Domain Names - Implementation and Specification

● 1,035
Network Working Group                                     P. Mockapetris
Request for Comments: 1035                                           ISI
                                                           November 1987
Obsoletes: RFCs 882, 883, 973
            DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION
1. STATUS OF THIS MEMO
This RFC describes the details of the domain system and protocol, and
assumes that the reader is familiar with the concepts discussed in a
companion RFC, "Domain Names - Concepts and Facilities" [RFC-1034].
The domain system is a mixture of functions and data types which are an
official protocol and functions and data types which are still
experimental.  Since the domain system is intentionally extensible, new
data types and experimental behavior should always be expected in parts
of the system beyond the official protocol.  The official protocol parts
include standard queries, responses and the Internet class RR data
formats (e.g., host addresses).  Since the previous RFC set, several
definitions have changed, so some previous definitions are obsolete.
Experimental or obsolete features are clearly marked in these RFCs, and
such information should be used with caution.
The reader is especially cautioned not to depend on the values which
appear in examples to be current or complete, since their purpose is
primarily pedagogical.  Distribution of this memo is unlimited.
                           Table of Contents
  1. STATUS OF THIS MEMO                                              1
  2. INTRODUCTION                                                     3
      2.1. Overview                                                   3
      2.2. Common configurations                                      4
      2.3. Conventions                                                7
          2.3.1. Preferred name syntax                                7
          2.3.2. Data Transmission Order                              8
          2.3.3. Character Case                                       9
          2.3.4. Size limits                                         10
  3. DOMAIN NAME SPACE AND RR DEFINITIONS                            10
      3.1. Name space definitions                                    10
      3.2. RR definitions                                            11
          3.2.1. Format                                              11
          3.2.2. TYPE values                                         12
          3.2.3. QTYPE values                                        12
          3.2.4. CLASS values                                        13
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RFC 1035        Domain Implementation and Specification    November 1987
          3.2.5. QCLASS values                                       13
      3.3. Standard RRs                                              13
          3.3.1. CNAME RDATA format                                  14
          3.3.2. HINFO RDATA format                                  14
          3.3.3. MB RDATA format (EXPERIMENTAL)                      14
          3.3.4. MD RDATA format (Obsolete)                          15
          3.3.5. MF RDATA format (Obsolete)                          15
          3.3.6. MG RDATA format (EXPERIMENTAL)                      16
          3.3.7. MINFO RDATA format (EXPERIMENTAL)                   16
          3.3.8. MR RDATA format (EXPERIMENTAL)                      17
          3.3.9. MX RDATA format                                     17
          3.3.10. NULL RDATA format (EXPERIMENTAL)                   17
          3.3.11. NS RDATA format                                    18
          3.3.12. PTR RDATA format                                   18
          3.3.13. SOA RDATA format                                   19
          3.3.14. TXT RDATA format                                   20
      3.4. ARPA Internet specific RRs                                20
          3.4.1. A RDATA format                                      20
          3.4.2. WKS RDATA format                                    21
      3.5. IN-ADDR.ARPA domain                                       22
      3.6. Defining new types, classes, and special namespaces       24
  4. MESSAGES                                                        25
      4.1. Format                                                    25
          4.1.1. Header section format                               26
          4.1.2. Question section format                             28
          4.1.3. Resource record format                              29
          4.1.4. Message compression                                 30
      4.2. Transport                                                 32
          4.2.1. UDP usage                                           32
          4.2.2. TCP usage                                           32
  5. MASTER FILES                                                    33
      5.1. Format                                                    33
      5.2. Use of master files to define zones                       35
      5.3. Master file example                                       36
  6. NAME SERVER IMPLEMENTATION                                      37
      6.1. Architecture                                              37
          6.1.1. Control                                             37
          6.1.2. Database                                            37
          6.1.3. Time                                                39
      6.2. Standard query processing                                 39
      6.3. Zone refresh and reload processing                        39
      6.4. Inverse queries (Optional)                                40
          6.4.1. The contents of inverse queries and responses       40
          6.4.2. Inverse query and response example                  41
          6.4.3. Inverse query processing                            42
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RFC 1035        Domain Implementation and Specification    November 1987
      6.5. Completion queries and responses                          42
  7. RESOLVER IMPLEMENTATION                                         43
      7.1. Transforming a user request into a query                  43
      7.2. Sending the queries                                       44
      7.3. Processing responses                                      46
      7.4. Using the cache                                           47
  8. MAIL SUPPORT                                                    47
      8.1. Mail exchange binding                                     48
      8.2. Mailbox binding (Experimental)                            48
  9. REFERENCES and BIBLIOGRAPHY                                     50
  Index                                                              54
2. INTRODUCTION
2.1. Overview
The goal of domain names is to provide a mechanism for naming resources
in such a way that the names are usable in different hosts, networks,
protocol families, internets, and administrative organizations.
From the user's point of view, domain names are useful as arguments to a
local agent, called a resolver, which retrieves information associated
with the domain name.  Thus a user might ask for the host address or
mail information associated with a particular domain name.  To enable
the user to request a particular type of information, an appropriate
query type is passed to the resolver with the domain name.  To the user,
the domain tree is a single information space; the resolver is
responsible for hiding the distribution of data among name servers from
the user.
From the resolver's point of view, the database that makes up the domain
space is distributed among various name servers.  Different parts of the
domain space are stored in different name servers, although a particular
data item will be stored redundantly in two or more name servers.  The
resolver starts with knowledge of at least one name server.  When the
resolver processes a user query it asks a known name server for the
information; in return, the resolver either receives the desired
information or a referral to another name server.  Using these
referrals, resolvers learn the identities and contents of other name
servers.  Resolvers are responsible for dealing with the distribution of
the domain space and dealing with the effects of name server failure by
consulting redundant databases in other servers.
Name servers manage two kinds of data.  The first kind of data held in
sets called zones; each zone is the complete database for a particular
"pruned" subtree of the domain space.  This data is called
authoritative.  A name server periodically checks to make sure that its
zones are up to date, and if not, obtains a new copy of updated zones
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from master files stored locally or in another name server.  The second
kind of data is cached data which was acquired by a local resolver.
This data may be incomplete, but improves the performance of the
retrieval process when non-local data is repeatedly accessed.  Cached
data is eventually discarded by a timeout mechanism.
This functional structure isolates the problems of user interface,
failure recovery, and distribution in the resolvers and isolates the
database update and refresh problems in the name servers.
2.2. Common configurations
A host can participate in the domain name system in a number of ways,
depending on whether the host runs programs that retrieve information
from the domain system, name servers that answer queries from other
hosts, or various combinations of both functions.  The simplest, and
perhaps most typical, configuration is shown below:
                 Local Host                        |  Foreign
                                                   |
    +---------+               +----------+         |  +--------+
    |         | user queries  |          |queries  |  |        |
    |  User   |-------------->|          |---------|->|Foreign |
    | Program |               | Resolver |         |  |  Name  |
    |         |<--------------|          |<--------|--| Server |
    |         | user responses|          |responses|  |        |
    +---------+               +----------+         |  +--------+
                                |     A            |
                cache additions |     | references |
                                V     |            |
                              +----------+         |
                              |  cache   |         |
                              +----------+         |
User programs interact with the domain name space through resolvers; the
format of user queries and user responses is specific to the host and
its operating system.  User queries will typically be operating system
calls, and the resolver and its cache will be part of the host operating
system.  Less capable hosts may choose to implement the resolver as a
subroutine to be linked in with every program that needs its services.
Resolvers answer user queries with information they acquire via queries
to foreign name servers and the local cache.
Note that the resolver may have to make several queries to several
different foreign name servers to answer a particular user query, and
hence the resolution of a user query may involve several network
accesses and an arbitrary amount of time.  The queries to foreign name
servers and the corresponding responses have a standard format described
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in this memo, and may be datagrams.
Depending on its capabilities, a name server could be a stand alone
program on a dedicated machine or a process or processes on a large
timeshared host.  A simple configuration might be:
                 Local Host                        |  Foreign
                                                   |
      +---------+                                  |
     /         /|                                  |
    +---------+ |             +----------+         |  +--------+
    |         | |             |          |responses|  |        |
    |         | |             |   Name   |---------|->|Foreign |
    |  Master |-------------->|  Server  |         |  |Resolver|
    |  files  | |             |          |<--------|--|        |
    |         |/              |          | queries |  +--------+
    +---------+               +----------+         |
Here a primary name server acquires information about one or more zones
by reading master files from its local file system, and answers queries
about those zones that arrive from foreign resolvers.
The DNS requires that all zones be redundantly supported by more than
one name server.  Designated secondary servers can acquire zones and
check for updates from the primary server using the zone transfer
protocol of the DNS.  This configuration is shown below:
                 Local Host                        |  Foreign
                                                   |
      +---------+                                  |
     /         /|                                  |
    +---------+ |             +----------+         |  +--------+
    |         | |             |          |responses|  |        |
    |         | |             |   Name   |---------|->|Foreign |
    |  Master |-------------->|  Server  |         |  |Resolver|
    |  files  | |             |          |<--------|--|        |
    |         |/              |          | queries |  +--------+
    +---------+               +----------+         |
                                A     |maintenance |  +--------+
                                |     +------------|->|        |
                                |      queries     |  |Foreign |
                                |                  |  |  Name  |
                                +------------------|--| Server |
                             maintenance responses |  +--------+
In this configuration, the name server periodically establishes a
virtual circuit to a foreign name server to acquire a copy of a zone or
to check that an existing copy has not changed.  The messages sent for
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these maintenance activities follow the same form as queries and
responses, but the message sequences are somewhat different.
