< draft-ietf-dnsind-clarify-03.txt   draft-ietf-dnsind-clarify-04.txt >
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Network Working Group Robert Elz Network Working Group Robert Elz
Internet Draft University of Melbourne Internet Draft University of Melbourne
Expiration Date: July 1997 Expiration Date: July 1997
Randy Bush Randy Bush
RGnet, Inc. RGnet, Inc.
January 1997 January 1997
Clarifications to the DNS Specification Clarifications to the DNS Specification
draft-ietf-dnsind-clarify-03.txt draft-ietf-dnsind-clarify-04.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts. working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
To learn the current status of any Internet-Draft, please check the To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast). ftp.isi.edu (US West Coast).
1. Abstract 1. Abstract
Please do not bother with this draft, it was intended to be posted
before the San Jose IETF, but missed the deadline by minutes. It was
(other than this paragraph) later posted to the namedroppers mailing
list. A new version, which responds to comments received from that
mailing list posting will be posted to the I-D directories within
days, please wait for that one, and don't waste time on this. This
version is appearing solely to keep the I-D numbering sequence sane,
the mailing list version was called -03, so we must have a -03 so the
next real version can be -04 ...
This draft considers some areas that have been identified as problems This draft considers some areas that have been identified as problems
with the specification of the Domain Name System, and proposes with the specification of the Domain Name System, and proposes
remedies for the defects identified. Five separate issues are remedies for the defects identified. Five separate issues are
considered: considered:
+ IP packet header address usage from multi-homed servers, + IP packet header address usage from multi-homed servers,
+ TTLs in sets of records with the same name, class, and type, + TTLs in sets of records with the same name, class, and type,
+ correct handling of zone cuts, + correct handling of zone cuts,
+ the issue of what is an authoritative, or canonical, name, + the issue of what is an authoritative, or canonical, name,
+ and the issue of what makes a valid DNS label. + and the issue of what makes a valid DNS label.
The first three of these are areas where the correct behaviour has The first three of these are areas where the correct behaviour has
been somewhat unclear, we seek to rectify that. The other two are been somewhat unclear, we seek to rectify that. The other two are
already adequately specified, however the specifications seem to be already adequately specified, however the specifications seem to be
sometimes ignored. We seek to reinforce the existing specification. sometimes ignored. We seek to reinforce the existing specifications.
This version contains corrections and clarifications as suggested on
the mailing list. Most notable is a change to the order of
preference of similar information from multiple sources.
Contents Contents
1 Abstract ................................................... 1 1 Abstract ................................................... 1
2 Introduction ............................................... 2 2 Introduction ............................................... 2
3 Server Reply Source Address Selection ...................... 3 3 Terminology ................................................ 2
4 Resource Record Sets ....................................... 3 4 Server Reply Source Address Selection ...................... 3
5 Zone Cuts .................................................. 6 5 Resource Record Sets ....................................... 4
6 Naming issues .............................................. 7 6 Zone Cuts .................................................. 6
7 Name syntax ................................................ 9 7 Naming issues .............................................. 7
8 Security Considerations .................................... 10 8 Name syntax ................................................ 9
9 References ................................................. 10 9 Security Considerations .................................... 10
10 Acknowledgements ........................................... 10 10 References ................................................. 10
11 Authors' addresses ......................................... 11 11 Acknowledgements ........................................... 10
12 Authors' addresses ......................................... 11
2. Introduction 2. Introduction
Several problem areas in the Domain Name System specification Several problem areas in the Domain Name System specification
[RFC1034, RFC1035] have been noted through the years [RFC1123]. This [RFC1034, RFC1035] have been noted through the years [RFC1123]. This
draft addresses several additional problem areas. The issues here draft addresses several additional problem areas. The issues here
are independent. Those issues are the question of which source are independent. Those issues are the question of which source
address a multi-homed DNS server should use when replying to a query, address a multi-homed DNS server should use when replying to a query,
the issue of differing TTLs for DNS records with the same label, the issue of differing TTLs for DNS records with the same label,
class and type, and the issue of canonical names, what they are, how class and type, and the issue of canonical names, what they are, how
CNAME records relate, what names are legal in what parts of the DNS, CNAME records relate, what names are legal in what parts of the DNS,
and what is the valid syntax of a DNS name. and what is the valid syntax of a DNS name.
