name of a host.

   Remember that many resource records (abbreviated RR) take on more
   than one argument.  HINFO requires two arguments, as does RP.  If you
   don't supply enough arguments, servers sometime return garbage for
   the missing fields.  If you need to include whitespace within any
   data, you must put the string in quotes.

2.2 SOA records

   In the SOA record of every zone, remember to fill in the e-mail
   address that will get to the person who maintains the DNS at your
   site (commonly referred to as "hostmaster").  The `@' in the e-mail
   must be replaced by a `.' first.  Do not try to put an `@' sign in
   this address.  If the local part of the address already contains a
   `.' (e.g., John.Smith@widget.xx), then you need to quote the `.' by
   preceding it with `\' character.  (e.g., to become
   John\.Smith.widget.xx) Alternately (and preferred), you can just use
   the generic name `hostmaster', and use a mail alias to redirect it to
   the appropriate persons.  There exists software which uses this field
   to automatically generate the e-mail address for the zone contact.
   This software will break if this field is improperly formatted.  It
   is imperative that this address get to one or more real persons,
   because it is often used for everything from reporting bad DNS data
   to reporting security incidents.

   Even though some BIND versions allow you to use a decimal in a serial
   number, don't.  A decimal serial number is converted to an unsigned
   32-bit integer internally anyway.  The formula for a n.m serial
   number is n*10^(3+int(0.9+log10(m))) + m which translates to
   something rather unexpected.  For example it's routinely possible
   with a decimal serial number (perhaps automatically generated by
   SCCS) to be incremented such that it is numerically larger, but after
   the above conversion yield a serial number which is LOWER than
   before.  Decimal serial numbers have been officially deprecated in
   recent BIND versions.  The recommended syntax is YYYYMMDDnn
   (YYYY=year, MM=month, DD=day, nn=revision number.  This won't
   overflow until the year 4294.



Barr                         Informational                      [Page 3]

RFC 1912                   Common DNS Errors               February 1996


   Choose logical values for the timer values in the SOA record (note
   values below must be expressed as seconds in the zone data):

      Refresh: How often a secondary will poll the primary server to see
          if the serial number for the zone has increased (so it knows
          to request a new copy of the data for the zone).  Set this to
          how long your secondaries can comfortably contain out-of-date
          data.  You can keep it short (20 mins to 2 hours) if you
          aren't worried about a small increase in bandwidth used, or
          longer (2-12 hours) if your Internet connection is slow or is
          started on demand.  Recent BIND versions (4.9.3) have optional
          code to automatically notify secondaries that data has
          changed, allowing you to set this TTL to a long value (one
          day, or more).

      Retry: If a secondary was unable to contact the primary at the
          last refresh, wait the retry value before trying again.  This
          value isn't as important as others, unless the secondary is on
          a distant network from the primary or the primary is more
          prone to outages.  It's typically some fraction of the refresh
          interval.


      Expire: How long a secondary will still treat its copy of the zone
          data as valid if it can't contact the primary.  This value
          should be greater than how long a major outage would typically
          last, and must be greater than the minimum and retry
          intervals, to avoid having a secondary expire the data before
          it gets a chance to get a new copy.  After a zone is expired a
          secondary will still continue to try to contact the primary,
          but it will no longer provide nameservice for the zone.  2-4
          weeks are suggested values.

      Minimum: The default TTL (time-to-live) for resource records --
          how long data will remain in other nameservers' cache.  ([RFC
          1035] defines this to be the minimum value, but servers seem
          to always implement this as the default value)  This is by far
          the most important timer.  Set this as large as is comfortable
          given how often you update your nameserver.  If you plan to
          make major changes, it's a good idea to turn this value down
          temporarily beforehand.  Then wait the previous minimum value,
          make your changes, verify their correctness, and turn this
          value back up.  1-5 days are typical values.  Remember this
          value can be overridden on individual resource records.







Barr                         Informational                      [Page 4]

RFC 1912                   Common DNS Errors               February 1996


   As you can see, the typical values above for the timers vary widely.
   Popular documentation like [RFC 1033] recommended a day for the
   minimum TTL, which is now considered too low except for zones with
   data that vary regularly.  Once a DNS stabilizes, values on the order
   of 3 or more days are recommended.  It is also recommended that you
   individually override the TTL on certain RRs which are often
   referenced and don't often change to have very large values (1-2
   weeks).  Good examples of this are the MX, A, and PTR records of your
   mail host(s), the NS records of your zone, and the A records of your
   nameservers.

