From a9dd10ae2a355ef40f8a54b773596cbefc0b248e Mon Sep 17 00:00:00 2001
From: brain <brain@e03df62e-2008-0410-955e-edbf42e46eb7>
Date: Fri, 27 Oct 2006 09:26:51 +0000
Subject: RFC1035 - dns rfc

git-svn-id: http://svn.inspircd.org/repository/trunk/inspircd@5550 e03df62e-2008-0410-955e-edbf42e46eb7
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+Network Working Group                                     P. Mockapetris
+Request for Comments: 1035                                           ISI
+                                                           November 1987
+Obsoletes: RFCs 882, 883, 973
+
+            DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION
+
+
+1. STATUS OF THIS MEMO
+
+This RFC describes the details of the domain system and protocol, and
+assumes that the reader is familiar with the concepts discussed in a
+companion RFC, "Domain Names - Concepts and Facilities" [RFC-1034].
+
+The domain system is a mixture of functions and data types which are an
+official protocol and functions and data types which are still
+experimental.  Since the domain system is intentionally extensible, new
+data types and experimental behavior should always be expected in parts
+of the system beyond the official protocol.  The official protocol parts
+include standard queries, responses and the Internet class RR data
+formats (e.g., host addresses).  Since the previous RFC set, several
+definitions have changed, so some previous definitions are obsolete.
+
+Experimental or obsolete features are clearly marked in these RFCs, and
+such information should be used with caution.
+
+The reader is especially cautioned not to depend on the values which
+appear in examples to be current or complete, since their purpose is
+primarily pedagogical.  Distribution of this memo is unlimited.
+
+                           Table of Contents
+
+  1. STATUS OF THIS MEMO                                              1
+  2. INTRODUCTION                                                     3
+      2.1. Overview                                                   3
+      2.2. Common configurations                                      4
+      2.3. Conventions                                                7
+          2.3.1. Preferred name syntax                                7
+          2.3.2. Data Transmission Order                              8
+          2.3.3. Character Case                                       9
+          2.3.4. Size limits                                         10
+  3. DOMAIN NAME SPACE AND RR DEFINITIONS                            10
+      3.1. Name space definitions                                    10
+      3.2. RR definitions                                            11
+          3.2.1. Format                                              11
+          3.2.2. TYPE values                                         12
+          3.2.3. QTYPE values                                        12
+          3.2.4. CLASS values                                        13
+
+
+
+Mockapetris                                                     [Page 1]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+          3.2.5. QCLASS values                                       13
+      3.3. Standard RRs                                              13
+          3.3.1. CNAME RDATA format                                  14
+          3.3.2. HINFO RDATA format                                  14
+          3.3.3. MB RDATA format (EXPERIMENTAL)                      14
+          3.3.4. MD RDATA format (Obsolete)                          15
+          3.3.5. MF RDATA format (Obsolete)                          15
+          3.3.6. MG RDATA format (EXPERIMENTAL)                      16
+          3.3.7. MINFO RDATA format (EXPERIMENTAL)                   16
+          3.3.8. MR RDATA format (EXPERIMENTAL)                      17
+          3.3.9. MX RDATA format                                     17
+          3.3.10. NULL RDATA format (EXPERIMENTAL)                   17
+          3.3.11. NS RDATA format                                    18
+          3.3.12. PTR RDATA format                                   18
+          3.3.13. SOA RDATA format                                   19
+          3.3.14. TXT RDATA format                                   20
+      3.4. ARPA Internet specific RRs                                20
+          3.4.1. A RDATA format                                      20
+          3.4.2. WKS RDATA format                                    21
+      3.5. IN-ADDR.ARPA domain                                       22
+      3.6. Defining new types, classes, and special namespaces       24
+  4. MESSAGES                                                        25
+      4.1. Format                                                    25
+          4.1.1. Header section format                               26
+          4.1.2. Question section format                             28
+          4.1.3. Resource record format                              29
+          4.1.4. Message compression                                 30
+      4.2. Transport                                                 32
+          4.2.1. UDP usage                                           32
+          4.2.2. TCP usage                                           32
+  5. MASTER FILES                                                    33
+      5.1. Format                                                    33
+      5.2. Use of master files to define zones                       35
+      5.3. Master file example                                       36
+  6. NAME SERVER IMPLEMENTATION                                      37
+      6.1. Architecture                                              37