Suggestions for clarifications to the DNS specification to avoid Clarifications to the DNS specification to avoid these problems are
these problems are made in this memo. The solutions proposed herein made in this memo.
are intended to stimulate discussion. It is possible that the sense
of either may be reversed before the next iteration of this draft,
but less likely now than it was before the previous version.
3. Server Reply Source Address Selection 3. Terminology
This memo does not use the oft used expressions MUST, SHOULD, MAY, or
their negative forms. In some sections it may seem that a
specification is worded mildly, and hence some may infer that the
specification is optional. That is not correct. Anywhere that this
memo suggests that some action should be carried out, or must be
carried out, or that some behaviour is acceptable, or not, that is to
be considered as a fundamental aspect of this specification,
regardless of the specific words used. If some behaviour or action
is truly optional, that will be clearly specified by the text.
4. Server Reply Source Address Selection
Most, if not all, DNS clients, whether servers acting as clients for Most, if not all, DNS clients, whether servers acting as clients for
the purposes of recursive query resolution, or resolvers, expect the the purposes of recursive query resolution, or resolvers, expect the
address from which a reply is received to be the same address as that address from which a reply is received to be the same address as that
to which the query eliciting the reply was sent. This, along with to which the query eliciting the reply was sent. This, along with
the identifier (ID) in the reply is used for disambiguating replies, the identifier (ID) in the reply is used for disambiguating replies,
and filtering spurious responses. This may, or may not, have been and filtering spurious responses. This may, or may not, have been
intended when the DNS was designed, but is now a fact of life. intended when the DNS was designed, but is now a fact of life.
Some multi-homed hosts running DNS servers fail to anticipate this Some multi-homed hosts running DNS servers fail to expect this usage.
usage, and consequently send replies from the "wrong" source address, Consequently they send replies from the "wrong" source address,
causing the reply to be discarded by the client. causing the reply to be discarded by the client.
3.1. UDP Source Address Selection 4.1. UDP Source Address Selection
To avoid these problems, servers when responding to queries using UDP To avoid these problems, servers when responding to queries using UDP
must cause the reply to be sent with the source address field in the must cause the reply to be sent with the source address field in the
IP header set to the address that was in the destination address IP header set to the address that was in the destination address
field of the IP header of the packet containing the query causing the field of the IP header of the packet containing the query causing the
response. If this would cause the response to be sent from an IP response. If this would cause the response to be sent from an IP
address which is not permitted for this purpose, then the response address that is not permitted for this purpose, then the response may
may be sent from any legal IP address allocated to the server. That be sent from any legal IP address allocated to the server. That
address should be chosen to maximise the possibility that the client address should be chosen to maximise the possibility that the client
will be able to use it for further queries. Servers configured in will be able to use it for further queries. Servers configured in
such a way that not all their addresses are equally reachable from such a way that not all their addresses are equally reachable from
all potential clients need take particular care when responding to all potential clients need take particular care when responding to
queries sent to anycast, multicast, or similar, addresses. queries sent to anycast, multicast, or similar, addresses.
3.2. Port Number Selection 4.2. Port Number Selection
Replies to all queries must be directed to the port from which they Replies to all queries must be directed to the port from which they
were sent. With queries received via TCP this is an inherent part of were sent. When queries are received via TCP this is an inherent
the transport protocol, for queries received by UDP the server must part of the transport protocol. For queries received by UDP the
take note of the source port and use that as the destination port in server must take note of the source port and use that as the
the response. Replies should always be sent from the port to which destination port in the response. Replies should always be sent from
they were directed. Except in extraordinary circumstances, this will the port to which they were directed. Except in extraordinary
be the well known port assigned for DNS queries [RFC1700]. circumstances, this will be the well known port assigned for DNS
queries [RFC1700].
