RFC2181 - Clarifications to the DNS Specification

Network Working Group R. Elz

Request for Comments: 2181 University of Melbourne

Updates: 1034, 1035, 1123 R. Bush

Category: Standards Track RGnet, Inc.

July 1997

Clarifications to the DNS Specification

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

1. Abstract

This document considers some areas that have been identified as

problems with the specification of the Domain Name System, and

proposes remedies for the defects identified. Eight separate issues

are considered:

+ IP packet header address usage from multi-homed servers,

+ TTLs in sets of records with the same name, class, and type,

+ correct handling of zone cuts,

+ three minor issues concerning SOA records and their use,

+ the precise definition of the Time to Live (TTL)

+ Use of the TC (truncated) header bit

+ the issue of what is an authoritative, or canonical, name,

+ and the issue of what makes a valid DNS label.

The first six of these are areas where the correct behaviour has been

somewhat unclear, we seek to rectify that. The other two are already

adequately specified, however the specifications seem to be sometimes

ignored. We seek to reinforce the existing specifications.

Contents

1 Abstract ................................................... 1

2 IntrodUCtion ............................................... 2

3 Terminology ................................................ 3

4 Server Reply Source Address Selection ...................... 3

5 Resource Record Sets ....................................... 4

6 Zone Cuts .................................................. 8

7 SOA RRs .................................................... 10

8 Time to Live (TTL) ......................................... 10

9 The TC (truncated) header bit .............................. 11

10 Naming issues .............................................. 11

11 Name syntax ................................................ 13

12 Security Considerations .................................... 14

13 References ................................................. 14

14 Acknowledgements ........................................... 15

15 Authors' Addresses ......................................... 15

2. Introduction

Several problem areas in the Domain Name System specification

[RFC1034, RFC1035] have been noted through the years [RFC1123]. This

document addresses several additional problem areas. The issues here

are independent. Those issues are the question of which source

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,

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,

and what is the valid syntax of a DNS name.

Clarifications to the DNS specification to avoid these problems are

made in this memo. A minor ambiguity in RFC1034 concerned with SOA

records is also corrected, as is one in the definition of the TTL

(Time To Live) and some possible confusion in use of the TC bit.

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, expect the address from which a reply

is received to be the same address as that to which the query

eliciting the reply was sent. This is true for servers acting as

clients for the purposes of recursive query resolution, as well as

simple resolver clients. The address, along with the identifier (ID)

in the reply is used for disambiguating replies, and filtering

spurious responses. This may, or may not, have been intended when

the DNS was designed, but is now a fact of life.

Some multi-homed hosts running DNS servers generate a reply using a

source address that is not the same as the destination address from

the client's request packet. Such replies will be discarded by the

client because the source address of the reply does not match that of

a host to which the client sent the original request. That is, it

appears to be an unsolicited response.

4.1. UDP Source Address Selection

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

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

response. If this would cause the response to be sent from an IP

address that is not permitted for this purpose, then the response may

be sent from any legal IP address allocated to the server. That

address should be chosen to maximise the possibility that the client

will be able to use it for further queries. Servers configured in

such a way that not all their addresses are equally reachable from

all potential clients need take particular care when responding to

queries sent to anycast, multicast, or similar, addresses.

4.2. Port Number Selection

Replies to all queries must be directed to the port from which they

were sent. When queries are received via TCP this is an inherent

part of the transport protocol. For queries received by UDP the

server must take note of the source port and use that as the

destination port in the response. Replies should always be sent from

the port to which they were directed. Except in extraordinary

circumstances, this will be the well known port assigned for DNS

queries [RFC1700].

5. Resource Record Sets

Each DNS Resource Record (RR) has a label, class, type, and data. It

is meaningless for two records to ever have label, class, type and

data all equal - servers should suppress such duplicates if

encountered. It is however possible for most record types to exist

with the same label, class and type, but with different data. Such a

group of records is hereby defined to be a Resource Record Set

(RRSet).