+          6.1.1. Control                                             37
+          6.1.2. Database                                            37
+          6.1.3. Time                                                39
+      6.2. Standard query processing                                 39
+      6.3. Zone refresh and reload processing                        39
+      6.4. Inverse queries (Optional)                                40
+          6.4.1. The contents of inverse queries and responses       40
+          6.4.2. Inverse query and response example                  41
+          6.4.3. Inverse query processing                            42
+
+
+
+
+
+
+Mockapetris                                                     [Page 2]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+      6.5. Completion queries and responses                          42
+  7. RESOLVER IMPLEMENTATION                                         43
+      7.1. Transforming a user request into a query                  43
+      7.2. Sending the queries                                       44
+      7.3. Processing responses                                      46
+      7.4. Using the cache                                           47
+  8. MAIL SUPPORT                                                    47
+      8.1. Mail exchange binding                                     48
+      8.2. Mailbox binding (Experimental)                            48
+  9. REFERENCES and BIBLIOGRAPHY                                     50
+  Index                                                              54
+
+2. INTRODUCTION
+
+2.1. Overview
+
+The goal of domain names is to provide a mechanism for naming resources
+in such a way that the names are usable in different hosts, networks,
+protocol families, internets, and administrative organizations.
+
+From the user's point of view, domain names are useful as arguments to a
+local agent, called a resolver, which retrieves information associated
+with the domain name.  Thus a user might ask for the host address or
+mail information associated with a particular domain name.  To enable
+the user to request a particular type of information, an appropriate
+query type is passed to the resolver with the domain name.  To the user,
+the domain tree is a single information space; the resolver is
+responsible for hiding the distribution of data among name servers from
+the user.
+
+From the resolver's point of view, the database that makes up the domain
+space is distributed among various name servers.  Different parts of the
+domain space are stored in different name servers, although a particular
+data item will be stored redundantly in two or more name servers.  The
+resolver starts with knowledge of at least one name server.  When the
+resolver processes a user query it asks a known name server for the
+information; in return, the resolver either receives the desired
+information or a referral to another name server.  Using these
+referrals, resolvers learn the identities and contents of other name
+servers.  Resolvers are responsible for dealing with the distribution of
+the domain space and dealing with the effects of name server failure by
+consulting redundant databases in other servers.
+
+Name servers manage two kinds of data.  The first kind of data held in
+sets called zones; each zone is the complete database for a particular
+"pruned" subtree of the domain space.  This data is called
+authoritative.  A name server periodically checks to make sure that its
+zones are up to date, and if not, obtains a new copy of updated zones
+
+
+
+Mockapetris                                                     [Page 3]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+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
+
+
+
+Mockapetris                                                     [Page 4]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+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
+
+
+
+Mockapetris                                                     [Page 5]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+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.
+
+
+
+
+Mockapetris                                                     [Page 6]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+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.
+
+
+
+Mockapetris                                                     [Page 7]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+However, when assigning a domain name for an object, the prudent user
+will select a name which satisfies both the rules of the domain system
+and any existing rules for the object, whether these rules are published
+or implied by existing programs.
+
+For example, when naming a mail domain, the user should satisfy both the
+rules of this memo and those in RFC-822.  When creating a new host name,
+the old rules for HOSTS.TXT should be followed.  This avoids problems
+when old software is converted to use domain names.
+
+The following syntax will result in fewer problems with many
+
+applications that use domain names (e.g., mail, TELNET).
+
+<domain> ::= <subdomain> | " "
+
+<subdomain> ::= <label> | <subdomain> "." <label>
+
+<label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
+
+<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
+
+<let-dig-hyp> ::= <let-dig> | "-"
+
+<let-dig> ::= <letter> | <digit>
+
+<letter> ::= any one of the 52 alphabetic characters A through Z in
+upper case and a through z in lower case
+
+<digit> ::= any one of the ten digits 0 through 9
+
+Note that while upper and lower case letters are allowed in domain
+names, no significance is attached to the case.  That is, two names with
+the same spelling but different case are to be treated as if identical.