4. Resource Record Sets 5. Resource Record Sets
Each DNS Resource Record (RR) has a label, class, type, and data. Each DNS Resource Record (RR) has a label, class, type, and data. It
While it is meaningless for two records to ever have label, class, is meaningless for two records to ever have label, class, type and
type and data all equal (servers should suppress such duplicates if data all equal - servers should suppress such duplicates if
encountered), it is possible for many record types to exist with the encountered. It is however possible for most record types to exist
same label class and type, but with different data. Such a group of with the same label class and type, but with different data. Such a
records is hereby defined to be a Resource Record Set (RRSet). group of records is hereby defined to be a Resource Record Set
(RRSet).
4.1. Sending RRs from an RRSet 5.1. Sending RRs from an RRSet
A query for a specific (or non-specific) label, class, and type, will A query for a specific (or non-specific) label, class, and type, will
always return all records in the associated RRSet - whether that be always return all records in the associated RRSet - whether that be
one or more RRs, or the response shall be marked as "truncated" if one or more RRs. The response must be marked as "truncated" if the
the entire RRSet will not fit in the response. entire RRSet will not fit in the response.
4.2. TTLs of RRs in an RRSet 5.2. TTLs of RRs in an RRSet
Resource Records also have a time to live (TTL). It is possible for Resource Records also have a time to live (TTL). It is possible for
the RRs in an RRSet to have different TTLs, however no uses for this the RRs in an RRSet to have different TTLs. No uses for this have
have been found which cannot be better accomplished in other ways. been found that cannot be better accomplished in other ways. This
This can, however, cause partial replies (not marked "truncated") can, however, cause partial replies (not marked "truncated") from a
from a caching server, where the TTLs for some but not all of the RRs caching server, where the TTLs for some but not all the RRs in the
in the RRSet have expired. RRSet have expired.
Consequently the use of differing TTLs in an RRSet is hereby Consequently the use of differing TTLs in an RRSet is hereby
deprecated, the TTLs of all RRs in an RRSet must be the same. deprecated, the TTLs of all RRs in an RRSet must be the same.
Should a client receive a response containing RRs from an RRSet with Should a client receive a response containing RRs from an RRSet with
differing TTLs, it should treat the RRs for all purposes as if all differing TTLs, it should treat the RRs for all purposes as if all
TTLs in the RRSet had been set to the value of the lowest TTL in the TTLs in the RRSet had been set to the value of the lowest TTL in the
RRSet. RRSet.
4.3. Receiving RRSets 5.3. Receiving RRSets
Servers must never merge RRs from a response with RRs in their cache Servers must never merge RRs from a response with RRs in their cache
to form an RRSet. If a response contains data which would form an to form an RRSet. If a response contains data that would form an
RRSet with data in a server's cache the server must either ignore the RRSet with data in a server's cache the server must either ignore the
RRs in the response, or use those to replace the existing RRSet in RRs in the response, or discard the entire the existing RRSet in the
the cache, as appropriate. Consequently the issue of TTLs varying cache, as appropriate. Consequently the issue of TTLs varying
between the cache and a response does not cause concern, one will be between the cache and a response does not cause concern, one will be
ignored. That is, one of the data sets is always incorrect if the ignored. That is, one of the data sets is always incorrect if the
data from an answer differs from the data in the cache. The data from an answer differs from the data in the cache. The
challenge for the server is to determine which of the data sets is challenge for the server is to determine which of the data sets is
correct, assuming that one is, and retain that, while ignoring the correct, if one is, and retain that, while ignoring the other. Note
other. Note that if a server receives an answer containing an RRSet that if a server receives an answer containing an RRSet that is
that is identical to that in its cache, with the possible exception identical to that in its cache, with the possible exception of the
of the TTL value, it may update the TTL in its cache with the TTL of TTL value, it may, optionally, update the TTL in its cache with the
the received answer. It should do this if the received answer would TTL of the received answer. It should do this if the received answer
be considered more authoritative (as discussed in the next section) would be considered more authoritative (as discussed in the next
than the previously cached answer. section) than the previously cached answer.
4.3.1. Ranking data 5.3.1. Ranking data
When considering whether to accept an RRSet in a reply, or retain an When considering whether to accept an RRSet in a reply, or retain an
RRSet already in its cache instead, a server should consider the RRSet already in its cache instead, a server should consider the
relative likely trustworthiness of the various data. That is, an relative likely trustworthiness of the various data. An
authoritative answer from a reply should replace cached data that had authoritative answer from a reply should replace cached data that had
been obtained from additional information in an earlier reply, but been obtained from additional information in an earlier reply.
additional information from a reply will be ignored if the cache However additional information from a reply will be ignored if the
contains data from an authoritative answer or a zone file. cache contains data from an authoritative answer or a zone file.