5.1. Sending RRs from an RRSet

A query for a specific (or non-specific) label, class, and type, will

always return all records in the associated RRSet - whether that be

one or more RRs. The response must be marked as "truncated" if the

entire RRSet will not fit in the response.

5.2. TTLs of RRs in an RRSet

Resource Records also have a time to live (TTL). It is possible for

the RRs in an RRSet to have different TTLs. No uses for this have

been found that cannot be better accomplished in other ways. This

can, however, cause partial replies (not marked "truncated") from a

caching server, where the TTLs for some but not all the RRs in the

RRSet have expired.

Consequently the use of differing TTLs in an RRSet is hereby

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

differing TTLs, it should treat this as an error. If the RRSet

concerned is from a non-authoritative source for this data, the

client should simply ignore the RRSet, and if the values were

required, seek to acquire them from an authoritative source. Clients

that are configured to send all queries to one, or more, particular

servers should treat those servers as authoritative for this purpose.

Should an authoritative source send such a malformed RRSet, the

client 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 RRSet. In

no case may a server send an RRSet with TTLs not all equal.

5.3. DNSSEC Special Cases

Two of the record types added by DNS Security (DNSSEC) [RFC2065]

require special attention when considering the formation of Resource

Record Sets. Those are the SIG and NXT records. It should be noted

that DNS Security is still very new, and there is, as yet, little

experience with it. Readers should be prepared for the information

related to DNSSEC contained in this document to become outdated as

the DNS Security specification matures.

5.3.1. SIG records and RRSets

A SIG record provides signature (validation) data for another RRSet

in the DNS. Where a zone has been signed, every RRSet in the zone

will have had a SIG record associated with it. The data type of the

RRSet is included in the data of the SIG RR, to indicate with which

particular RRSet this SIG record is associated. Were the rules above

applied, whenever a SIG record was included with a response to

validate that response, the SIG records for all other RRSets

associated with the appropriate node would also need to be included.

In some cases, this could be a very large number of records, not

helped by their being rather large RRs.

Thus, it is specifically permitted for the authority section to

contain only those SIG RRs with the "type covered" field equal to the

type field of an answer being returned. However, where SIG records

are being returned in the answer section, in response to a query for

SIG records, or a query for all records associated with a name

(type=ANY) the entire SIG RRSet must be included, as for any other RR

type.

Servers that receive responses containing SIG records in the

authority section, or (probably incorrectly) as additional data, must

understand that the entire RRSet has almost certainly not been

included. Thus, they must not cache that SIG record in a way that

would permit it to be returned should a query for SIG records be

received at that server. RFC2065 actually requires that SIG queries

be directed only to authoritative servers to avoid the problems that

could be caused here, and while servers exist that do not understand

the special properties of SIG records, this will remain necessary.

However, careful design of SIG record processing in new

implementations should permit this restriction to be relaxed in the

future, so resolvers do not need to treat SIG record queries

specially.

It has been occasionally stated that a received request for a SIG

record should be forwarded to an authoritative server, rather than

being answered from data in the cache. This is not necessary - a

server that has the knowledge of SIG as a special case for processing

this way would be better to correctly cache SIG records, taking into

account their characteristics. Then the server can determine when it

is safe to reply from the cache, and when the answer is not available

and the query must be forwarded.

5.3.2. NXT RRs

Next Resource Records (NXT) are even more peculiar. There will only

ever be one NXT record in a zone for a particular label, so

superficially, the RRSet problem is trivial. However, at a zone cut,

both the parent zone, and the child zone (superzone and subzone in

RFC2065 terminology) will have NXT records for the same name. Those

two NXT records do not form an RRSet, even where both zones are

housed at the same server. NXT RRSets always contain just a single

RR. Where both NXT records are visible, two RRSets exist. However,

servers are not required to treat this as a special case when

receiving NXT records in a response. They may elect to notice the

existence of two different NXT RRSets, and treat that as they would

two different RRSets of any other type. That is, cache one, and

ignore the other. Security aware servers will need to correctly

process the NXT record in the received response though.