2.3 Glue A Records

   Glue records are A records that are associated with NS records to
   provide "bootstrapping" information to the nameserver.  For example:

           podunk.xx.      in      ns      ns1.podunk.xx.
                           in      ns      ns2.podunk.xx.
           ns1.podunk.xx.  in      a       1.2.3.4
           ns2.podunk.xx.  in      a       1.2.3.5

   Here, the A records are referred to as "Glue records".

   Glue records are required only in forward zone files for nameservers
   that are located in the subdomain of the current zone that is being
   delegated.  You shouldn't have any A records in an in-addr.arpa zone
   file (unless you're using RFC 1101-style encoding of subnet masks).

   If your nameserver is multi-homed (has more than one IP address), you
   must list all of its addresses in the glue to avoid cache
   inconsistency due to differing TTL values, causing some lookups to
   not find all addresses for your nameserver.

   Some people get in the bad habit of putting in a glue record whenever
   they add an NS record "just to make sure".  Having duplicate glue
   records in your zone files just makes it harder when a nameserver
   moves to a new IP address, or is removed. You'll spend hours trying
   to figure out why random people still see the old IP address for some
   host, because someone forgot to change or remove a glue record in
   some other file.  Newer BIND versions will ignore these extra glue
   records in local zone files.

   Older BIND versions (4.8.3 and previous) have a problem where it
   inserts these extra glue records in the zone transfer data to
   secondaries.  If one of these glues is wrong, the error can be
   propagated to other nameservers.  If two nameservers are secondaries
   for other zones of each other, it's possible for one to continually
   pass old glue records back to the other.  The only way to get rid of



Barr                         Informational                      [Page 5]

RFC 1912                   Common DNS Errors               February 1996


   the old data is to kill both of them, remove the saved backup files,
   and restart them.  Combined with that those same versions also tend
   to become infected more easily with bogus data found in other non-
   secondary nameservers (like the root zone data).

2.4 CNAME records

   A CNAME record is not allowed to coexist with any other data.  In
   other words, if suzy.podunk.xx is an alias for sue.podunk.xx, you
   can't also have an MX record for suzy.podunk.edu, or an A record, or
   even a TXT record.  Especially do not try to combine CNAMEs and NS
   records like this!:


           podunk.xx.      IN      NS      ns1
                           IN      NS      ns2
                           IN      CNAME   mary
           mary            IN      A       1.2.3.4


   This is often attempted by inexperienced administrators as an obvious
   way to allow your domain name to also be a host.  However, DNS
   servers like BIND will see the CNAME and refuse to add any other
   resources for that name.  Since no other records are allowed to
   coexist with a CNAME, the NS entries are ignored.  Therefore all the
   hosts in the podunk.xx domain are ignored as well!

   If you want to have your domain also be a host, do the following:

           podunk.xx.      IN      NS      ns1
                           IN      NS      ns2
                           IN      A       1.2.3.4
           mary            IN      A       1.2.3.4

   Don't go overboard with CNAMEs.  Use them when renaming hosts, but
   plan to get rid of them (and inform your users).  However CNAMEs are
   useful (and encouraged) for generalized names for servers -- `ftp'
   for your ftp server, `www' for your Web server, `gopher' for your
   Gopher server, `news' for your Usenet news server, etc.

   Don't forget to delete the CNAMEs associated with a host if you
   delete the host it is an alias for.  Such "stale CNAMEs" are a waste
   of resources.








Barr                         Informational                      [Page 6]

RFC 1912                   Common DNS Errors               February 1996


   Don't use CNAMEs in combination with RRs which point to other names
   like MX, CNAME, PTR and NS.  (PTR is an exception if you want to
   implement classless in-addr delegation.)  For example, this is
   strongly discouraged:

           podunk.xx.      IN      MX      mailhost
           mailhost        IN      CNAME   mary
           mary            IN      A       1.2.3.4


   [RFC 1034] in section 3.6.2 says this should not be done, and [RFC
   974] explicitly states that MX records shall not point to an alias
   defined by a CNAME.  This results in unnecessary indirection in
   accessing the data, and DNS resolvers and servers need to work more
   to get the answer.  If you really want to do this, you can accomplish
   the same thing by using a preprocessor such as m4 on your host files.

   Also, having chained records such as CNAMEs pointing to CNAMEs may
   make administration issues easier, but is known to tickle bugs in
   some resolvers that fail to check loops correctly.  As a result some
   hosts may not be able to resolve such names.