The accuracy of data available is assumed from its source. The accuracy of data available is assumed from its source.
Trustworthiness shall be, in order from most to least: Trustworthiness shall be, in order from most to least:
+ Data from a primary zone file, other than glue data, + Data from a primary zone file, other than glue data,
+ Data from a zone transfer, other than glue, + Data from a zone transfer, other than glue,
+ That from the answer section of an authoritative reply, + That from the answer section of an authoritative reply,
+ Glue from a primary zone, or glue from a zone transfer,
+ Data from the authority section of an authoritative answer, + Data from the authority section of an authoritative answer,
+ Glue from a primary zone, or glue from a zone transfer,
+ Data from the answer section of a non-authoritative answer, + Data from the answer section of a non-authoritative answer,
+ Additional information from an authoritative answer, + Additional information from an authoritative answer,
+ Data from the authority section of a non-authoritative answer, + Data from the authority section of a non-authoritative answer,
+ Additional information from non-authoritative answers. + Additional information from non-authoritative answers.
When DNS security [DNSSEC] is in use, and authenticated data has been When DNS security [RFC2065] is in use, and an authenticated reply has
received and verified, it shall be considered more trustworthy than been received and verified, the data thus authenticated shall be
unauthenticated data of the same type. Note that throughout this considered more trustworthy than unauthenticated data of the same
document, "authoritative" is used to mean a reply with the AA bit type. Note that throughout this document, "authoritative" means a
set. DNSSEC uses trusted chains of SIG and KEY records to determine reply with the AA bit set. DNSSEC uses trusted chains of SIG and KEY
what data is authenticated, the AA bit is almost irrelevant. However records to determine the authenticity of data, the AA bit is almost
DNSSEC aware servers must still correctly set the AA bit in responses irrelevant. However DNSSEC aware servers must still correctly set
to enable correct operation with servers that are not security aware the AA bit in responses to enable correct operation with servers that
(almost all currently). are not security aware (almost all currently).
Note that, glue excluded, it is impossible for data from two primary Note that, glue excluded, it is impossible for data from two primary
zone files, two secondary zones (data from zone transfers) or data zone files, two secondary zones (data from zone transfers) or data
from primary and secondary zones to ever conflict. Where glue for from primary and secondary zones to ever conflict. Where glue for
the same name exists in multiple zones, and differs in value, the the same name exists in multiple zones, and differs in value, the
nameserver should select data from a primary zone file in preference nameserver should select data from a primary zone file in preference
to secondary, but otherwise may choose any single set of such data. to secondary, but otherwise may choose any single set of such data.
Choosing that which appears to come from a source nearer the Choosing that which appears to come from a source nearer the
authoritative data source may make sense where that can be authoritative data source may make sense where that can be
determined. Choosing primary data over secondary allows the source determined. Choosing primary data over secondary allows the source
of incorrect glue data to be discovered more readily, when such data of incorrect glue data to be discovered more readily, when a problem
does exist. with such data exists.
"Glue" above includes any record in a zone file that is not properly "Glue" above includes any record in a zone file that is not properly
part of that zone, including nameserver records of delegated sub- part of that zone, including nameserver records of delegated sub-
zones (NS records), address records that accompany those NS records zones (NS records), address records that accompany those NS records
(A, AAAA, etc), and any other stray data that might appear. (A, AAAA, etc), and any other stray data that might appear.
4.4. Sending RRSets (reprise) 5.4. Sending RRSets (reprise)
A Resource Record Set should only be included once in any DNS reply. A Resource Record Set should only be included once in any DNS reply.