5.4. Receiving RRSets

Servers must never merge RRs from a response with RRs in their cache

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

RRs in the response, or discard the entire RRSet currently in the

cache, as appropriate. Consequently the issue of TTLs varying

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

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

correct, if one is, and retain that, while ignoring the other. Note

that if a server receives an answer containing an RRSet that is

identical to that in its cache, with the possible exception of the

TTL value, it may, optionally, update the TTL in its cache with the

TTL of the received answer. It should do this if the received answer

would be considered more authoritative (as discussed in the next

section) than the previously cached answer.

5.4.1. Ranking data

When considering whether to accept an RRSet in a reply, or retain an

RRSet already in its cache instead, a server should consider the

relative likely trustworthiness of the various data. An

authoritative answer from a reply should replace cached data that had

been oBTained from additional information in an earlier reply.

However additional information from a reply will be ignored if the

cache contains data from an authoritative answer or a zone file.

The accuracy of data available is assumed from its source.

Trustworthiness shall be, in order from most to least:

+ Data from a primary zone file, other than glue data,

+ Data from a zone transfer, other than glue,

+ The authoritative data included in the answer section of an

authoritative reply.

+ 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, and

non-authoritative data from the answer section of authoritative

answers,

+ Additional information from an authoritative answer,

Data from the authority section of a non-authoritative answer,

Additional information from non-authoritative answers.

Note that the answer section of an authoritative answer normally

contains only authoritative data. However when the name sought is an

alias (see section 10.1.1) only the record describing that alias is

necessarily authoritative. Clients should assume that other records

may have come from the server's cache. Where authoritative answers

are required, the client should query again, using the canonical name

associated with the alias.

Unauthenticated RRs received and cached from the least trustworthy of

those groupings, that is data from the additional data section, and

data from the authority section of a non-authoritative answer, should

not be cached in such a way that they would ever be returned as

answers to a received query. They may be returned as additional

information where appropriate. Ignoring this would allow the

trustworthiness of relatively untrustworthy data to be increased

without cause or excuse.

When DNS security [RFC2065] is in use, and an authenticated reply has

been received and verified, the data thus authenticated shall be

considered more trustworthy than unauthenticated data of the same

type. Note that throughout this document, "authoritative" means a

reply with the AA bit set. DNSSEC uses trusted chains of SIG and KEY

records to determine the authenticity of data, the AA bit is almost

irrelevant. However DNSSEC aware servers must still correctly set

the AA bit in responses to enable correct operation with servers that

are not security aware (almost all currently).

Note that, glue excluded, it is impossible for data from two

correctly configured primary zone files, two correctly configured

secondary zones (data from zone transfers) or data from correctly

configured primary and secondary zones to ever conflict. Where glue

for the same name exists in multiple zones, and differs in value, the

nameserver should select data from a primary zone file in preference

to secondary, but otherwise may choose any single set of such data.

Choosing that which appears to come from a source nearer the

authoritative data source may make sense where that can be

determined. Choosing primary data over secondary allows the source

of incorrect glue data to be discovered more readily, when a problem

with such data exists. Where a server can detect from two zone files

that one or more are incorrectly configured, so as to create

conflicts, it should refuse to load the zones determined to be

erroneous, and issue suitable diagnostics.

"Glue" above includes any record in a zone file that is not properly

part of that zone, including nameserver records of delegated sub-

zones (NS records), address records that accompany those NS records

(A, AAAA, etc), and any other stray data that might appear.

5.5. Sending RRSets (reprise)

A Resource Record Set should only be included once in any DNS reply.

It may occur in any of the Answer, Authority, or Additional

Information sections, as required. However it should not be repeated

in the same, or any other, section, except where explicitly required

by a specification. For example, an AXFR response requires the SOA

record (always an RRSet containing a single RR) be both the first and

last record of the reply. Where duplicates are required this way,

the TTL transmitted in each case must be the same.

6. Zone Cuts

The DNS tree is divided into "zones", which are collections of

domains that are treated as a unit for certain management purposes.

Zones are delimited by "zone cuts". Each zone cut separates a

"child" zone (below the cut) from a "parent" zone (above the cut).