   Having NS records pointing to a CNAME is bad and may conflict badly
   with current BIND servers.  In fact, current BIND implementations
   will ignore such records, possibly leading to a lame delegation.
   There is a certain amount of security checking done in BIND to
   prevent spoofing DNS NS records.  Also, older BIND servers reportedly
   will get caught in an infinite query loop trying to figure out the
   address for the aliased nameserver, causing a continuous stream of
   DNS requests to be sent.

2.5 MX records

   It is a good idea to give every host an MX record, even if it points
   to itself!  Some mailers will cache MX records, but will always need
   to check for an MX before sending mail.  If a site does not have an
   MX, then every piece of mail may result in one more resolver query,
   since the answer to the MX query often also contains the IP addresses
   of the MX hosts.  Internet SMTP mailers are required by [RFC 1123] to
   support the MX mechanism.

   Put MX records even on hosts that aren't intended to send or receive
   e-mail.  If there is a security problem involving one of these hosts,
   some people will mistakenly send mail to postmaster or root at the
   site without checking first to see if it is a "real" host or just a
   terminal or personal computer that's not set up to accept e-mail.  If
   you give it an MX record, then the e-mail can be redirected to a real
   person.  Otherwise mail can just sit in a queue for hours or days



Barr                         Informational                      [Page 7]

RFC 1912                   Common DNS Errors               February 1996


   until the mailer gives up trying to send it.

   Don't forget that whenever you add an MX record, you need to inform
   the target mailer if it is to treat the first host as "local".  (The
   "Cw" flag in sendmail, for example)

   If you add an MX record which points to an external host (e.g., for
   the purposes of backup mail routing) be sure to ask permission from
   that site first.  Otherwise that site could get rather upset and take
   action (like throw your mail away, or appeal to higher authorities
   like your parent DNS administrator or network provider.)

2.6 Other Resource Records

2.6.1 WKS

   WKS records are deprecated in [RFC 1123].  They serve no known useful
   function, except internally among LISP machines.  Don't use them.

2.6.2 HINFO

   On the issue HINFO records, some will argue that these is a security
   problem (by broadcasting what vendor hardware and operating system
   you so people can run systematic attacks on known vendor security
   holes).  If you do use them, you should keep up to date with known
   vendor security problems.  However, they serve a useful purpose.
   Don't forget that HINFO requires two arguments, the hardware type,
   and the operating system.

   HINFO is sometimes abused to provide other information.  The record
   is meant to provide specific information about the machine itself.
   If you need to express other information about the host in the DNS,
   use TXT.

2.6.3 TXT

   TXT records have no specific definition.  You can put most anything
   in them.  Some use it for a generic description of the host, some put
   specific information like its location, primary user, or maybe even a
   phone number.

2.6.4 RP

   RP records are relatively new.  They are used to specify an e-mail
   address (see first paragraph of section 2.2)  of the "Responsible
   Person" of the host, and the name of a TXT record where you can get
   more information.  See [RFC 1183].




Barr                         Informational                      [Page 8]

RFC 1912                   Common DNS Errors               February 1996


2.7 Wildcard records

   Wildcard MXs are useful mostly for non IP-connected sites.  A common
   mistake is thinking that a wildcard MX for a zone will apply to all
   hosts in the zone.  A wildcard MX will apply only to names in the
   zone which aren't listed in the DNS at all.  e.g.,

           podunk.xx.      IN      NS      ns1
                           IN      NS      ns2
           mary            IN      A       1.2.3.4
           *.podunk.xx.    IN      MX      5 sue

   Mail for mary.podunk.xx will be sent to itself for delivery.  Only
   mail for jane.podunk.xx or any hosts you don't see above will be sent
   to the MX.  For most Internet sites, wildcard MX records are not
   useful.  You need to put explicit MX records on every host.

   Wildcard MXs can be bad, because they make some operations succeed
   when they should fail instead.  Consider the case where someone in
   the domain "widget.com" tries to send mail to "joe@larry".  If the
   host "larry" doesn't actually exist, the mail should in fact bounce
   immediately.  But because of domain searching the address gets
   resolved to "larry.widget.com", and because of the wildcard MX this
   is a valid address according to DNS.  Or perhaps someone simply made
   a typo in the hostname portion of the address.  The mail message then
   gets routed to the mail host, which then rejects the mail with
   strange error messages like "I refuse to talk to myself" or "Local
   configuration error".