The information flow in a host that supports all aspects of the domain
name system is shown below:
                 Local Host                        |  Foreign
                                                   |
    +---------+               +----------+         |  +--------+
    |         | user queries  |          |queries  |  |        |
    |  User   |-------------->|          |---------|->|Foreign |
    | Program |               | Resolver |         |  |  Name  |
    |         |<--------------|          |<--------|--| Server |
    |         | user responses|          |responses|  |        |
    +---------+               +----------+         |  +--------+
                                |     A            |
                cache additions |     | references |
                                V     |            |
                              +----------+         |
                              |  Shared  |         |
                              | database |         |
                              +----------+         |
                                A     |            |
      +---------+     refreshes |     | references |
     /         /|               |     V            |
    +---------+ |             +----------+         |  +--------+
    |         | |             |          |responses|  |        |
    |         | |             |   Name   |---------|->|Foreign |
    |  Master |-------------->|  Server  |         |  |Resolver|
    |  files  | |             |          |<--------|--|        |
    |         |/              |          | queries |  +--------+
    +---------+               +----------+         |
                                A     |maintenance |  +--------+
                                |     +------------|->|        |
                                |      queries     |  |Foreign |
                                |                  |  |  Name  |
                                +------------------|--| Server |
                             maintenance responses |  +--------+
The shared database holds domain space data for the local name server
and resolver.  The contents of the shared database will typically be a
mixture of authoritative data maintained by the periodic refresh
operations of the name server and cached data from previous resolver
requests.  The structure of the domain data and the necessity for
synchronization between name servers and resolvers imply the general
characteristics of this database, but the actual format is up to the
local implementor.
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Information flow can also be tailored so that a group of hosts act
together to optimize activities.  Sometimes this is done to offload less
capable hosts so that they do not have to implement a full resolver.
This can be appropriate for PCs or hosts which want to minimize the
amount of new network code which is required.  This scheme can also
allow a group of hosts can share a small number of caches rather than
maintaining a large number of separate caches, on the premise that the
centralized caches will have a higher hit ratio.  In either case,
resolvers are replaced with stub resolvers which act as front ends to
resolvers located in a recursive server in one or more name servers
known to perform that service:
                   Local Hosts                     |  Foreign
                                                   |
    +---------+                                    |
    |         | responses                          |
    | Stub    |<--------------------+              |
    | Resolver|                     |              |
    |         |----------------+    |              |
    +---------+ recursive      |    |              |
                queries        |    |              |
                               V    |              |
    +---------+ recursive     +----------+         |  +--------+
    |         | queries       |          |queries  |  |        |
    | Stub    |-------------->| Recursive|---------|->|Foreign |
    | Resolver|               | Server   |         |  |  Name  |
    |         |<--------------|          |<--------|--| Server |
    +---------+ responses     |          |responses|  |        |
                              +----------+         |  +--------+
                              |  Central |         |
                              |   cache  |         |
                              +----------+         |
In any case, note that domain components are always replicated for
reliability whenever possible.
2.3. Conventions
The domain system has several conventions dealing with low-level, but
fundamental, issues.  While the implementor is free to violate these
conventions WITHIN HIS OWN SYSTEM, he must observe these conventions in
ALL behavior observed from other hosts.
2.3.1. Preferred name syntax
The DNS specifications attempt to be as general as possible in the rules
for constructing domain names.  The idea is that the name of any
existing object can be expressed as a domain name with minimal changes.
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However, when assigning a domain name for an object, the prudent user
will select a name which satisfies both the rules of the domain system
and any existing rules for the object, whether these rules are published
or implied by existing programs.
For example, when naming a mail domain, the user should satisfy both the
rules of this memo and those in RFC-822.  When creating a new host name,
the old rules for HOSTS.TXT should be followed.  This avoids problems
when old software is converted to use domain names.
The following syntax will result in fewer problems with many
applications that use domain names (e.g., mail, TELNET).
<domain> ::= <subdomain> | " "
<subdomain> ::= <label> | <subdomain> "." <label>
<label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
<let-dig-hyp> ::= <let-dig> | "-"
<let-dig> ::= <letter> | <digit>
<letter> ::= any one of the 52 alphabetic characters A through Z in
upper case and a through z in lower case
<digit> ::= any one of the ten digits 0 through 9
Note that while upper and lower case letters are allowed in domain
names, no significance is attached to the case.  That is, two names with
the same spelling but different case are to be treated as if identical.
The labels must follow the rules for ARPANET host names.  They must
start with a letter, end with a letter or digit, and have as interior
characters only letters, digits, and hyphen.  There are also some
restrictions on the length.  Labels must be 63 characters or less.
For example, the following strings identify hosts in the Internet:
A.ISI.EDU XX.LCS.MIT.EDU SRI-NIC.ARPA
2.3.2. Data Transmission Order
The order of transmission of the header and data described in this
document is resolved to the octet level.  Whenever a diagram shows a
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group of octets, the order of transmission of those octets is the normal
order in which they are read in English.  For example, in the following
diagram, the octets are transmitted in the order they are numbered.
     0                   1
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       1       |       2       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       3       |       4       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       5       |       6       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Whenever an octet represents a numeric quantity, the left most bit in
the diagram is the high order or most significant bit.  That is, the bit
labeled 0 is the most significant bit.  For example, the following
diagram represents the value 170 (decimal).
     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |1 0 1 0 1 0 1 0|
    +-+-+-+-+-+-+-+-+
Similarly, whenever a multi-octet field represents a numeric quantity
the left most bit of the whole field is the most significant bit.  When
a multi-octet quantity is transmitted the most significant octet is
transmitted first.
2.3.3. Character Case
For all parts of the DNS that are part of the official protocol, all
comparisons between character strings (e.g., labels, domain names, etc.)
are done in a case-insensitive manner.  At present, this rule is in
force throughout the domain system without exception.  However, future
additions beyond current usage may need to use the full binary octet
capabilities in names, so attempts to store domain names in 7-bit ASCII
or use of special bytes to terminate labels, etc., should be avoided.
When data enters the domain system, its original case should be
preserved whenever possible.  In certain circumstances this cannot be
done.  For example, if two RRs are stored in a database, one at x.y and
one at X.Y, they are actually stored at the same place in the database,
and hence only one casing would be preserved.  The basic rule is that
case can be discarded only when data is used to define structure in a
database, and two names are identical when compared in a case
insensitive manner.
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Loss of case sensitive data must be minimized.  Thus while data for x.y
and X.Y may both be stored under a single location x.y or X.Y, data for
a.x and B.X would never be stored under A.x, A.X, b.x, or b.X.  In
general, this preserves the case of the first label of a domain name,
but forces standardization of interior node labels.
Systems administrators who enter data into the domain database should
take care to represent the data they supply to the domain system in a
case-consistent manner if their system is case-sensitive.  The data
distribution system in the domain system will ensure that consistent
representations are preserved.
2.3.4. Size limits
Various objects and parameters in the DNS have size limits.  They are
listed below.  Some could be easily changed, others are more
fundamental.
labels          63 octets or less
names           255 octets or less
TTL             positive values of a signed 32 bit number.
UDP messages    512 octets or less
3. DOMAIN NAME SPACE AND RR DEFINITIONS
3.1. Name space definitions
Domain names in messages are expressed in terms of a sequence of labels.
Each label is represented as a one octet length field followed by that
number of octets.  Since every domain name ends with the null label of
the root, a domain name is terminated by a length byte of zero.  The
high order two bits of every length octet must be zero, and the
remaining six bits of the length field limit the label to 63 octets or
less.
To simplify implementations, the total length of a domain name (i.e.,
label octets and label length octets) is restricted to 255 octets or
less.
Although labels can contain any 8 bit values in octets that make up a
label, it is strongly recommended that labels follow the preferred
syntax described elsewhere in this memo, which is compatible with
existing host naming conventions.  Name servers and resolvers must
compare labels in a case-insensitive manner (i.e., A=a), assuming ASCII
with zero parity.  Non-alphabetic codes must match exactly.
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3.2. RR definitions
3.2.1. Format
All RRs have the same top level format shown below:
                                    1  1  1  1  1  1
      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                                               |
    /                                               /
    /                      NAME                     /
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                      TYPE                     |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                     CLASS                     |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                      TTL                      |
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                   RDLENGTH                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
    /                     RDATA                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
NAME            an owner name, i.e., the name of the node to which this
                resource record pertains.
TYPE            two octets containing one of the RR TYPE codes.
CLASS           two octets containing one of the RR CLASS codes.
TTL             a 32 bit signed integer that specifies the time interval
                that the resource record may be cached before the source
                of the information should again be consulted.  Zero
                values are interpreted to mean that the RR can only be
                used for the transaction in progress, and should not be
                cached.  For example, SOA records are always distributed
                with a zero TTL to prohibit caching.  Zero values can
                also be used for extremely volatile data.
RDLENGTH        an unsigned 16 bit integer that specifies the length in
                octets of the RDATA field.
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RDATA           a variable length string of octets that describes the
                resource.  The format of this information varies
                according to the TYPE and CLASS of the resource record.
3.2.2. TYPE values
TYPE fields are used in resource records.  Note that these types are a
subset of QTYPEs.
TYPE            value and meaning
A               1 a host address
NS              2 an authoritative name server
MD              3 a mail destination (Obsolete - use MX)
MF              4 a mail forwarder (Obsolete - use MX)
CNAME           5 the canonical name for an alias
SOA             6 marks the start of a zone of authority
MB              7 a mailbox domain name (EXPERIMENTAL)
MG              8 a mail group member (EXPERIMENTAL)
MR              9 a mail rename domain name (EXPERIMENTAL)
NULL            10 a null RR (EXPERIMENTAL)
WKS             11 a well known service description
PTR             12 a domain name pointer
HINFO           13 host information
MINFO           14 mailbox or mail list information
MX              15 mail exchange
TXT             16 text strings
3.2.3. QTYPE values
QTYPE fields appear in the question part of a query.  QTYPES are a
superset of TYPEs, hence all TYPEs are valid QTYPEs.  In addition, the
following QTYPEs are defined:
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AXFR            252 A request for a transfer of an entire zone
MAILB           253 A request for mailbox-related records (MB, MG or MR)
MAILA           254 A request for mail agent RRs (Obsolete - see MX)
*               255 A request for all records
3.2.4. CLASS values
CLASS fields appear in resource records.  The following CLASS mnemonics
and values are defined:
IN              1 the Internet
CS              2 the CSNET class (Obsolete - used only for examples in
                some obsolete RFCs)
CH              3 the CHAOS class
HS              4 Hesiod [Dyer 87]
3.2.5. QCLASS values
QCLASS fields appear in the question section of a query.  QCLASS values
are a superset of CLASS values; every CLASS is a valid QCLASS.  In
addition to CLASS values, the following QCLASSes are defined:
*               255 any class
3.3. Standard RRs
The following RR definitions are expected to occur, at least
potentially, in all classes.  In particular, NS, SOA, CNAME, and PTR
will be used in all classes, and have the same format in all classes.