It may occur in any of the Answer, Authority, or Additional It may occur in any of the Answer, Authority, or Additional
Information sections, as required, however should not be repeated in Information sections, as required, however should not be repeated in
the same, or any other, section, except where explicitly required by the same, or any other, section, except where explicitly required by
a specification. For example, an AXFR response requires the SOA a specification. For example, an AXFR response requires the SOA
record (always an RRSet containing a single RR) be both the first and record (always an RRSet containing a single RR) be both the first and
last record of the reply. Where duplicates are required this way, last record of the reply. Where duplicates are required this way,
the TTL transmitted in each case must be the same. the TTL transmitted in each case must be the same.
5. Zone Cuts 6. Zone Cuts
A "Zone" is a set of one, or usually, more, domains collected and A "Zone" is a set of one, or usually, more, domains collected and
treated as a unit. A "Zone Cut" is the division between one zone and treated as a unit. A "Zone Cut" is the division between one zone and
another. A zone comprises some subset of the DNS tree, rooted at a another. A zone comprises some subset of the DNS tree, rooted at a
domain known as the "origin" of the zone. The origin domain itself, domain known as the "origin" of the zone. The origin domain itself,
and some, or all, of its sub-domains, form the zone. The existence and some, or all, of its sub-domains, form the zone. The existence
of a zone cut is indicated by the presence, in the zone, of a of a zone cut is indicated by the presence, in the zone, of a
NameServer (NS) record for any domain other than the origin of the NameServer (NS) record for any domain other than the origin of the
zone. zone.
5.1. Zone authority 6.1. Zone authority
The authoritative servers for a zone are listed in the NS records for The authoritative servers for a zone are enumerated in the NS records
the origin of the zone, which, along with a Start of Authority (SOA) for the origin of the zone, which, along with a Start of Authority
record are the mandatory records in every zone. Such a server is (SOA) record are the mandatory records in every zone. Such a server
authoritative for all resource records in a zone which are not in is authoritative for all resource records in a zone that are not in
another zone. The NS records that indicate a zone cut are the another zone. The NS records that indicate a zone cut are the
property of the child zone created, as are any other records for the property of the child zone created, as are any other records for the
origin of that child zone, or any sub-domains of it. A server for origin of that child zone, or any sub-domains of it. A server for a
the parent zone should not return authoritative answers for queries zone should not return authoritative answers for queries related to
related to names in a child zone, which includes the NS records at names in another zone, which includes the NS, and perhaps A, records
the zone cut, unless it also happens to be a server for the child at a zone cut, unless it also happens to be a server for the other
zone of course. zone.
Other than the DNSSEC cases mentioned immediately below, servers Other than the DNSSEC cases mentioned immediately below, servers
should ignore data other than NS records, and necessary A records to should ignore data other than NS records, and necessary A records to
locate the servers listed in the NS records, that may happen to be locate the servers listed in the NS records, that may happen to be
configured in a zone at a zone cut. configured in a zone at a zone cut.
5.2. DNSSEC issues 6.2. DNSSEC issues
The DNS security mechanisms [DNSSEC] complicate this somewhat, as The DNS security mechanisms [RFC2065] complicate this somewhat, as
some of the new resource record types added are very unusual when some of the new resource record types added are very unusual when
compared with other DNS RRs. In particular the NXT ("next") RR type compared with other DNS RRs. In particular the NXT ("next") RR type
contains information about which names exist in a zone, and hence contains information about which names exist in a zone, and hence
which do not, and thus must necessarily relate to the zone in which which do not, and thus must necessarily relate to the zone in which
it exists. In fact, the same domain name may have different NXT it exists. The same domain name may have different NXT records in
records in the parent zone and the child zone, and both are valid, the parent zone and the child zone, and both are valid, and are not
and are not an RRSet. an RRSet.
Since NXT records are intended to be automatically generated, rather Since NXT records are intended to be automatically generated, rather
than configured by DNS operators, servers may, but are not required than configured by DNS operators, servers may, but are not required
to, retain all differing NXT records they receive regardless of the to, retain all differing NXT records they receive regardless of the
rules in section 4.3. rules in section 5.3.