   Wildcard MX records are good for when you have a large number of
   hosts which are not directly Internet-connected (for example, behind
   a firewall) and for administrative or political reasons it is too
   difficult to have individual MX records for every host, or to force
   all e-mail addresses to be "hidden" behind one or more domain names.
   In that case, you must divide your DNS into two parts, an internal
   DNS, and an external DNS.  The external DNS will have only a few
   hosts and explicit MX records, and one or more wildcard MXs for each
   internal domain.  Internally the DNS will be complete, with all
   explicit MX records and no wildcards.

   Wildcard As and CNAMEs are possible too, and are really confusing to
   users, and a potential nightmare if used without thinking first.  It
   could result (due again to domain searching) in any telnet/ftp
   attempts from within the domain to unknown hosts to be directed to
   one address.  One such wildcard CNAME (in *.edu.com) caused
   Internet-wide loss of services and potential security nightmares due
   to unexpected interactions with domain searching.  It resulted in
   swift fixes, and even an RFC ([RFC 1535]) documenting the problem.



Barr                         Informational                      [Page 9]

RFC 1912                   Common DNS Errors               February 1996


2.8 Authority and Delegation Errors (NS records)

   You are required to have at least two nameservers for every domain,
   though more is preferred.  Have secondaries outside your network.  If
   the secondary isn't under your control, periodically check up on them
   and make sure they're getting current zone data from you.  Queries to
   their nameserver about your hosts should always result in an
   "authoritative" response.  If not, this is called a "lame
   delegation".  A lame delegations exists when a nameserver is
   delegated responsibility for providing nameservice for a zone (via NS
   records) but is not performing nameservice for that zone (usually
   because it is not set up as a primary or secondary for the zone).

   The "classic" lame delegation can be illustrated in this example:

           podunk.xx.      IN      NS      ns1.podunk.xx.
                           IN      NS      ns0.widget.com.

   "podunk.xx" is a new domain which has recently been created, and
   "ns1.podunk.xx" has been set up to perform nameservice for the zone.
   They haven't quite finished everything yet and haven't made sure that
   the hostmaster at "ns0.widget.com" has set up to be a proper
   secondary, and thus has no information about the podunk.xx domain,
   even though the DNS says it is supposed to.  Various things can
   happen depending on which nameserver is used.  At best, extra DNS
   traffic will result from a lame delegation.  At worst, you can get
   unresolved hosts and bounced e-mail.

   Also, sometimes a nameserver is moved to another host or removed from
   the list of secondaries.  Unfortunately due to caching of NS records,
   many sites will still think that a host is a secondary after that
   host has stopped providing nameservice.  In order to prevent lame
   delegations while the cache is being aged, continue to provide
   nameservice on the old nameserver for the length of the maximum of
   the minimum plus refresh times for the zone and the parent zone.
   (See section 2.2)

   Whenever a primary or secondary is removed or changed, it takes a
   fair amount of human coordination among the parties involved.  (The
   site itself, it's parent, and the site hosting the secondary)  When a
   primary moves, make sure all secondaries have their named.boot files
   updated and their servers reloaded.  When a secondary moves, make
   sure the address records at both the primary and parent level are
   changed.

   It's also been reported that some distant sites like to pick popular
   nameservers like "ns.uu.net" and just add it to their list of NS
   records in hopes that they will magically perform additional



Barr                         Informational                     [Page 10]

RFC 1912                   Common DNS Errors               February 1996


   nameservice for them.  This is an even worse form of lame delegation,
   since this adds traffic to an already busy nameserver.  Please
   contact the hostmasters of sites which have lame delegations.
   Various tools can be used to detect or actively find lame
   delegations.  See the list of contributed software in the BIND
   distribution.

   Make sure your parent domain has the same NS records for your zone as
   you do.  (Don't forget your in-addr.arpa zones too!).  Do not list
   too many (7 is the recommended maximum), as this just makes things
   harder to manage and is only really necessary for very popular top-
   level or root zones.  You also run the risk of overflowing the 512-
   byte limit of a UDP packet in the response to an NS query.  If this
   happens, resolvers will "fall back" to using TCP requests, resulting
   in increased load on your nameserver.

   It's important when picking geographic locations for secondary
   nameservers to minimize latency as well as increase reliability.
   Keep in mind network topologies.  For example if your site is on the
   other end of a slow local or international link, consider a secondary
   on the other side of the link to decrease average latency.  Contact
   your Internet service provider or parent domain contact for more
   information about secondaries which may be available to you.