+
+The labels must follow the rules for ARPANET host names.  They must
+start with a letter, end with a letter or digit, and have as interior
+characters only letters, digits, and hyphen.  There are also some
+restrictions on the length.  Labels must be 63 characters or less.
+
+For example, the following strings identify hosts in the Internet:
+
+A.ISI.EDU XX.LCS.MIT.EDU SRI-NIC.ARPA
+
+2.3.2. Data Transmission Order
+
+The order of transmission of the header and data described in this
+document is resolved to the octet level.  Whenever a diagram shows a
+
+
+
+Mockapetris                                                     [Page 8]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+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.
+
+
+
+
+Mockapetris                                                     [Page 9]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+Loss of case sensitive data must be minimized.  Thus while data for x.y
+and X.Y may both be stored under a single location x.y or X.Y, data for
+a.x and B.X would never be stored under A.x, A.X, b.x, or b.X.  In
+general, this preserves the case of the first label of a domain name,
+but forces standardization of interior node labels.
+
+Systems administrators who enter data into the domain database should
+take care to represent the data they supply to the domain system in a
+case-consistent manner if their system is case-sensitive.  The data
+distribution system in the domain system will ensure that consistent
+representations are preserved.
+
+2.3.4. Size limits
+
+Various objects and parameters in the DNS have size limits.  They are
+listed below.  Some could be easily changed, others are more
+fundamental.
+
+labels          63 octets or less
+
+names           255 octets or less
+
+TTL             positive values of a signed 32 bit number.
+
+UDP messages    512 octets or less
+
+3. DOMAIN NAME SPACE AND RR DEFINITIONS
+
+3.1. Name space definitions
+
+Domain names in messages are expressed in terms of a sequence of labels.
+Each label is represented as a one octet length field followed by that
+number of octets.  Since every domain name ends with the null label of
+the root, a domain name is terminated by a length byte of zero.  The
+high order two bits of every length octet must be zero, and the
+remaining six bits of the length field limit the label to 63 octets or
+less.
+
+To simplify implementations, the total length of a domain name (i.e.,
+label octets and label length octets) is restricted to 255 octets or
+less.
+
+Although labels can contain any 8 bit values in octets that make up a
+label, it is strongly recommended that labels follow the preferred
+syntax described elsewhere in this memo, which is compatible with
+existing host naming conventions.  Name servers and resolvers must
+compare labels in a case-insensitive manner (i.e., A=a), assuming ASCII
+with zero parity.  Non-alphabetic codes must match exactly.
+
+
+
+Mockapetris                                                    [Page 10]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+3.2. RR definitions
+
+3.2.1. Format
+
+All RRs have the same top level format shown below:
+
+                                    1  1  1  1  1  1
+      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                                               |
+    /                                               /
+    /                      NAME                     /
+    |                                               |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                      TYPE                     |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                     CLASS                     |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                      TTL                      |
+    |                                               |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                   RDLENGTH                    |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
+    /                     RDATA                     /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+
+where:
+
+NAME            an owner name, i.e., the name of the node to which this
+                resource record pertains.
+
+TYPE            two octets containing one of the RR TYPE codes.
+
+CLASS           two octets containing one of the RR CLASS codes.
+
+TTL             a 32 bit signed integer that specifies the time interval
+                that the resource record may be cached before the source
+                of the information should again be consulted.  Zero
+                values are interpreted to mean that the RR can only be
+                used for the transaction in progress, and should not be
+                cached.  For example, SOA records are always distributed
+                with a zero TTL to prohibit caching.  Zero values can
+                also be used for extremely volatile data.
+
+RDLENGTH        an unsigned 16 bit integer that specifies the length in
+                octets of the RDATA field.
+
+
+
+Mockapetris                                                    [Page 11]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+RDATA           a variable length string of octets that describes the
+                resource.  The format of this information varies
+                according to the TYPE and CLASS of the resource record.
+
+3.2.2. TYPE values
+
+TYPE fields are used in resource records.  Note that these types are a
+subset of QTYPEs.