To indicate that a subzone is insecure, securely, that is, from a For a secure parent zone to securely indicate that a subzone is
secure parent zone, DNSSEC requires that a KEY RR indicating that the insecure, DNSSEC requires that a KEY RR indicating that the subzone
subzone is insecure, and the parent zone's authenticating SIG RR(s) is insecure, and the parent zone's authenticating SIG RR(s) be
be present in the parent zone, as they by definition cannot be in the present in the parent zone, as they by definition cannot be in the
subzone. Where a subzone is secure, the KEY and SIG can be subzone. Where a subzone is secure, the KEY and SIG can be
duplicated in both zone files, but should always be present in the duplicated in both zone files, but should always be present in the
subzone. subzone.
Note that in none of these cases should a server for the parent zone, Note that in none of these cases should a server for the parent zone,
not also being a server for the subzone, set the AA bit in any not also being a server for the subzone, set the AA bit in any
response for a label at a zone cut. response for a label at a zone cut.
6. Naming issues 7. Naming issues
It has sometimes been inferred from some sections of the DNS It has sometimes been inferred from some sections of the DNS
specification [RFC1034, RFC1035] that a host, or perhaps an interface specification [RFC1034, RFC1035] that a host, or perhaps an interface
of a host, is permitted exactly one authoritative, or official, name, of a host, is permitted exactly one authoritative, or official, name,
called the canonical name. There is no such requirement in the DNS. called the canonical name. There is no such requirement in the DNS.
6.1. CNAME records 7.1. CNAME records
The DNS CNAME ("canonical name") record exists to provide the The DNS CNAME ("canonical name") record exists to provide the
canonical name associated with an alias name. There may be only one canonical name associated with an alias name. There may be only one
such canonical name for any one alias. That name should generally be such canonical name for any one alias. That name should generally be
a name that exists elsewhere in the DNS, though some applications for a name that exists elsewhere in the DNS, though some applications for
aliases with no accompanying canonical name exist. An alias name aliases with no accompanying canonical name exist. An alias name
(label of a CNAME record) may, if DNSSEC is in use, have SIG, NXT, (label of a CNAME record) may, if DNSSEC is in use, have SIG, NXT,
and KEY RRs, but may have no other data. That is, for any label in and KEY RRs, but may have no other data. That is, for any label in
the DNS (any domain name) exactly one of the following is true: the DNS (any domain name) exactly one of the following is true:
+ one CNAME record exists, optionally accompanied by SIG, NXT, and + one CNAME record exists, optionally accompanied by SIG, NXT, and
KEY RRs, KEY RRs,
+ other records exist, possibly many records, none of them being + one or more records exist, none being CNAME records,
CNAME records
+ the name does not exist at all. + the name does not exist at all.
If the canonical name associated with an alias does not exist, a 7.1.1. CNAME terminology
lookup of the alias seeking anything but one of the CNAME, SIG, NXT,
or KEY RR (or the pseudo-type ANY) should indicate that the name does
not exist, just as if the alias itself did not exist. A CNAME (or
ANY) type lookup should return the CNAME RR itself. Lookups for SIG,
NXT or KEY records should return any such associated RR's that the
alias may own (as would an ANY lookup).
6.1.1. CNAME terminology
It has been traditional to refer to the label of a CNAME record as "a It has been traditional to refer to the label of a CNAME record as "a
CNAME". This is unfortunate, as "CNAME" is an abbreviation of CNAME". This is unfortunate, as "CNAME" is an abbreviation of
"canonical name", and the label of a CNAME record is most certainly "canonical name", and the label of a CNAME record is most certainly
not a canonical name. It is, however, an entrenched usage, care must not a canonical name. It is, however, an entrenched usage. Care
therefore be taken to be very clear whether the label, or the value must therefore be taken to be very clear whether the label, or the
(the canonical name) of a CNAME resource record is intended. In this value (the canonical name) of a CNAME resource record is intended.
document, the label of a CNAME resource record will always be In this document, the label of a CNAME resource record will always be
referred to as an alias. referred to as an alias.
6.2. PTR records 7.2. PTR records
Confusion about canonical names has lead to a belief that a PTR Confusion about canonical names has lead to a belief that a PTR
record should have exactly one RR in its RRSet. This is incorrect, record should have exactly one RR in its RRSet. This is incorrect,
the relevant section of RFC1034 (section 3.6.2) indicates that the the relevant section of RFC1034 (section 3.6.2) indicates that the
value of a PTR record should be a canonical name. That is, it should value of a PTR record should be a canonical name. That is, it should
not be an alias. There is no implication in that section that only not be an alias. There is no implication in that section that only
one PTR record is permitted for a name, and no such restriction one PTR record is permitted for a name. No such restriction should
should be inferred. be inferred.