Because their RDATA format is known, all domain names in the RDATA
section of these RRs may be compressed.
<domain-name> is a domain name represented as a series of labels, and
terminated by a label with zero length.  <character-string> is a single
length octet followed by that number of characters.  <character-string>
is treated as binary information, and can be up to 256 characters in
length (including the length octet).
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3.3.1. CNAME RDATA format
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                     CNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
CNAME           A <domain-name> which specifies the canonical or primary
                name for the owner.  The owner name is an alias.
CNAME RRs cause no additional section processing, but name servers may
choose to restart the query at the canonical name in certain cases.  See
the description of name server logic in [RFC-1034] for details.
3.3.2. HINFO RDATA format
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                      CPU                      /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                       OS                      /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
CPU             A <character-string> which specifies the CPU type.
OS              A <character-string> which specifies the operating
                system type.
Standard values for CPU and OS can be found in [RFC-1010].
HINFO records are used to acquire general information about a host.  The
main use is for protocols such as FTP that can use special procedures
when talking between machines or operating systems of the same type.
3.3.3. MB RDATA format (EXPERIMENTAL)
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   MADNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
MADNAME         A <domain-name> which specifies a host which has the
                specified mailbox.
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MB records cause additional section processing which looks up an A type
RRs corresponding to MADNAME.
3.3.4. MD RDATA format (Obsolete)
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   MADNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
MADNAME         A <domain-name> which specifies a host which has a mail
                agent for the domain which should be able to deliver
                mail for the domain.
MD records cause additional section processing which looks up an A type
record corresponding to MADNAME.
MD is obsolete.  See the definition of MX and [RFC-974] for details of
the new scheme.  The recommended policy for dealing with MD RRs found in
a master file is to reject them, or to convert them to MX RRs with a
preference of 0.
3.3.5. MF RDATA format (Obsolete)
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   MADNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
MADNAME         A <domain-name> which specifies a host which has a mail
                agent for the domain which will accept mail for
                forwarding to the domain.
MF records cause additional section processing which looks up an A type
record corresponding to MADNAME.
MF is obsolete.  See the definition of MX and [RFC-974] for details ofw
the new scheme.  The recommended policy for dealing with MD RRs found in
a master file is to reject them, or to convert them to MX RRs with a
preference of 10.
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3.3.6. MG RDATA format (EXPERIMENTAL)
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   MGMNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
MGMNAME         A <domain-name> which specifies a mailbox which is a
                member of the mail group specified by the domain name.
MG records cause no additional section processing.
3.3.7. MINFO RDATA format (EXPERIMENTAL)
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                    RMAILBX                    /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                    EMAILBX                    /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
RMAILBX         A <domain-name> which specifies a mailbox which is
                responsible for the mailing list or mailbox.  If this
                domain name names the root, the owner of the MINFO RR is
                responsible for itself.  Note that many existing mailing
                lists use a mailbox X-request for the RMAILBX field of
                mailing list X, e.g., Msgroup-request for Msgroup.  This
                field provides a more general mechanism.
EMAILBX         A <domain-name> which specifies a mailbox which is to
                receive error messages related to the mailing list or
                mailbox specified by the owner of the MINFO RR (similar
                to the ERRORS-TO: field which has been proposed).  If
                this domain name names the root, errors should be
                returned to the sender of the message.
MINFO records cause no additional section processing.  Although these
records can be associated with a simple mailbox, they are usually used
with a mailing list.
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3.3.8. MR RDATA format (EXPERIMENTAL)
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   NEWNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
NEWNAME         A <domain-name> which specifies a mailbox which is the
                proper rename of the specified mailbox.
MR records cause no additional section processing.  The main use for MR
is as a forwarding entry for a user who has moved to a different
mailbox.
3.3.9. MX RDATA format
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                  PREFERENCE                   |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   EXCHANGE                    /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
PREFERENCE      A 16 bit integer which specifies the preference given to
                this RR among others at the same owner.  Lower values
                are preferred.
EXCHANGE        A <domain-name> which specifies a host willing to act as
                a mail exchange for the owner name.
MX records cause type A additional section processing for the host
specified by EXCHANGE.  The use of MX RRs is explained in detail in
[RFC-974].
3.3.10. NULL RDATA format (EXPERIMENTAL)
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                  <anything>                   /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Anything at all may be in the RDATA field so long as it is 65535 octets
or less.
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NULL records cause no additional section processing.  NULL RRs are not
allowed in master files.  NULLs are used as placeholders in some
experimental extensions of the DNS.
3.3.11. NS RDATA format
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   NSDNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
NSDNAME         A <domain-name> which specifies a host which should be
                authoritative for the specified class and domain.
NS records cause both the usual additional section processing to locate
a type A record, and, when used in a referral, a special search of the
zone in which they reside for glue information.
The NS RR states that the named host should be expected to have a zone
starting at owner name of the specified class.  Note that the class may
not indicate the protocol family which should be used to communicate
with the host, although it is typically a strong hint.  For example,
hosts which are name servers for either Internet (IN) or Hesiod (HS)
class information are normally queried using IN class protocols.
3.3.12. PTR RDATA format
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   PTRDNAME                    /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
PTRDNAME        A <domain-name> which points to some location in the
                domain name space.
PTR records cause no additional section processing.  These RRs are used
in special domains to point to some other location in the domain space.
These records are simple data, and don't imply any special processing
similar to that performed by CNAME, which identifies aliases.  See the
description of the IN-ADDR.ARPA domain for an example.
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3.3.13. SOA RDATA format
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                     MNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                     RNAME                     /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    SERIAL                     |
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    REFRESH                    |
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                     RETRY                     |
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    EXPIRE                     |
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    MINIMUM                    |
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
MNAME           The <domain-name> of the name server that was the
                original or primary source of data for this zone.
RNAME           A <domain-name> which specifies the mailbox of the
                person responsible for this zone.
SERIAL          The unsigned 32 bit version number of the original copy
                of the zone.  Zone transfers preserve this value.  This
                value wraps and should be compared using sequence space
                arithmetic.
REFRESH         A 32 bit time interval before the zone should be
                refreshed.
RETRY           A 32 bit time interval that should elapse before a
                failed refresh should be retried.
EXPIRE          A 32 bit time value that specifies the upper limit on
                the time interval that can elapse before the zone is no
                longer authoritative.
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MINIMUM         The unsigned 32 bit minimum TTL field that should be
                exported with any RR from this zone.
SOA records cause no additional section processing.
All times are in units of seconds.
Most of these fields are pertinent only for name server maintenance
operations.  However, MINIMUM is used in all query operations that
retrieve RRs from a zone.  Whenever a RR is sent in a response to a
query, the TTL field is set to the maximum of the TTL field from the RR
and the MINIMUM field in the appropriate SOA.  Thus MINIMUM is a lower
bound on the TTL field for all RRs in a zone.  Note that this use of
MINIMUM should occur when the RRs are copied into the response and not
when the zone is loaded from a master file or via a zone transfer.  The
reason for this provison is to allow future dynamic update facilities to
change the SOA RR with known semantics.
3.3.14. TXT RDATA format
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   TXT-DATA                    /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
TXT-DATA        One or more <character-string>s.
TXT RRs are used to hold descriptive text.  The semantics of the text
depends on the domain where it is found.
3.4. Internet specific RRs
3.4.1. A RDATA format
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    ADDRESS                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
ADDRESS         A 32 bit Internet address.
Hosts that have multiple Internet addresses will have multiple A
records.
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A records cause no additional section processing.  The RDATA section of
an A line in a master file is an Internet address expressed as four
decimal numbers separated by dots without any imbedded spaces (e.g.,
"10.2.0.52" or "192.0.5.6").
3.4.2. WKS RDATA format
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    ADDRESS                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |       PROTOCOL        |                       |
    +--+--+--+--+--+--+--+--+                       |
    |                                               |
    /                   <BIT MAP>                   /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
ADDRESS         An 32 bit Internet address
PROTOCOL        An 8 bit IP protocol number
<BIT MAP>       A variable length bit map.  The bit map must be a
                multiple of 8 bits long.
The WKS record is used to describe the well known services supported by
a particular protocol on a particular internet address.  The PROTOCOL
field specifies an IP protocol number, and the bit map has one bit per
port of the specified protocol.  The first bit corresponds to port 0,
the second to port 1, etc.  If the bit map does not include a bit for a
protocol of interest, that bit is assumed zero.  The appropriate values
and mnemonics for ports and protocols are specified in [RFC-1010].
For example, if PROTOCOL=TCP (6), the 26th bit corresponds to TCP port
25 (SMTP).  If this bit is set, a SMTP server should be listening on TCP
port 25; if zero, SMTP service is not supported on the specified
address.
The purpose of WKS RRs is to provide availability information for
servers for TCP and UDP.  If a server supports both TCP and UDP, or has
multiple Internet addresses, then multiple WKS RRs are used.
WKS RRs cause no additional section processing.
In master files, both ports and protocols are expressed using mnemonics
or decimal numbers.
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3.5. IN-ADDR.ARPA domain
The Internet uses a special domain to support gateway location and
Internet address to host mapping.  Other classes may employ a similar
strategy in other domains.  The intent of this domain is to provide a
guaranteed method to perform host address to host name mapping, and to
facilitate queries to locate all gateways on a particular network in the
Internet.
Note that both of these services are similar to functions that could be
performed by inverse queries; the difference is that this part of the
domain name space is structured according to address, and hence can
guarantee that the appropriate data can be located without an exhaustive
search of the domain space.
The domain begins at IN-ADDR.ARPA and has a substructure which follows
the Internet addressing structure.
Domain names in the IN-ADDR.ARPA domain are defined to have up to four
labels in addition to the IN-ADDR.ARPA suffix.  Each label represents
one octet of an Internet address, and is expressed as a character string
for a decimal value in the range 0-255 (with leading zeros omitted
except in the case of a zero octet which is represented by a single
zero).
Host addresses are represented by domain names that have all four labels
specified.  Thus data for Internet address 10.2.0.52 is located at
domain name 52.0.2.10.IN-ADDR.ARPA.  The reversal, though awkward to
read, allows zones to be delegated which are exactly one network of
address space.  For example, 10.IN-ADDR.ARPA can be a zone containing
data for the ARPANET, while 26.IN-ADDR.ARPA can be a separate zone for
MILNET.  Address nodes are used to hold pointers to primary host names
in the normal domain space.
