--- /dev/null
+
+
+
+
+
+
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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
+
+
+
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+\f
+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:
+
+
+
+
+
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+\f
+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.)
+
+
+
+
+
+
+
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+ 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
+
+
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+ 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.
+
+
+
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+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.
+
+
+
+
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+ [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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projects / thirdparty / bind9.git / commitdiff