6.3. MX and NS records Note that while the value of a PTR record must not be an alias, there
is no requirement that the process of resolving a PTR record not
encounter any aliases. The label that is being looked up for a PTR
value might have a CNAME record. That is, it might be an alias. The
value of that CNAME RR, if not another alias, will give the location
where the PTR record is found. That record gives the result of the
PTR type lookup. This final result, the value of the PTR RR, is the
label which must not be an alias.
7.3. MX and NS records
The domain name used as the value of a NS resource record, or part of The domain name used as the value of a NS resource record, or part of
the value of a MX resource record should not be an alias. Not only the value of a MX resource record must not be an alias. Not only is
is the specification quite clear on this point, but using an alias in the specification clear on this point, but using an alias in either
either of these positions neither works as well as might be hoped, of these positions neither works as well as might be hoped, nor well
nor well fulfills the ambition that may have led to this approach. fulfills the ambition that may have led to this approach. This
domain name must have as its value one or more address records.
Currently those will be A records, however in the future other record
types giving addressing information may be acceptable. It can also
have other RRs, but never a CNAME RR.
Searching for either NS or MX records causes "additional section Searching for either NS or MX records causes "additional section
processing" in which address records associated with the value of the processing" in which address records associated with the value of the
record sought are appended to the answer. This helps avoid needless record sought are appended to the answer. This helps avoid needless
extra queries which are easily anticipated when the first was made. extra queries that are easily anticipated when the first was made.
Additional section processing does not include CNAME records, let Additional section processing does not include CNAME records, let
alone the address records that may be associated with the canonical alone the address records that may be associated with the canonical
name derived from the alias. Thus, if an alias is used as the value name derived from the alias. Thus, if an alias is used as the value
of an NS or MX record, no address will be returned together with the of an NS or MX record, no address will be returned with the NS or MX
NS or MX value. This can cause extra queries, and extra network value. This can cause extra queries, and extra network burden, on
burden, on every query, that could have been trivially avoided by every query. It is trivial to avoid this by resolving the alias and
resolving the alias and placing the canonical name directly in the placing the canonical name directly in the affected record just once
affected record just once when it was updated or installed. In some when it is updated or installed. In some particular hard cases the
particular hard cases the lack of the additional section address lack of the additional section address records in the results of a NS
records in the results of a NS lookup can actually cause the request lookup can cause the request to fail.
to fail.
7. Name syntax 8. Name syntax
Occasionally it is assumed that the Domain Name System serves only Occasionally it is assumed that the Domain Name System serves only
the purpose of mapping Internet host names to data, and mapping the purpose of mapping Internet host names to data, and mapping
Internet addresses to host names. This is not correct, the DNS is a Internet addresses to host names. This is not correct, the DNS is a
general (if somewhat limited) hierarchical database, and can store general (if somewhat limited) hierarchical database, and can store
almost any kind of data, for almost any purpose. almost any kind of data, for almost any purpose.
The DNS itself places only one restriction upon the particular labels The DNS itself places only one restriction on the particular labels
that can be used to identify resource records. That one restriction that can be used to identify resource records. That one restriction
relates to the length of the label and the full name. Any one label relates to the length of the label and the full name. The length of
is limited to 63 octets, and a full name is limited to 255 octets any one label is limited to between 1 and 63 octets. A full domain
(including the separators). That restriction aside, any binary name is limited to 255 octets (including the separators). The zero
string whatever can be used as the label of any resource record, and length full name is defined as representing the root of the DNS tree,
as the value of one of the records that includes a domain name as and is typically written and displayed as ".". Those restrictions
some or all of its value (SOA, NS, MX, PTR, CNAME, SRV, and any aside, any binary string whatever can be used as the label of any
others that may be added). Implementations of the DNS protocols must resource record. Similarly, any binary string can serve as the value
not place any restrictions on the labels that can be used. of any record that includes a domain name as some or all of its value
(SOA, NS, MX, PTR, CNAME, and any others that may be added).