The domain name that appears at the top of a zone (just below the cut

that separates the zone from its parent) is called the zone's

"origin". The name of the zone is the same as the name of the domain

at the zone's origin. Each zone comprises that subset of the DNS

tree that is at or below the zone's origin, and that is above the

cuts that separate the zone from its children (if any). The

existence of a zone cut is indicated in the parent zone by the

existence of NS records specifying the origin of the child zone. A

child zone does not contain any explicit reference to its parent.

6.1. Zone authority

The authoritative servers for a zone are enumerated in the NS records

for the origin of the zone, which, along with a Start of Authority

(SOA) record are the mandatory records in every zone. Such a server

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

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 a

zone should not return authoritative answers for queries related to

names in another zone, which includes the NS, and perhaps A, records

at a zone cut, unless it also happens to be a server for the other

zone.

Other than the DNSSEC cases mentioned immediately below, servers

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

configured in a zone at a zone cut.

6.2. DNSSEC issues

The DNS security mechanisms [RFC2065] complicate this somewhat, as

some of the new resource record types added are very unusual when

compared with other DNS RRs. In particular the NXT ("next") RR type

contains information about which names exist in a zone, and hence

which do not, and thus must necessarily relate to the zone in which

it exists. The same domain name may have different NXT records in

the parent zone and the child zone, and both are valid, and are not

an RRSet. See also section 5.3.2.

Since NXT records are intended to be automatically generated, rather

than configured by DNS operators, servers may, but are not required

to, retain all differing NXT records they receive regardless of the

rules in section 5.4.

For a secure parent zone to securely indicate that a subzone is

insecure, DNSSEC requires that a KEY RR indicating that the subzone

is insecure, and the parent zone's authenticating SIG RR(s) be

present in the parent zone, as they by definition cannot be in the

subzone. Where a subzone is secure, the KEY and SIG records will be

present, and authoritative, in that zone, but should also always be

present in the parent zone (if secure).

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

response for a label at a zone cut.

7. SOA RRs

Three minor issues concerning the Start of Zone of Authority (SOA)

Resource Record need some clarification.

7.1. Placement of SOA RRs in authoritative answers

RFC1034, in section 3.7, indicates that the authority section of an

authoritative answer may contain the SOA record for the zone from

which the answer was obtained. When discussing negative caching,

RFC1034 section 4.3.4 refers to this technique but mentions the

additional section of the response. The former is correct, as is

implied by the example shown in section 6.2.5 of RFC1034. SOA

records, if added, are to be placed in the authority section.

7.2. TTLs on SOA RRs

It may be observed that in section 3.2.1 of RFC1035, which defines

the format of a Resource Record, that the definition of the TTL field

contains a throw away line which states that the TTL of an SOA record

should always be sent as zero to prevent caching. This is mentioned

nowhere else, and has not generally been implemented.

Implementations should not assume that SOA records will have a TTL of

zero, nor are they required to send SOA records with a TTL of zero.

7.3. The SOA.MNAME field

It is quite clear in the specifications, yet seems to have been

widely ignored, that the MNAME field of the SOA record should contain

the name of the primary (master) server for the zone identified by

the SOA. It should not contain the name of the zone itself. That

information would be useless, as to discover it, one needs to start

with the domain name of the SOA record - that is the name of the

zone.

8. Time to Live (TTL)

The definition of values appropriate to the TTL field in STD 13 is

not as clear as it could be, with respect to how many significant

bits exist, and whether the value is signed or unsigned. It is

hereby specified that a TTL value is an unsigned number, with a

minimum value of 0, and a maximum value of 2147483647. That is, a

maximum of 2^31 - 1. When transmitted, this value shall be encoded

in the less significant 31 bits of the 32 bit TTL field, with the

most significant, or sign, bit set to zero.

Implementations should treat TTL values received with the most

significant bit set as if the entire value received was zero.

Implementations are always free to place an upper bound on any TTL

received, and treat any larger values as if they were that upper

bound. The TTL specifies a maximum time to live, not a mandatory

time to live.

