Network Working Group
Request for Comments: 1035
Obsoletes: RFCs 882, 883, 973
P. Mockapetris
November 1987



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
          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
      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


2.1. 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 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

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

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.

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.

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:


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

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.

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


3.1. 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.

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                     /
    /                                               /


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.

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)


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:

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).

3.3.1. CNAME RDATA format

    /                     CNAME                     /
    /                                               /


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                      /


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.


    /                   MADNAME                     /
    /                                               /


MADNAME A <domain-name> which specifies a host which has the

specified mailbox.

MB records cause additional section processing which looks up an A type

RRs corresponding to MADNAME.

3.3.4. MD RDATA format (Obsolete)

    /                   MADNAME                     /
    /                                               /


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                     /
    /                                               /


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.


    /                   MGMNAME                     /
    /                                               /


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.


    /                    RMAILBX                    /
    /                    EMAILBX                    /


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.


    /                   NEWNAME                     /
    /                                               /


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                    /
    /                                               /


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].


    /                  <anything>                   /
    /                                               /

Anything at all may be in the RDATA field so long as it is 65535 octets

or less.

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                     /
    /                                               /


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                    /


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.

3.3.13. SOA RDATA format

    /                     MNAME                     /
    /                                               /
    /                     RNAME                     /
    |                    SERIAL                     |
    |                                               |
    |                    REFRESH                    |
    |                                               |
    |                     RETRY                     |
    |                                               |
    |                    EXPIRE                     |
    |                                               |
    |                    MINIMUM                    |
    |                                               |


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

REFRESH A 32 bit time interval before the zone should be


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.

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                    /


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                    |


ADDRESS A 32 bit Internet address.

Hosts that have multiple Internet addresses will have multiple A


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., "" or "").

3.4.2. WKS RDATA format

    |                    ADDRESS                    |
    |       PROTOCOL        |                       |
    +--+--+--+--+--+--+--+--+                       |
    |                                               |
    /                   <BIT MAP>                   /
    /                                               /


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.

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 is located at domain name 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

net 18, and hosts A.ISI.EDU and MULTICS.MIT.EDU. Assuming that ISI gateway has addresses and, and a name MILNET- GW.ISI.EDU, and the MIT gateway has addresses and and a name GW.LCS.MIT.EDU, the domain database would contain:

    10.IN-ADDR.ARPA.           PTR GW.LCS.MIT.EDU.
    18.IN-ADDR.ARPA.           PTR GW.LCS.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 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 would pursue a query of the form QTYPE=PTR, QCLASS=IN, QNAME=, and would receive:     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.

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


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 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.


4.1. 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.

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                    |


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 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

                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

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

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                    |


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.

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                     /
    /                                               /


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

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.

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

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

defined by a single octet of zeros at 92; the root domain name has no


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

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.


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:

    $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.

<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.
  1. Exactly one SOA RR should be present at the top of the zone.
  1. If delegations are present and glue information is required, it should be present.
  1. 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







Where the file <SUBSYS>ISI-MAILBOXES.TXT is:

    MOE     MB      A.ISI.EDU.
    LARRY   MB      A.ISI.EDU.
    CURLEY  MB      A.ISI.EDU.
            MG      LARRY
            MG      CURLEY

Note the use of the \ character in the SOA RR to specify the responsible

person mailbox "".


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

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

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


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

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 "" 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
  1. 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 is shown below:

           Header        |          OPCODE=IQUERY, ID=997          |
          Question       |                 <empty>                 |
           Answer        |        <anyname> A IN         |
          Authority      |                 <empty>                 |
         Additional      |                 <empty>                 |

This query asks for a question whose answer is the Internet style address 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:

           Header        |         OPCODE=RESPONSE, ID=997         |
          Question       |QTYPE=A, QCLASS=IN, QNAME=VENERA.ISI.EDU |
           Answer        |  VENERA.ISI.EDU  A IN         |
          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.


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 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

  • 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


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.
  • 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.

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.


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.

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 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:

  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.

  1. 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.

  1. 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.

  1. 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.

  1. 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.

New fields may be added to this RR in the future.


[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

[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.

Obsolete. See RFC-953.

[RFC-812]       K. Harrenstien, and V. White, "NICNAME/WHOIS", RFC-812,
                Network Information Center, SRI International, March

[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.

[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.

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.


          *   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
          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