3. BIND operation

   This section discusses common problems people have in the actual
   operation of the nameserver (specifically, BIND).  Not only must the
   data be correct as explained above, but the nameserver must be
   operated correctly for the data to be made available.

3.1 Serial numbers

   Each zone has a serial number associated with it.  Its use is for
   keeping track of who has the most current data.  If and only if the
   primary's serial number of the zone is greater will the secondary ask
   the primary for a copy of the new zone data (see special case below).

   Don't forget to change the serial number when you change data!  If
   you don't, your secondaries will not transfer the new zone
   information.  Automating the incrementing of the serial number with
   software is also a good idea.

   If you make a mistake and increment the serial number too high, and
   you want to reset the serial number to a lower value, use the
   following procedure:





Barr                         Informational                     [Page 11]

RFC 1912                   Common DNS Errors               February 1996


      Take the `incorrect' serial number and add 2147483647 to it.  If
      the number exceeds 4294967296, subtract 4294967296.  Load the
      resulting number.  Then wait 2 refresh periods to allow the zone
      to propagate to all servers.

      Repeat above until the resulting serial number is less than the
      target serial number.

      Up the serial number to the target serial number.

   This procedure won't work if one of your secondaries is running an
   old version of BIND (4.8.3 or earlier).  In this case you'll have to
   contact the hostmaster for that secondary and have them kill the
   secondary servers, remove the saved backup file, and restart the
   server.  Be careful when editing the serial number -- DNS admins
   don't like to kill and restart nameservers because you lose all that
   cached data.

3.2 Zone file style guide

   Here are some useful tips in structuring your zone files.  Following
   these will help you spot mistakes, and avoid making more.

   Be consistent with the style of entries in your DNS files. If your
   $ORIGIN is podunk.xx., try not to write entries like:

           mary            IN      A       1.2.3.1
           sue.podunk.xx.  IN      A       1.2.3.2

   or:

           bobbi           IN      A       1.2.3.2
                           IN      MX      mary.podunk.xx.


   Either use all FQDNs (Fully Qualified Domain Names) everywhere or
   used unqualified names everywhere.  Or have FQDNs all on the right-
   hand side but unqualified names on the left.  Above all, be
   consistent.

   Use tabs between fields, and try to keep columns lined up.  It makes
   it easier to spot missing fields (note some fields such as "IN" are
   inherited from the previous record and may be left out in certain
   circumstances.)







Barr                         Informational                     [Page 12]

RFC 1912                   Common DNS Errors               February 1996


   Remember you don't need to repeat the name of the host when you are
   defining multiple records for one host.  Be sure also to keep all
   records associated with a host together in the file.  It will make
   things more straightforward when it comes time to remove or rename a
   host.

   Always remember your $ORIGIN.  If you don't put a `.' at the end of
   an FQDN, it's not recognized as an FQDN.  If it is not an FQDN, then
   the nameserver will append $ORIGIN to the name.  Double check, triple
   check, those trailing dots, especially in in-addr.arpa zone files,
   where they are needed the most.

   Be careful with the syntax of the SOA and WKS records (the records
   which use parentheses).  BIND is not very flexible in how it parses
   these records.  See the documentation for BIND.

3.3 Verifying data

   Verify the data you just entered or changed by querying the resolver
   with dig (or your favorite DNS tool, many are included in the BIND
   distribution) after a change.  A few seconds spent double checking
   can save hours of trouble, lost mail, and general headaches.  Also be
   sure to check syslog output when you reload the nameserver.  If you
   have grievous errors in your DNS data or boot file, named will report
   it via syslog.

   It is also highly recommended that you automate this checking, either
   with software which runs sanity checks on the data files before they
   are loaded into the nameserver, or with software which checks the
   data already loaded in the nameserver.  Some contributed software to
   do this is included in the BIND distribution.

4. Miscellaneous Topics

4.1 Boot file setup

   Certain zones should always be present in nameserver configurations:

           primary         localhost               localhost
           primary         0.0.127.in-addr.arpa    127.0
           primary         255.in-addr.arpa        255
           primary         0.in-addr.arpa          0

   These are set up to either provide nameservice for "special"
   addresses, or to help eliminate accidental queries for broadcast or
   local address to be sent off to the root nameservers.  All of these
   files will contain NS and SOA records just like the other zone files
   you maintain, the exception being that you can probably make the SOA



Barr                         Informational                     [Page 13]

RFC 1912                   Common DNS Errors               February 1996


   timers very long, since this data will never change.

