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+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
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
+Barr                         Informational                     [Page 16]
+