Network numbers correspond to some non-terminal nodes at various depths
in the IN-ADDR.ARPA domain, since Internet network numbers are either 1,
2, or 3 octets.  Network nodes are used to hold pointers to the primary
host names of gateways attached to that network.  Since a gateway is, by
definition, on more than one network, it will typically have two or more
network nodes which point at it.  Gateways will also have host level
pointers at their fully qualified addresses.
Both the gateway pointers at network nodes and the normal host pointers
at full address nodes use the PTR RR to point back to the primary domain
names of the corresponding hosts.
For example, the IN-ADDR.ARPA domain will contain information about the
ISI gateway between net 10 and 26, an MIT gateway from net 10 to MIT's
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net 18, and hosts A.ISI.EDU and MULTICS.MIT.EDU.  Assuming that ISI
gateway has addresses 10.2.0.22 and 26.0.0.103, and a name MILNET-
GW.ISI.EDU, and the MIT gateway has addresses 10.0.0.77 and 18.10.0.4
and a name GW.LCS.MIT.EDU, the domain database would contain:
    10.IN-ADDR.ARPA.           PTR MILNET-GW.ISI.EDU.
    10.IN-ADDR.ARPA.           PTR GW.LCS.MIT.EDU.
    18.IN-ADDR.ARPA.           PTR GW.LCS.MIT.EDU.
    26.IN-ADDR.ARPA.           PTR MILNET-GW.ISI.EDU.
    22.0.2.10.IN-ADDR.ARPA.    PTR MILNET-GW.ISI.EDU.
    103.0.0.26.IN-ADDR.ARPA.   PTR MILNET-GW.ISI.EDU.
    77.0.0.10.IN-ADDR.ARPA.    PTR GW.LCS.MIT.EDU.
    4.0.10.18.IN-ADDR.ARPA.    PTR GW.LCS.MIT.EDU.
    103.0.3.26.IN-ADDR.ARPA.   PTR A.ISI.EDU.
    6.0.0.10.IN-ADDR.ARPA.     PTR MULTICS.MIT.EDU.
Thus a program which wanted to locate gateways on net 10 would originate
a query of the form QTYPE=PTR, QCLASS=IN, QNAME=10.IN-ADDR.ARPA.  It
would receive two RRs in response:
    10.IN-ADDR.ARPA.           PTR MILNET-GW.ISI.EDU.
    10.IN-ADDR.ARPA.           PTR GW.LCS.MIT.EDU.
The program could then originate QTYPE=A, QCLASS=IN queries for MILNET-
GW.ISI.EDU. and GW.LCS.MIT.EDU. to discover the Internet addresses of
these gateways.
A resolver which wanted to find the host name corresponding to Internet
host address 10.0.0.6 would pursue a query of the form QTYPE=PTR,
QCLASS=IN, QNAME=6.0.0.10.IN-ADDR.ARPA, and would receive:
    6.0.0.10.IN-ADDR.ARPA.     PTR MULTICS.MIT.EDU.
Several cautions apply to the use of these services:
   - Since the IN-ADDR.ARPA special domain and the normal domain
     for a particular host or gateway will be in different zones,
     the possibility exists that that the data may be inconsistent.
   - Gateways will often have two names in separate domains, only
     one of which can be primary.
   - Systems that use the domain database to initialize their
     routing tables must start with enough gateway information to
     guarantee that they can access the appropriate name server.
   - The gateway data only reflects the existence of a gateway in a
     manner equivalent to the current HOSTS.TXT file.  It doesn't
     replace the dynamic availability information from GGP or EGP.
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3.6. Defining new types, classes, and special namespaces
The previously defined types and classes are the ones in use as of the
date of this memo.  New definitions should be expected.  This section
makes some recommendations to designers considering additions to the
existing facilities.  The mailing list NAMEDROPPERS@SRI-NIC.ARPA is the
forum where general discussion of design issues takes place.
In general, a new type is appropriate when new information is to be
added to the database about an existing object, or we need new data
formats for some totally new object.  Designers should attempt to define
types and their RDATA formats that are generally applicable to all
classes, and which avoid duplication of information.  New classes are
appropriate when the DNS is to be used for a new protocol, etc which
requires new class-specific data formats, or when a copy of the existing
name space is desired, but a separate management domain is necessary.
New types and classes need mnemonics for master files; the format of the
master files requires that the mnemonics for type and class be disjoint.
TYPE and CLASS values must be a proper subset of QTYPEs and QCLASSes
respectively.
The present system uses multiple RRs to represent multiple values of a
type rather than storing multiple values in the RDATA section of a
single RR.  This is less efficient for most applications, but does keep
RRs shorter.  The multiple RRs assumption is incorporated in some
experimental work on dynamic update methods.
The present system attempts to minimize the duplication of data in the
database in order to insure consistency.  Thus, in order to find the
address of the host for a mail exchange, you map the mail domain name to
a host name, then the host name to addresses, rather than a direct
mapping to host address.  This approach is preferred because it avoids
the opportunity for inconsistency.
In defining a new type of data, multiple RR types should not be used to
create an ordering between entries or express different formats for
equivalent bindings, instead this information should be carried in the
body of the RR and a single type used.  This policy avoids problems with
caching multiple types and defining QTYPEs to match multiple types.
For example, the original form of mail exchange binding used two RR
types one to represent a "closer" exchange (MD) and one to represent a
"less close" exchange (MF).  The difficulty is that the presence of one
RR type in a cache doesn't convey any information about the other
because the query which acquired the cached information might have used
a QTYPE of MF, MD, or MAILA (which matched both).  The redesigned
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service used a single type (MX) with a "preference" value in the RDATA
section which can order different RRs.  However, if any MX RRs are found
in the cache, then all should be there.
4. MESSAGES
4.1. Format
All communications inside of the domain protocol are carried in a single
format called a message.  The top level format of message is divided
into 5 sections (some of which are empty in certain cases) shown below:
    +---------------------+
    |        Header       |
    +---------------------+
    |       Question      | the question for the name server
    +---------------------+
    |        Answer       | RRs answering the question
    +---------------------+
    |      Authority      | RRs pointing toward an authority
    +---------------------+
    |      Additional     | RRs holding additional information
    +---------------------+
The header section is always present.  The header includes fields that
specify which of the remaining sections are present, and also specify
whether the message is a query or a response, a standard query or some
other opcode, etc.
The names of the sections after the header are derived from their use in
standard queries.  The question section contains fields that describe a
question to a name server.  These fields are a query type (QTYPE), a
query class (QCLASS), and a query domain name (QNAME).  The last three
sections have the same format: a possibly empty list of concatenated
resource records (RRs).  The answer section contains RRs that answer the
question; the authority section contains RRs that point toward an
authoritative name server; the additional records section contains RRs
which relate to the query, but are not strictly answers for the
question.
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4.1.1. Header section format
The header contains the following fields:
                                    1  1  1  1  1  1
      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                      ID                       |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |QR|   Opcode  |AA|TC|RD|RA|   Z    |   RCODE   |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    QDCOUNT                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    ANCOUNT                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    NSCOUNT                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    ARCOUNT                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
ID              A 16 bit identifier assigned by the program that
                generates any kind of query.  This identifier is copied
                the corresponding reply and can be used by the requester
                to match up replies to outstanding queries.
QR              A one bit field that specifies whether this message is a
                query (0), or a response (1).
OPCODE          A four bit field that specifies kind of query in this
                message.  This value is set by the originator of a query
                and copied into the response.  The values are:
                0               a standard query (QUERY)
                1               an inverse query (IQUERY)
                2               a server status request (STATUS)
                3-15            reserved for future use
AA              Authoritative Answer - this bit is valid in responses,
                and specifies that the responding name server is an
                authority for the domain name in question section.
                Note that the contents of the answer section may have
                multiple owner names because of aliases.  The AA bit
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                corresponds to the name which matches the query name, or
                the first owner name in the answer section.
TC              TrunCation - specifies that this message was truncated
                due to length greater than that permitted on the
                transmission channel.
RD              Recursion Desired - this bit may be set in a query and
                is copied into the response.  If RD is set, it directs
                the name server to pursue the query recursively.
                Recursive query support is optional.
RA              Recursion Available - this be is set or cleared in a
                response, and denotes whether recursive query support is
                available in the name server.
Z               Reserved for future use.  Must be zero in all queries
                and responses.
RCODE           Response code - this 4 bit field is set as part of
                responses.  The values have the following
                interpretation:
                0               No error condition
                1               Format error - The name server was
                                unable to interpret the query.
                2               Server failure - The name server was
                                unable to process this query due to a
                                problem with the name server.
                3               Name Error - Meaningful only for
                                responses from an authoritative name
                                server, this code signifies that the
                                domain name referenced in the query does
                                not exist.
                4               Not Implemented - The name server does
                                not support the requested kind of query.
                5               Refused - The name server refuses to
                                perform the specified operation for
                                policy reasons.  For example, a name
                                server may not wish to provide the
                                information to the particular requester,
                                or a name server may not wish to perform
                                a particular operation (e.g., zone
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                                transfer) for particular data.
                6-15            Reserved for future use.
QDCOUNT         an unsigned 16 bit integer specifying the number of
                entries in the question section.
ANCOUNT         an unsigned 16 bit integer specifying the number of
                resource records in the answer section.
NSCOUNT         an unsigned 16 bit integer specifying the number of name
                server resource records in the authority records
                section.
ARCOUNT         an unsigned 16 bit integer specifying the number of
                resource records in the additional records section.
4.1.2. Question section format
The question section is used to carry the "question" in most queries,
i.e., the parameters that define what is being asked.  The section
contains QDCOUNT (usually 1) entries, each of the following format:
                                    1  1  1  1  1  1
      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                                               |
    /                     QNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                     QTYPE                     |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                     QCLASS                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
QNAME           a domain name represented as a sequence of labels, where
                each label consists of a length octet followed by that
                number of octets.  The domain name terminates with the
                zero length octet for the null label of the root.  Note
                that this field may be an odd number of octets; no
                padding is used.
QTYPE           a two octet code which specifies the type of the query.
                The values for this field include all codes valid for a
                TYPE field, together with some more general codes which
                can match more than one type of RR.
