RFC 1912

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.

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.

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.

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.

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

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.

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

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].

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.

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

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:

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.)

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

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.

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.

[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