Implementations of the DNS protocols must not place any restrictions
on the labels that can be used. In particular, DNS servers must not
refuse to serve a zone because it contains labels that might not be
acceptable to some DNS client programs. A DNS server may be
configurable to issue warnings when loading, or even to refuse to
load, a primary zone containing labels that might be considered
questionable, however this should not happen by default.
Note however, that the various applications that make use of DNS data Note however, that the various applications that make use of DNS data
can have restrictions imposed upon what particular data is acceptable can have restrictions imposed on what particular values are
in their environment. For example, that any binary label can have an acceptable in their environment. For example, that any binary label
MX record does not imply that any binary name can be used as the host can have an MX record does not imply that any binary name can be used
part of an e-mail address. Clients of the DNS can impose whatever as the host part of an e-mail address. Clients of the DNS can impose
restrictions are appropriate to their circumstances to the values whatever restrictions are appropriate to their circumstances on the
they use as keys for DNS lookup requests, and to the values returned values they use as keys for DNS lookup requests, and on the values
by the DNS. returned by the DNS.
See also [RFC1123] section 6.1.3.5. See also [RFC1123] section 6.1.3.5.
8. Security Considerations 9. Security Considerations
This document does not consider security. This document does not consider security.
In particular, nothing in section 3 is any way related to, or useful In particular, nothing in section 4 is any way related to, or useful
for, any security related purposes. for, any security related purposes.
Section 4.3.1 is also not related to security. Security of DNS data Section 5.3.1 is also not related to security. Security of DNS data
will be obtained by the Secure DNS [DNSSEC], which is orthogonal to will be obtained by the Secure DNS [RFC2065], which is mostly
this memo. orthogonal to this memo.
It is not believed that anything in this document adds to any It is not believed that anything in this document adds to any
security issues that may exist with the DNS, nor does it do anything security issues that may exist with the DNS, nor does it do anything
to lessen them. to lessen them.
9. References 10. References
[RFC1034] Domain Names - Concepts and Facilities, (STD 13) [RFC1034] Domain Names - Concepts and Facilities, (STD 13)
P. Mockapetris, ISI, November 1987. P. Mockapetris, ISI, November 1987.
[RFC1035] Domain Names - Implementation and Specification (STD 13) [RFC1035] Domain Names - Implementation and Specification (STD 13)
P. Mockapetris, ISI, November 1987 P. Mockapetris, ISI, November 1987.
[RFC1123] Requirements for Internet hosts - application and support, [RFC1123] Requirements for Internet hosts - application and support,
(STD 3) R. Braden, January 1989 (STD 3) R. Braden, January 1989.
[RFC1700] Assigned Numbers (STD 2) [RFC1700] Assigned Numbers (STD 2)
J. Reynolds, J. Postel, October 1994. J. Reynolds, J. Postel, October 1994.
[DNSSEC] Domain Name System Security Extensions, [RFC2065] Domain Name System Security Extensions,
D. E. Eastlake, 3rd, C. W. Kaufman, D. E. Eastlake, 3rd, C. W. Kaufman, January 1997.
Work in Progress (soon to be an RFC), August 1996.
10. Acknowledgements 11. Acknowledgements
This memo arose from discussions in the DNSIND working group of the This memo arose from discussions in the DNSIND working group of the
IETF in 1995 and 1996, the members of that working group are largely IETF in 1995 and 1996, the members of that working group are largely
responsible for the ideas captured herein. Particular thanks to responsible for the ideas captured herein. Particular thanks to
Donald E. Eastlake, 3rd, for assistance with the DNSSEC issues in Donald E. Eastlake, 3rd, for help with the DNSSEC issues in this
this document. document, and to John Gilmore for pointing out where the
clarifications were not necessarily clarifying.
11. Authors' addresses 12. Authors' addresses
Robert Elz Robert Elz
Computer Science Computer Science
University of Melbourne University of Melbourne
Parkville, Victoria, 3052 Parkville, Victoria, 3052
Australia. Australia.
EMail: kre@munnari.OZ.AU EMail: kre@munnari.OZ.AU
Randy Bush Randy Bush
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