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QCLASS          a two octet code that specifies the class of the query.
                For example, the QCLASS field is IN for the Internet.
4.1.3. Resource record format
The answer, authority, and additional sections all share the same
format: a variable number of resource records, where the number of
records is specified in the corresponding count field in the header.
Each resource record has the following format:
                                    1  1  1  1  1  1
      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                                               |
    /                                               /
    /                      NAME                     /
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                      TYPE                     |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                     CLASS                     |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                      TTL                      |
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                   RDLENGTH                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
    /                     RDATA                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
NAME            a domain name to which this resource record pertains.
TYPE            two octets containing one of the RR type codes.  This
                field specifies the meaning of the data in the RDATA
                field.
CLASS           two octets which specify the class of the data in the
                RDATA field.
TTL             a 32 bit unsigned integer that specifies the time
                interval (in seconds) that the resource record may be
                cached before it should be discarded.  Zero values are
                interpreted to mean that the RR can only be used for the
                transaction in progress, and should not be cached.
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RDLENGTH        an unsigned 16 bit integer that specifies the length in
                octets of the RDATA field.
RDATA           a variable length string of octets that describes the
                resource.  The format of this information varies
                according to the TYPE and CLASS of the resource record.
                For example, the if the TYPE is A and the CLASS is IN,
                the RDATA field is a 4 octet ARPA Internet address.
4.1.4. Message compression
In order to reduce the size of messages, the domain system utilizes a
compression scheme which eliminates the repetition of domain names in a
message.  In this scheme, an entire domain name or a list of labels at
the end of a domain name is replaced with a pointer to a prior occurance
of the same name.
The pointer takes the form of a two octet sequence:
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    | 1  1|                OFFSET                   |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
The first two bits are ones.  This allows a pointer to be distinguished
from a label, since the label must begin with two zero bits because
labels are restricted to 63 octets or less.  (The 10 and 01 combinations
are reserved for future use.)  The OFFSET field specifies an offset from
the start of the message (i.e., the first octet of the ID field in the
domain header).  A zero offset specifies the first byte of the ID field,
etc.
The compression scheme allows a domain name in a message to be
represented as either:
   - a sequence of labels ending in a zero octet
   - a pointer
   - a sequence of labels ending with a pointer
Pointers can only be used for occurances of a domain name where the
format is not class specific.  If this were not the case, a name server
or resolver would be required to know the format of all RRs it handled.
As yet, there are no such cases, but they may occur in future RDATA
formats.
If a domain name is contained in a part of the message subject to a
length field (such as the RDATA section of an RR), and compression is
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used, the length of the compressed name is used in the length
calculation, rather than the length of the expanded name.
Programs are free to avoid using pointers in messages they generate,
although this will reduce datagram capacity, and may cause truncation.
However all programs are required to understand arriving messages that
contain pointers.
For example, a datagram might need to use the domain names F.ISI.ARPA,
FOO.F.ISI.ARPA, ARPA, and the root.  Ignoring the other fields of the
message, these domain names might be represented as:
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    20 |           1           |           F           |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    22 |           3           |           I           |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    24 |           S           |           I           |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    26 |           4           |           A           |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    28 |           R           |           P           |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    30 |           A           |           0           |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    40 |           3           |           F           |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    42 |           O           |           O           |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    44 | 1  1|                20                       |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    64 | 1  1|                26                       |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    92 |           0           |                       |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
The domain name for F.ISI.ARPA is shown at offset 20.  The domain name
FOO.F.ISI.ARPA is shown at offset 40; this definition uses a pointer to
concatenate a label for FOO to the previously defined F.ISI.ARPA.  The
domain name ARPA is defined at offset 64 using a pointer to the ARPA
component of the name F.ISI.ARPA at 20; note that this pointer relies on
ARPA being the last label in the string at 20.  The root domain name is
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defined by a single octet of zeros at 92; the root domain name has no
labels.
4.2. Transport
The DNS assumes that messages will be transmitted as datagrams or in a
byte stream carried by a virtual circuit.  While virtual circuits can be
used for any DNS activity, datagrams are preferred for queries due to
their lower overhead and better performance.  Zone refresh activities
must use virtual circuits because of the need for reliable transfer.
The Internet supports name server access using TCP [RFC-793] on server
port 53 (decimal) as well as datagram access using UDP [RFC-768] on UDP
port 53 (decimal).
4.2.1. UDP usage
Messages sent using UDP user server port 53 (decimal).
Messages carried by UDP are restricted to 512 bytes (not counting the IP
or UDP headers).  Longer messages are truncated and the TC bit is set in
the header.
UDP is not acceptable for zone transfers, but is the recommended method
for standard queries in the Internet.  Queries sent using UDP may be
lost, and hence a retransmission strategy is required.  Queries or their
responses may be reordered by the network, or by processing in name
servers, so resolvers should not depend on them being returned in order.
The optimal UDP retransmission policy will vary with performance of the
Internet and the needs of the client, but the following are recommended:
   - The client should try other servers and server addresses
     before repeating a query to a specific address of a server.
   - The retransmission interval should be based on prior
     statistics if possible.  Too aggressive retransmission can
     easily slow responses for the community at large.  Depending
     on how well connected the client is to its expected servers,
     the minimum retransmission interval should be 2-5 seconds.
More suggestions on server selection and retransmission policy can be
found in the resolver section of this memo.
4.2.2. TCP usage
Messages sent over TCP connections use server port 53 (decimal).  The
message is prefixed with a two byte length field which gives the message
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length, excluding the two byte length field.  This length field allows
the low-level processing to assemble a complete message before beginning
to parse it.
Several connection management policies are recommended:
   - The server should not block other activities waiting for TCP
     data.
   - The server should support multiple connections.
   - The server should assume that the client will initiate
     connection closing, and should delay closing its end of the
     connection until all outstanding client requests have been
     satisfied.
   - If the server needs to close a dormant connection to reclaim
     resources, it should wait until the connection has been idle
     for a period on the order of two minutes.  In particular, the
     server should allow the SOA and AXFR request sequence (which
     begins a refresh operation) to be made on a single connection.
     Since the server would be unable to answer queries anyway, a
     unilateral close or reset may be used instead of a graceful
     close.
5. MASTER FILES
Master files are text files that contain RRs in text form.  Since the
contents of a zone can be expressed in the form of a list of RRs a
master file is most often used to define a zone, though it can be used
to list a cache's contents.  Hence, this section first discusses the
format of RRs in a master file, and then the special considerations when
a master file is used to create a zone in some name server.
5.1. Format
The format of these files is a sequence of entries.  Entries are
predominantly line-oriented, though parentheses can be used to continue
a list of items across a line boundary, and text literals can contain
CRLF within the text.  Any combination of tabs and spaces act as a
delimiter between the separate items that make up an entry.  The end of
any line in the master file can end with a comment.  The comment starts
with a ";" (semicolon).
The following entries are defined:
    <blank>[<comment>]
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    $ORIGIN <domain-name> [<comment>]
    $INCLUDE <file-name> [<domain-name>] [<comment>]
    <domain-name><rr> [<comment>]
    <blank><rr> [<comment>]
Blank lines, with or without comments, are allowed anywhere in the file.
Two control entries are defined: $ORIGIN and $INCLUDE.  $ORIGIN is
followed by a domain name, and resets the current origin for relative
domain names to the stated name.  $INCLUDE inserts the named file into
the current file, and may optionally specify a domain name that sets the
relative domain name origin for the included file.  $INCLUDE may also
have a comment.  Note that a $INCLUDE entry never changes the relative
origin of the parent file, regardless of changes to the relative origin
made within the included file.
The last two forms represent RRs.  If an entry for an RR begins with a
blank, then the RR is assumed to be owned by the last stated owner.  If
an RR entry begins with a <domain-name>, then the owner name is reset.
<rr> contents take one of the following forms:
    [<TTL>] [<class>] <type> <RDATA>
    [<class>] [<TTL>] <type> <RDATA>
The RR begins with optional TTL and class fields, followed by a type and
RDATA field appropriate to the type and class.  Class and type use the
standard mnemonics, TTL is a decimal integer.  Omitted class and TTL
values are default to the last explicitly stated values.  Since type and
class mnemonics are disjoint, the parse is unique.  (Note that this
order is different from the order used in examples and the order used in
the actual RRs; the given order allows easier parsing and defaulting.)
<domain-name>s make up a large share of the data in the master file.
The labels in the domain name are expressed as character strings and
separated by dots.  Quoting conventions allow arbitrary characters to be
stored in domain names.  Domain names that end in a dot are called
absolute, and are taken as complete.  Domain names which do not end in a
dot are called relative; the actual domain name is the concatenation of
the relative part with an origin specified in a $ORIGIN, $INCLUDE, or as
an argument to the master file loading routine.  A relative name is an
error when no origin is available.
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<character-string> is expressed in one or two ways: as a contiguous set
of characters without interior spaces, or as a string beginning with a "
and ending with a ".  Inside a " delimited string any character can
occur, except for a " itself, which must be quoted using \ (back slash).
Because these files are text files several special encodings are
necessary to allow arbitrary data to be loaded.  In particular:
                of the root.
@               A free standing @ is used to denote the current origin.
\X              where X is any character other than a digit (0-9), is
                used to quote that character so that its special meaning
                does not apply.  For example, "\." can be used to place
                a dot character in a label.
\DDD            where each D is a digit is the octet corresponding to
                the decimal number described by DDD.  The resulting
                octet is assumed to be text and is not checked for
                special meaning.
( )             Parentheses are used to group data that crosses a line
                boundary.  In effect, line terminations are not
                recognized within parentheses.
;               Semicolon is used to start a comment; the remainder of
                the line is ignored.
5.2. Use of master files to define zones
When a master file is used to load a zone, the operation should be
suppressed if any errors are encountered in the master file.  The
rationale for this is that a single error can have widespread
consequences.  For example, suppose that the RRs defining a delegation
have syntax errors; then the server will return authoritative name
errors for all names in the subzone (except in the case where the
subzone is also present on the server).
Several other validity checks that should be performed in addition to
insuring that the file is syntactically correct:
   1. All RRs in the file should have the same class.
   2. Exactly one SOA RR should be present at the top of the zone.
   3. If delegations are present and glue information is required,
      it should be present.
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   4. Information present outside of the authoritative nodes in the
      zone should be glue information, rather than the result of an
      origin or similar error.