9. The TC (truncated) header bit

The TC bit should be set in responses only when an RRSet is required

as a part of the response, but could not be included in its entirety.

The TC bit should not be set merely because some extra information

could have been included, but there was insufficient room. This

includes the results of additional section processing. In such cases

the entire RRSet that will not fit in the response should be omitted,

and the reply sent as is, with the TC bit clear. If the recipient of

the reply needs the omitted data, it can construct a query for that

data and send that separately.

Where TC is set, the partial RRSet that would not completely fit may

be left in the response. When a DNS client receives a reply with TC

set, it should ignore that response, and query again, using a

mechanism, such as a TCP connection, that will permit larger replies.

10. Naming issues

It has sometimes been inferred from some sections of the DNS

specification [RFC1034, RFC1035] that a host, or perhaps an interface

of a host, is permitted exactly one authoritative, or official, name,

called the canonical name. There is no such requirement in the DNS.

10.1. CNAME resource records

The DNS CNAME ("canonical name") record exists to provide the

canonical name associated with an alias name. There may be only one

such canonical name for any one alias. That name should generally be

a name that exists elsewhere in the DNS, though there are some rare

applications for aliases with the accompanying canonical name

undefined in the DNS. An alias name (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 the DNS (any domain name)

exactly one of the following is true:

+ one CNAME record exists, optionally accompanied by SIG, NXT, and

KEY RRs,

+ one or more records exist, none being CNAME records,

+ the name exists, but has no associated RRs of any type,

+ the name does not exist at all.

10.1.1. CNAME terminology

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

"canonical name", and the label of a CNAME record is most certainly

not a canonical name. It is, however, an entrenched usage. Care

must therefore be taken to be very clear whether the label, or the

value (the canonical name) of a CNAME resource record is intended.

In this document, the label of a CNAME resource record will always be

referred to as an alias.

10.2. PTR records

Confusion about canonical names has lead to a belief that a PTR

record should have exactly one RR in its RRSet. This is incorrect,

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

not be an alias. There is no implication in that section that only

one PTR record is permitted for a name. No such restriction should

be inferred.

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, which it should not be,

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.

10.3. MX and NS records

The domain name used as the value of a NS resource record, or part of

the value of a MX resource record must not be an alias. Not only is

the specification clear on this point, but using an alias in either

of these positions neither works as well as might be hoped, nor well

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

processing" in which address records associated with the value of the

record sought are appended to the answer. This helps avoid needless

extra queries that are easily anticipated when the first was made.

Additional section processing does not include CNAME records, let

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

of an NS or MX record, no address will be returned with the NS or MX

value. This can cause extra queries, and extra network burden, on

every query. It is trivial for the DNS administrator to avoid this

by resolving the alias and placing the canonical name directly in the

affected record just once when it is updated or installed. In some

particular hard cases the lack of the additional section address

records in the results of a NS lookup can cause the request to fail.

11. Name syntax

Occasionally it is assumed that the Domain Name System serves only

the purpose of mapping Internet host names to data, and mapping

Internet addresses to host names. This is not correct, the DNS is a

general (if somewhat limited) hierarchical database, and can store

almost any kind of data, for almost any purpose.

The DNS itself places only one restriction on the particular labels

that can be used to identify resource records. That one restriction

relates to the length of the label and the full name. The length of

any one label is limited to between 1 and 63 octets. A full domain

name is limited to 255 octets (including the separators). The zero

length full name is defined as representing the root of the DNS tree,

and is typically written and displayed as ".". Those restrictions

aside, any binary string whatever can be used as the label of any

resource record. Similarly, any binary string can serve as the value

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

can have restrictions imposed on what particular values are

acceptable in their environment. For example, that any binary label

can have an MX record does not imply that any binary name can be used

as the host part of an e-mail address. Clients of the DNS can impose

whatever restrictions are appropriate to their circumstances on the

values they use as keys for DNS lookup requests, and on the values

returned by the DNS. If the client has such restrictions, it is

solely responsible for validating the data from the DNS to ensure

that it conforms before it makes any use of that data.