   The "localhost" address is a "special" address which always refers to
   the local host.  It should contain the following line:

           localhost.      IN      A       127.0.0.1

   The "127.0" file should contain the line:

           1    PTR     localhost.

   There has been some extensive discussion about whether or not to
   append the local domain to it.  The conclusion is that "localhost."
   would be the best solution.  The reasons given include:

      "localhost" by itself is used and expected to work in some
      systems.

      Translating 127.0.0.1 into "localhost.dom.ain" can cause some
      software to connect back to the loopback interface when it didn't
      want to because "localhost" is not equal to "localhost.dom.ain".

   The "255" and "0" files should not contain any additional data beyond
   the NS and SOA records.

   Note that future BIND versions may include all or some of this data
   automatically without additional configuration.

4.2 Other Resolver and Server bugs

   Very old versions of the DNS resolver have a bug that cause queries
   for names that look like IP addresses to go out, because the user
   supplied an IP address and the software didn't realize that it didn't
   need to be resolved.  This has been fixed but occasionally it still
   pops up.  It's important because this bug means that these queries
   will be sent directly to the root nameservers, adding to an already
   heavy DNS load.

   While running a secondary nameserver off another secondary nameserver
   is possible, it is not recommended unless necessary due to network
   topologies.  There are known cases where it has led to problems like
   bogus TTL values.  While this may be caused by older or flawed DNS
   implementations, you should not chain secondaries off of one another
   since this builds up additional reliability dependencies as well as
   adds additional delays in updates of new zone data.






Barr                         Informational                     [Page 14]

RFC 1912                   Common DNS Errors               February 1996


4.3 Server issues

   DNS operates primarily via UDP (User Datagram Protocol) messages.
   Some UNIX operating systems, in an effort to save CPU cycles, run
   with UDP checksums turned off.  The relative merits of this have long
   been debated.  However, with the increase in CPU speeds, the
   performance considerations become less and less important.  It is
   strongly encouraged that you turn on UDP checksumming to avoid
   corrupted data not only with DNS but with other services that use UDP
   (like NFS).  Check with your operating system documentation to verify
   that UDP checksumming is enabled.

References

   [RFC 974] Partridge, C., "Mail routing and the domain system", STD
              14, RFC 974, CSNET CIC BBN Laboratories Inc, January 1986.

   [RFC 1033] Lottor, M, "Domain Administrators Operations Guide", RFC
              1033, USC/Information Sciences Institute, November 1987.

   [RFC 1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
              STD 13, RFC 1034, USC/Information Sciences Institute,
              November 1987.

   [RFC 1035] Mockapetris, P., "Domain Names - Implementation and
              Specification", STD 13, RFC 1035, USC/Information Sciences
              Institute, November 1987.

   [RFC 1123] Braden, R., "Requirements for Internet Hosts --
              Application and Support", STD 3, RFC 1123, IETF, October
              1989.

   [RFC 1178] Libes, D., "Choosing a Name for Your Computer", FYI 5, RFC
              1178, Integrated Systems Group/NIST, August 1990.

   [RFC 1183] Ullman, R., Mockapetris, P., Mamakos, L, and C. Everhart,
              "New DNS RR Definitions", RFC 1183, October 1990.

   [RFC 1535] Gavron, E., "A Security Problem and Proposed Correction
              With Widely Deployed DNS Software", RFC 1535, ACES
              Research Inc., October 1993.

   [RFC 1536] 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.





Barr                         Informational                     [Page 15]

RFC 1912                   Common DNS Errors               February 1996


   [RFC 1537] Beertema, P., "Common DNS Data File Configuration Errors",
              RFC 1537, CWI, October 1993.

   [RFC 1713] A. Romao, "Tools for DNS debugging", RFC 1713, FCCN,
              November 1994.

   [BOG] Vixie, P, et. al., "Name Server Operations Guide for BIND",
              Vixie Enterprises, July 1994.

5. Security Considerations

   Security issues are not discussed in this memo.

6. Author's Address

   David Barr
   The Pennsylvania State University
   Department of Mathematics
   334 Whitmore Building
   University Park, PA 16802

   Voice: +1 814 863 7374
   Fax: +1 814 863-8311
   EMail: barr@math.psu.edu



























Barr                         Informational                     [Page 16]