5.3. Master file example
The following is an example file which might be used to define the
ISI.EDU zone.and is loaded with an origin of ISI.EDU:
@   IN  SOA     VENERA      Action\.domains (
                                 20     ; SERIAL
                                 7200   ; REFRESH
                                 600    ; RETRY
                                 3600000; EXPIRE
                                 60)    ; MINIMUM
        NS      A.ISI.EDU.
        NS      VENERA
        NS      VAXA
        MX      10      VENERA
        MX      20      VAXA
A       A       26.3.0.103
VENERA  A       10.1.0.52
        A       128.9.0.32
VAXA    A       10.2.0.27
        A       128.9.0.33
$INCLUDE <SUBSYS>ISI-MAILBOXES.TXT
Where the file <SUBSYS>ISI-MAILBOXES.TXT is:
    MOE     MB      A.ISI.EDU.
    LARRY   MB      A.ISI.EDU.
    CURLEY  MB      A.ISI.EDU.
    STOOGES MG      MOE
            MG      LARRY
            MG      CURLEY
Note the use of the \ character in the SOA RR to specify the responsible
person mailbox "Action.domains@E.ISI.EDU".
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6. NAME SERVER IMPLEMENTATION
6.1. Architecture
The optimal structure for the name server will depend on the host
operating system and whether the name server is integrated with resolver
operations, either by supporting recursive service, or by sharing its
database with a resolver.  This section discusses implementation
considerations for a name server which shares a database with a
resolver, but most of these concerns are present in any name server.
6.1.1. Control
A name server must employ multiple concurrent activities, whether they
are implemented as separate tasks in the host's OS or multiplexing
inside a single name server program.  It is simply not acceptable for a
name server to block the service of UDP requests while it waits for TCP
data for refreshing or query activities.  Similarly, a name server
should not attempt to provide recursive service without processing such
requests in parallel, though it may choose to serialize requests from a
single client, or to regard identical requests from the same client as
duplicates.  A name server should not substantially delay requests while
it reloads a zone from master files or while it incorporates a newly
refreshed zone into its database.
6.1.2. Database
While name server implementations are free to use any internal data
structures they choose, the suggested structure consists of three major
parts:
   - A "catalog" data structure which lists the zones available to
     this server, and a "pointer" to the zone data structure.  The
     main purpose of this structure is to find the nearest ancestor
     zone, if any, for arriving standard queries.
   - Separate data structures for each of the zones held by the
     name server.
   - A data structure for cached data. (or perhaps separate caches
     for different classes)
All of these data structures can be implemented an identical tree
structure format, with different data chained off the nodes in different
parts: in the catalog the data is pointers to zones, while in the zone
and cache data structures, the data will be RRs.  In designing the tree
framework the designer should recognize that query processing will need
to traverse the tree using case-insensitive label comparisons; and that
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in real data, a few nodes have a very high branching factor (100-1000 or
more), but the vast majority have a very low branching factor (0-1).
One way to solve the case problem is to store the labels for each node
in two pieces: a standardized-case representation of the label where all
ASCII characters are in a single case, together with a bit mask that
denotes which characters are actually of a different case.  The
branching factor diversity can be handled using a simple linked list for
a node until the branching factor exceeds some threshold, and
transitioning to a hash structure after the threshold is exceeded.  In
any case, hash structures used to store tree sections must insure that
hash functions and procedures preserve the casing conventions of the
DNS.
The use of separate structures for the different parts of the database
is motivated by several factors:
   - The catalog structure can be an almost static structure that
     need change only when the system administrator changes the
     zones supported by the server.  This structure can also be
     used to store parameters used to control refreshing
     activities.
   - The individual data structures for zones allow a zone to be
     replaced simply by changing a pointer in the catalog.  Zone
     refresh operations can build a new structure and, when
     complete, splice it into the database via a simple pointer
     replacement.  It is very important that when a zone is
     refreshed, queries should not use old and new data
     simultaneously.
   - With the proper search procedures, authoritative data in zones
     will always "hide", and hence take precedence over, cached
     data.
   - Errors in zone definitions that cause overlapping zones, etc.,
     may cause erroneous responses to queries, but problem
     determination is simplified, and the contents of one "bad"
     zone can't corrupt another.
   - Since the cache is most frequently updated, it is most
     vulnerable to corruption during system restarts.  It can also
     become full of expired RR data.  In either case, it can easily
     be discarded without disturbing zone data.
A major aspect of database design is selecting a structure which allows
the name server to deal with crashes of the name server's host.  State
information which a name server should save across system crashes
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includes the catalog structure (including the state of refreshing for
each zone) and the zone data itself.
6.1.3. Time
Both the TTL data for RRs and the timing data for refreshing activities
depends on 32 bit timers in units of seconds.  Inside the database,
refresh timers and TTLs for cached data conceptually "count down", while
data in the zone stays with constant TTLs.
A recommended implementation strategy is to store time in two ways:  as
a relative increment and as an absolute time.  One way to do this is to
use positive 32 bit numbers for one type and negative numbers for the
other.  The RRs in zones use relative times; the refresh timers and
cache data use absolute times.  Absolute numbers are taken with respect
to some known origin and converted to relative values when placed in the
response to a query.  When an absolute TTL is negative after conversion
to relative, then the data is expired and should be ignored.
6.2. Standard query processing
The major algorithm for standard query processing is presented in
[RFC-1034].
When processing queries with QCLASS=*, or some other QCLASS which
matches multiple classes, the response should never be authoritative
unless the server can guarantee that the response covers all classes.
When composing a response, RRs which are to be inserted in the
additional section, but duplicate RRs in the answer or authority
sections, may be omitted from the additional section.
When a response is so long that truncation is required, the truncation
should start at the end of the response and work forward in the
datagram.  Thus if there is any data for the authority section, the
answer section is guaranteed to be unique.
The MINIMUM value in the SOA should be used to set a floor on the TTL of
data distributed from a zone.  This floor function should be done when
the data is copied into a response.  This will allow future dynamic
update protocols to change the SOA MINIMUM field without ambiguous
semantics.
6.3. Zone refresh and reload processing
In spite of a server's best efforts, it may be unable to load zone data
from a master file due to syntax errors, etc., or be unable to refresh a
zone within the its expiration parameter.  In this case, the name server
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should answer queries as if it were not supposed to possess the zone.
If a master is sending a zone out via AXFR, and a new version is created
during the transfer, the master should continue to send the old version
if possible.  In any case, it should never send part of one version and
part of another.  If completion is not possible, the master should reset
the connection on which the zone transfer is taking place.
6.4. Inverse queries (Optional)
Inverse queries are an optional part of the DNS.  Name servers are not
required to support any form of inverse queries.  If a name server
receives an inverse query that it does not support, it returns an error
response with the "Not Implemented" error set in the header.  While
inverse query support is optional, all name servers must be at least
able to return the error response.
6.4.1. The contents of inverse queries and responses          Inverse
queries reverse the mappings performed by standard query operations;
while a standard query maps a domain name to a resource, an inverse
query maps a resource to a domain name.  For example, a standard query
might bind a domain name to a host address; the corresponding inverse
query binds the host address to a domain name.
Inverse queries take the form of a single RR in the answer section of
the message, with an empty question section.  The owner name of the
query RR and its TTL are not significant.  The response carries
questions in the question section which identify all names possessing
the query RR WHICH THE NAME SERVER KNOWS.  Since no name server knows
about all of the domain name space, the response can never be assumed to
be complete.  Thus inverse queries are primarily useful for database
management and debugging activities.  Inverse queries are NOT an
acceptable method of mapping host addresses to host names; use the IN-
ADDR.ARPA domain instead.
Where possible, name servers should provide case-insensitive comparisons
for inverse queries.  Thus an inverse query asking for an MX RR of
"Venera.isi.edu" should get the same response as a query for
"VENERA.ISI.EDU"; an inverse query for HINFO RR "IBM-PC UNIX" should
produce the same result as an inverse query for "IBM-pc unix".  However,
this cannot be guaranteed because name servers may possess RRs that
contain character strings but the name server does not know that the
data is character.
When a name server processes an inverse query, it either returns:
   1. zero, one, or multiple domain names for the specified
      resource as QNAMEs in the question section
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   2. an error code indicating that the name server doesn't support
      inverse mapping of the specified resource type.
When the response to an inverse query contains one or more QNAMEs, the
owner name and TTL of the RR in the answer section which defines the
inverse query is modified to exactly match an RR found at the first
QNAME.
RRs returned in the inverse queries cannot be cached using the same
mechanism as is used for the replies to standard queries.  One reason
for this is that a name might have multiple RRs of the same type, and
only one would appear.  For example, an inverse query for a single
address of a multiply homed host might create the impression that only
one address existed.
6.4.2. Inverse query and response example          The overall structure
of an inverse query for retrieving the domain name that corresponds to
Internet address 10.1.0.52 is shown below:
                         +-----------------------------------------+
           Header        |          OPCODE=IQUERY, ID=997          |
                         +-----------------------------------------+
          Question       |                 <empty>                 |
                         +-----------------------------------------+
           Answer        |        <anyname> A IN 10.1.0.52         |
                         +-----------------------------------------+
          Authority      |                 <empty>                 |
                         +-----------------------------------------+
         Additional      |                 <empty>                 |
                         +-----------------------------------------+
This query asks for a question whose answer is the Internet style
address 10.1.0.52.  Since the owner name is not known, any domain name
can be used as a placeholder (and is ignored).  A single octet of zero,
signifying the root, is usually used because it minimizes the length of
the message.  The TTL of the RR is not significant.  The response to
this query might be:
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                         +-----------------------------------------+
           Header        |         OPCODE=RESPONSE, ID=997         |
                         +-----------------------------------------+
          Question       |QTYPE=A, QCLASS=IN, QNAME=VENERA.ISI.EDU |
                         +-----------------------------------------+
           Answer        |  VENERA.ISI.EDU  A IN 10.1.0.52         |
                         +-----------------------------------------+
          Authority      |                 <empty>                 |
                         +-----------------------------------------+
         Additional      |                 <empty>                 |
                         +-----------------------------------------+
Note that the QTYPE in a response to an inverse query is the same as the
TYPE field in the answer section of the inverse query.  Responses to
inverse queries may contain multiple questions when the inverse is not
unique.  If the question section in the response is not empty, then the
RR in the answer section is modified to correspond to be an exact copy
of an RR at the first QNAME.
6.4.3. Inverse query processing
Name servers that support inverse queries can support these operations
through exhaustive searches of their databases, but this becomes
impractical as the size of the database increases.  An alternative
approach is to invert the database according to the search key.
For name servers that support multiple zones and a large amount of data,
the recommended approach is separate inversions for each zone.  When a
particular zone is changed during a refresh, only its inversions need to
be redone.
Support for transfer of this type of inversion may be included in future
versions of the domain system, but is not supported in this version.
6.5. Completion queries and responses
The optional completion services described in RFC-882 and RFC-883 have
been deleted.  Redesigned services may become available in the future.
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7. RESOLVER IMPLEMENTATION
The top levels of the recommended resolver algorithm are discussed in
[RFC-1034].  This section discusses implementation details assuming the
database structure suggested in the name server implementation section
of this memo.
7.1. Transforming a user request into a query
The first step a resolver takes is to transform the client's request,
stated in a format suitable to the local OS, into a search specification
for RRs at a specific name which match a specific QTYPE and QCLASS.
Where possible, the QTYPE and QCLASS should correspond to a single type
and a single class, because this makes the use of cached data much
simpler.  The reason for this is that the presence of data of one type
in a cache doesn't confirm the existence or non-existence of data of
other types, hence the only way to be sure is to consult an
authoritative source.  If QCLASS=* is used, then authoritative answers
won't be available.
Since a resolver must be able to multiplex multiple requests if it is to
perform its function efficiently, each pending request is usually
represented in some block of state information.  This state block will
typically contain:
   - A timestamp indicating the time the request began.
     The timestamp is used to decide whether RRs in the database
     can be used or are out of date.  This timestamp uses the
     absolute time format previously discussed for RR storage in
     zones and caches.  Note that when an RRs TTL indicates a
     relative time, the RR must be timely, since it is part of a
     zone.  When the RR has an absolute time, it is part of a
     cache, and the TTL of the RR is compared against the timestamp
     for the start of the request.
     Note that using the timestamp is superior to using a current
     time, since it allows RRs with TTLs of zero to be entered in
     the cache in the usual manner, but still used by the current
     request, even after intervals of many seconds due to system
     load, query retransmission timeouts, etc.
   - Some sort of parameters to limit the amount of work which will
     be performed for this request.
     The amount of work which a resolver will do in response to a
     client request must be limited to guard against errors in the
     database, such as circular CNAME references, and operational
     problems, such as network partition which prevents the
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     resolver from accessing the name servers it needs.  While
     local limits on the number of times a resolver will retransmit
     a particular query to a particular name server address are
     essential, the resolver should have a global per-request
     counter to limit work on a single request.  The counter should
     be set to some initial value and decremented whenever the
     resolver performs any action (retransmission timeout,
     retransmission, etc.)  If the counter passes zero, the request
     is terminated with a temporary error.
     Note that if the resolver structure allows one request to
     start others in parallel, such as when the need to access a
     name server for one request causes a parallel resolve for the
     name server's addresses, the spawned request should be started
     with a lower counter.  This prevents circular references in
     the database from starting a chain reaction of resolver
     activity.
   - The SLIST data structure discussed in [RFC-1034].
     This structure keeps track of the state of a request if it
     must wait for answers from foreign name servers.
7.2. Sending the queries
As described in [RFC-1034], the basic task of the resolver is to
formulate a query which will answer the client's request and direct that
query to name servers which can provide the information.  The resolver
will usually only have very strong hints about which servers to ask, in
the form of NS RRs, and may have to revise the query, in response to
CNAMEs, or revise the set of name servers the resolver is asking, in
response to delegation responses which point the resolver to name
servers closer to the desired information.  In addition to the
information requested by the client, the resolver may have to call upon
its own services to determine the address of name servers it wishes to
contact.
In any case, the model used in this memo assumes that the resolver is
multiplexing attention between multiple requests, some from the client,
and some internally generated.  Each request is represented by some
state information, and the desired behavior is that the resolver
transmit queries to name servers in a way that maximizes the probability
that the request is answered, minimizes the time that the request takes,
and avoids excessive transmissions.  The key algorithm uses the state
information of the request to select the next name server address to
query, and also computes a timeout which will cause the next action
should a response not arrive.  The next action will usually be a
transmission to some other server, but may be a temporary error to the
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client.
The resolver always starts with a list of server names to query (SLIST).
This list will be all NS RRs which correspond to the nearest ancestor
zone that the resolver knows about.  To avoid startup problems, the
resolver should have a set of default servers which it will ask should
it have no current NS RRs which are appropriate.  The resolver then adds
to SLIST all of the known addresses for the name servers, and may start
parallel requests to acquire the addresses of the servers when the
resolver has the name, but no addresses, for the name servers.
To complete initialization of SLIST, the resolver attaches whatever
history information it has to the each address in SLIST.  This will
usually consist of some sort of weighted averages for the response time
of the address, and the batting average of the address (i.e., how often
the address responded at all to the request).  Note that this
information should be kept on a per address basis, rather than on a per
name server basis, because the response time and batting average of a
particular server may vary considerably from address to address.  Note
also that this information is actually specific to a resolver address /
server address pair, so a resolver with multiple addresses may wish to
keep separate histories for each of its addresses.  Part of this step
must deal with addresses which have no such history; in this case an
expected round trip time of 5-10 seconds should be the worst case, with
lower estimates for the same local network, etc.
Note that whenever a delegation is followed, the resolver algorithm
reinitializes SLIST.
The information establishes a partial ranking of the available name
server addresses.  Each time an address is chosen and the state should
be altered to prevent its selection again until all other addresses have
been tried.  The timeout for each transmission should be 50-100% greater
than the average predicted value to allow for variance in response.
Some fine points:
   - The resolver may encounter a situation where no addresses are
     available for any of the name servers named in SLIST, and
     where the servers in the list are precisely those which would
     normally be used to look up their own addresses.  This
     situation typically occurs when the glue address RRs have a
     smaller TTL than the NS RRs marking delegation, or when the
     resolver caches the result of a NS search.  The resolver
     should detect this condition and restart the search at the
     next ancestor zone, or alternatively at the root.
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   - If a resolver gets a server error or other bizarre response
     from a name server, it should remove it from SLIST, and may
     wish to schedule an immediate transmission to the next
     candidate server address.
7.3. Processing responses
The first step in processing arriving response datagrams is to parse the
response.  This procedure should include:
   - Check the header for reasonableness.  Discard datagrams which
     are queries when responses are expected.
   - Parse the sections of the message, and insure that all RRs are
     correctly formatted.
   - As an optional step, check the TTLs of arriving data looking
     for RRs with excessively long TTLs.  If a RR has an
     excessively long TTL, say greater than 1 week, either discard
     the whole response, or limit all TTLs in the response to 1
     week.
The next step is to match the response to a current resolver request.
The recommended strategy is to do a preliminary matching using the ID
field in the domain header, and then to verify that the question section
corresponds to the information currently desired.  This requires that
the transmission algorithm devote several bits of the domain ID field to
a request identifier of some sort.  This step has several fine points:
   - Some name servers send their responses from different
     addresses than the one used to receive the query.  That is, a
     resolver cannot rely that a response will come from the same
     address which it sent the corresponding query to.  This name
     server bug is typically encountered in UNIX systems.
   - If the resolver retransmits a particular request to a name
     server it should be able to use a response from any of the
     transmissions.  However, if it is using the response to sample
     the round trip time to access the name server, it must be able
     to determine which transmission matches the response (and keep
     transmission times for each outgoing message), or only
     calculate round trip times based on initial transmissions.
   - A name server will occasionally not have a current copy of a
     zone which it should have according to some NS RRs.  The
     resolver should simply remove the name server from the current
     SLIST, and continue.
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7.4. Using the cache
In general, we expect a resolver to cache all data which it receives in
responses since it may be useful in answering future client requests.
However, there are several types of data which should not be cached:
   - When several RRs of the same type are available for a
     particular owner name, the resolver should either cache them
     all or none at all.  When a response is truncated, and a
     resolver doesn't know whether it has a complete set, it should
     not cache a possibly partial set of RRs.
   - Cached data should never be used in preference to
     authoritative data, so if caching would cause this to happen
     the data should not be cached.
   - The results of an inverse query should not be cached.
   - The results of standard queries where the QNAME contains "*"
     labels if the data might be used to construct wildcards.  The
     reason is that the cache does not necessarily contain existing
     RRs or zone boundary information which is necessary to
     restrict the application of the wildcard RRs.
   - RR data in responses of dubious reliability.  When a resolver
     receives unsolicited responses or RR data other than that
     requested, it should discard it without caching it.  The basic
     implication is that all sanity checks on a packet should be
     performed before any of it is cached.
In a similar vein, when a resolver has a set of RRs for some name in a
response, and wants to cache the RRs, it should check its cache for
already existing RRs.  Depending on the circumstances, either the data
in the response or the cache is preferred, but the two should never be
combined.  If the data in the response is from authoritative data in the
answer section, it is always preferred.
8. MAIL SUPPORT
The domain system defines a standard for mapping mailboxes into domain
names, and two methods for using the mailbox information to derive mail
routing information.  The first method is called mail exchange binding
and the other method is mailbox binding.  The mailbox encoding standard
and mail exchange binding are part of the DNS official protocol, and are
the recommended method for mail routing in the Internet.  Mailbox
binding is an experimental feature which is still under development and
subject to change.
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The mailbox encoding standard assumes a mailbox name of the form
"<local-part>@<mail-domain>".  While the syntax allowed in each of these
sections varies substantially between the various mail internets, the
preferred syntax for the ARPA Internet is given in [RFC-822].
The DNS encodes the <local-part> as a single label, and encodes the
<mail-domain> as a domain name.  The single label from the <local-part>
is prefaced to the domain name from <mail-domain> to form the domain
name corresponding to the mailbox.  Thus the mailbox HOSTMASTER@SRI-
NIC.ARPA is mapped into the domain name HOSTMASTER.SRI-NIC.ARPA.  If the
<local-part> contains dots or other special characters, its
representation in a master file will require the use of backslash
quoting to ensure that the domain name is properly encoded.  For
example, the mailbox Action.domains@ISI.EDU would be represented as
Action\.domains.ISI.EDU.
8.1. Mail exchange binding
Mail exchange binding uses the <mail-domain> part of a mailbox
specification to determine where mail should be sent.  The <local-part>
is not even consulted.  [RFC-974] specifies this method in detail, and
should be consulted before attempting to use mail exchange support.
One of the advantages of this method is that it decouples mail
destination naming from the hosts used to support mail service, at the
cost of another layer of indirection in the lookup function.  However,
the addition layer should eliminate the need for complicated "%", "!",
etc encodings in <local-part>.
The essence of the method is that the <mail-domain> is used as a domain
name to locate type MX RRs which list hosts willing to accept mail for
<mail-domain>, together with preference values which rank the hosts
according to an order specified by the administrators for <mail-domain>.
In this memo, the <mail-domain> ISI.EDU is used in examples, together
with the hosts VENERA.ISI.EDU and VAXA.ISI.EDU as mail exchanges for
ISI.EDU.  If a mailer had a message for Mockapetris@ISI.EDU, it would
route it by looking up MX RRs for ISI.EDU.  The MX RRs at ISI.EDU name
VENERA.ISI.EDU and VAXA.ISI.EDU, and type A queries can find the host
addresses.
8.2. Mailbox binding (Experimental)
In mailbox binding, the mailer uses the entire mail destination
specification to construct a domain name.  The encoded domain name for
the mailbox is used as the QNAME field in a QTYPE=MAILB query.
Several outcomes are possible for this query:
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   1. The query can return a name error indicating that the mailbox
      does not exist as a domain name.
      In the long term, this would indicate that the specified
      mailbox doesn't exist.  However, until the use of mailbox
      binding is universal, this error condition should be
      interpreted to mean that the organization identified by the
      global part does not support mailbox binding.  The
      appropriate procedure is to revert to exchange binding at
      this point.
   2. The query can return a Mail Rename (MR) RR.
      The MR RR carries new mailbox specification in its RDATA
      field.  The mailer should replace the old mailbox with the
      new one and retry the operation.
   3. The query can return a MB RR.
      The MB RR carries a domain name for a host in its RDATA
      field.  The mailer should deliver the message to that host
      via whatever protocol is applicable, e.g., b,SMTP.
   4. The query can return one or more Mail Group (MG) RRs.
      This condition means that the mailbox was actually a mailing
      list or mail group, rather than a single mailbox.  Each MG RR
      has a RDATA field that identifies a mailbox that is a member
      of the group.  The mailer should deliver a copy of the
      message to each member.
   5. The query can return a MB RR as well as one or more MG RRs.
      This condition means the the mailbox was actually a mailing
      list.  The mailer can either deliver the message to the host
      specified by the MB RR, which will in turn do the delivery to
      all members, or the mailer can use the MG RRs to do the
      expansion itself.
In any of these cases, the response may include a Mail Information
(MINFO) RR.  This RR is usually associated with a mail group, but is
legal with a MB.  The MINFO RR identifies two mailboxes.  One of these
identifies a responsible person for the original mailbox name.  This
mailbox should be used for requests to be added to a mail group, etc.
The second mailbox name in the MINFO RR identifies a mailbox that should
receive error messages for mail failures.  This is particularly
appropriate for mailing lists when errors in member names should be
reported to a person other than the one who sends a message to the list.
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New fields may be added to this RR in the future.
9. REFERENCES and BIBLIOGRAPHY
[Dyer 87]       S. Dyer, F. Hsu, "Hesiod", Project Athena
                Technical Plan - Name Service, April 1987, version 1.9.
                Describes the fundamentals of the Hesiod name service.
[IEN-116]       J. Postel, "Internet Name Server", IEN-116,
                USC/Information Sciences Institute, August 1979.
                A name service obsoleted by the Domain Name System, but
                still in use.
[Quarterman 86] J. Quarterman, and J. Hoskins, "Notable Computer Networks",
                Communications of the ACM, October 1986, volume 29, number
                10.
[RFC-742]       K. Harrenstien, "NAME/FINGER", RFC-742, Network
                Information Center, SRI International, December 1977.
[RFC-768]       J. Postel, "User Datagram Protocol", RFC-768,
                USC/Information Sciences Institute, August 1980.
[RFC-793]       J. Postel, "Transmission Control Protocol", RFC-793,
                USC/Information Sciences Institute, September 1981.
[RFC-799]       D. Mills, "Internet Name Domains", RFC-799, COMSAT,
                September 1981.
                Suggests introduction of a hierarchy in place of a flat
                name space for the Internet.
[RFC-805]       J. Postel, "Computer Mail Meeting Notes", RFC-805,
                USC/Information Sciences Institute, February 1982.
[RFC-810]       E. Feinler, K. Harrenstien, Z. Su, and V. White, "DOD
                Internet Host Table Specification", RFC-810, Network
                Information Center, SRI International, March 1982.
                Obsolete.  See RFC-952.
[RFC-811]       K. Harrenstien, V. White, and E. Feinler, "Hostnames
                Server", RFC-811, Network Information Center, SRI
                International, March 1982.
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RFC 1035        Domain Implementation and Specification    November 1987
                Obsolete.  See RFC-953.
[RFC-812]       K. Harrenstien, and V. White, "NICNAME/WHOIS", RFC-812,
                Network Information Center, SRI International, March
                1982.
[RFC-819]       Z. Su, and J. Postel, "The Domain Naming Convention for
                Internet User Applications", RFC-819, Network
                Information Center, SRI International, August 1982.
                Early thoughts on the design of the domain system.
                Current implementation is completely different.
[RFC-821]       J. Postel, "Simple Mail Transfer Protocol", RFC-821,
                USC/Information Sciences Institute, August 1980.
[RFC-830]       Z. Su, "A Distributed System for Internet Name Service",
                RFC-830, Network Information Center, SRI International,
                October 1982.
                Early thoughts on the design of the domain system.
                Current implementation is completely different.
[RFC-882]       P. Mockapetris, "Domain names - Concepts and
                Facilities," RFC-882, USC/Information Sciences
                Institute, November 1983.
                Superceeded by this memo.
[RFC-883]       P. Mockapetris, "Domain names - Implementation and
                Specification," RFC-883, USC/Information Sciences
                Institute, November 1983.
                Superceeded by this memo.
[RFC-920]       J. Postel and J. Reynolds, "Domain Requirements",
                RFC-920, USC/Information Sciences Institute,
                October 1984.
                Explains the naming scheme for top level domains.
[RFC-952]       K. Harrenstien, M. Stahl, E. Feinler, "DoD Internet Host
                Table Specification", RFC-952, SRI, October 1985.
                Specifies the format of HOSTS.TXT, the host/address
                table replaced by the DNS.
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[RFC-953]       K. Harrenstien, M. Stahl, E. Feinler, "HOSTNAME Server",
                RFC-953, SRI, October 1985.
                This RFC contains the official specification of the
                hostname server protocol, which is obsoleted by the DNS.
                This TCP based protocol accesses information stored in
                the RFC-952 format, and is used to obtain copies of the
                host table.
[RFC-973]       P. Mockapetris, "Domain System Changes and
                Observations", RFC-973, USC/Information Sciences
                Institute, January 1986.
                Describes changes to RFC-882 and RFC-883 and reasons for
                them.
[RFC-974]       C. Partridge, "Mail routing and the domain system",
                RFC-974, CSNET CIC BBN Labs, January 1986.
                Describes the transition from HOSTS.TXT based mail
                addressing to the more powerful MX system used with the
                domain system.
[RFC-1001]      NetBIOS Working Group, "Protocol standard for a NetBIOS
                service on a TCP/UDP transport: Concepts and Methods",
                RFC-1001, March 1987.
                This RFC and RFC-1002 are a preliminary design for
                NETBIOS on top of TCP/IP which proposes to base NetBIOS
                name service on top of the DNS.
[RFC-1002]      NetBIOS Working Group, "Protocol standard for a NetBIOS
                service on a TCP/UDP transport: Detailed
                Specifications", RFC-1002, March 1987.
[RFC-1010]      J. Reynolds, and J. Postel, "Assigned Numbers", RFC-1010,
                USC/Information Sciences Institute, May 1987.
                Contains socket numbers and mnemonics for host names,
                operating systems, etc.
[RFC-1031]      W. Lazear, "MILNET Name Domain Transition", RFC-1031,
                November 1987.
                Describes a plan for converting the MILNET to the DNS.
[RFC-1032]      M. Stahl, "Establishing a Domain - Guidelines for
                Administrators", RFC-1032, November 1987.
Mockapetris                                                    [Page 52]
RFC 1035        Domain Implementation and Specification    November 1987
                Describes the registration policies used by the NIC to
                administer the top level domains and delegate subzones.
[RFC-1033]      M. Lottor, "Domain Administrators Operations Guide",
                RFC-1033, November 1987.
                A cookbook for domain administrators.
[Solomon 82]    M. Solomon, L. Landweber, and D. Neuhengen, "The CSNET
                Name Server", Computer Networks, vol 6, nr 3, July 1982.
                Describes a name service for CSNET which is independent
                from the DNS and DNS use in the CSNET.
Mockapetris                                                    [Page 53]
RFC 1035        Domain Implementation and Specification    November 1987
Index
          *   13
          ;   33, 35
          <character-string>   35
          <domain-name>   34
          @   35
          \   35
          A   12
          Byte order   8
          CH   13
          Character case   9
          CLASS   11
          CNAME   12
          Completion   42
          CS   13
          Hesiod   13
          HINFO   12
          HS   13
          IN   13
          IN-ADDR.ARPA domain   22
          Inverse queries   40
          Mailbox names   47
          MB   12
          MD   12
          MF   12
          MG   12
          MINFO   12
          MINIMUM   20
          MR   12
          MX   12
          NS   12
          NULL   12
          Port numbers   32
          Primary server   5
          PTR   12, 18
Mockapetris                                                    [Page 54]
RFC 1035        Domain Implementation and Specification    November 1987
          QCLASS   13
          QTYPE   12
          RDATA   12
          RDLENGTH  11
          Secondary server   5
          SOA   12
          Stub resolvers   7
          TCP   32
          TXT   12
          TYPE   11
          UDP   32
          WKS   12
Mockapetris                                                    [Page 55]
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