+
+TYPE            value and meaning
+
+A               1 a host address
+
+NS              2 an authoritative name server
+
+MD              3 a mail destination (Obsolete - use MX)
+
+MF              4 a mail forwarder (Obsolete - use MX)
+
+CNAME           5 the canonical name for an alias
+
+SOA             6 marks the start of a zone of authority
+
+MB              7 a mailbox domain name (EXPERIMENTAL)
+
+MG              8 a mail group member (EXPERIMENTAL)
+
+MR              9 a mail rename domain name (EXPERIMENTAL)
+
+NULL            10 a null RR (EXPERIMENTAL)
+
+WKS             11 a well known service description
+
+PTR             12 a domain name pointer
+
+HINFO           13 host information
+
+MINFO           14 mailbox or mail list information
+
+MX              15 mail exchange
+
+TXT             16 text strings
+
+3.2.3. QTYPE values
+
+QTYPE fields appear in the question part of a query.  QTYPES are a
+superset of TYPEs, hence all TYPEs are valid QTYPEs.  In addition, the
+following QTYPEs are defined:
+
+
+
+Mockapetris                                                    [Page 12]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+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).
+
+
+
+
+
+
+
+
+Mockapetris                                                    [Page 13]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+3.3.1. CNAME RDATA format
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                     CNAME                     /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+CNAME           A <domain-name> which specifies the canonical or primary
+                name for the owner.  The owner name is an alias.
+
+CNAME RRs cause no additional section processing, but name servers may
+choose to restart the query at the canonical name in certain cases.  See
+the description of name server logic in [RFC-1034] for details.
+
+3.3.2. HINFO RDATA format
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                      CPU                      /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                       OS                      /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+CPU             A <character-string> which specifies the CPU type.
+
+OS              A <character-string> which specifies the operating
+                system type.
+
+Standard values for CPU and OS can be found in [RFC-1010].
+
+HINFO records are used to acquire general information about a host.  The
+main use is for protocols such as FTP that can use special procedures
+when talking between machines or operating systems of the same type.
+
+3.3.3. MB RDATA format (EXPERIMENTAL)
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                   MADNAME                     /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+MADNAME         A <domain-name> which specifies a host which has the
+                specified mailbox.
+
+
+
+Mockapetris                                                    [Page 14]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+MB records cause additional section processing which looks up an A type
+RRs corresponding to MADNAME.
+
+3.3.4. MD RDATA format (Obsolete)
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                   MADNAME                     /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+MADNAME         A <domain-name> which specifies a host which has a mail
+                agent for the domain which should be able to deliver
+                mail for the domain.
+
+MD records cause additional section processing which looks up an A type
+record corresponding to MADNAME.
+
+MD is obsolete.  See the definition of MX and [RFC-974] for details of
+the new scheme.  The recommended policy for dealing with MD RRs found in
+a master file is to reject them, or to convert them to MX RRs with a
+preference of 0.
+
+3.3.5. MF RDATA format (Obsolete)
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                   MADNAME                     /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+MADNAME         A <domain-name> which specifies a host which has a mail
+                agent for the domain which will accept mail for
+                forwarding to the domain.
+
+MF records cause additional section processing which looks up an A type
+record corresponding to MADNAME.
+
+MF is obsolete.  See the definition of MX and [RFC-974] for details ofw
+the new scheme.  The recommended policy for dealing with MD RRs found in
+a master file is to reject them, or to convert them to MX RRs with a
+preference of 10.
+
+
+
+
+
+
+
+Mockapetris                                                    [Page 15]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+3.3.6. MG RDATA format (EXPERIMENTAL)
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                   MGMNAME                     /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+MGMNAME         A <domain-name> which specifies a mailbox which is a
+                member of the mail group specified by the domain name.
+
+MG records cause no additional section processing.
+
+3.3.7. MINFO RDATA format (EXPERIMENTAL)
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                    RMAILBX                    /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                    EMAILBX                    /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+RMAILBX         A <domain-name> which specifies a mailbox which is
+                responsible for the mailing list or mailbox.  If this
+                domain name names the root, the owner of the MINFO RR is
+                responsible for itself.  Note that many existing mailing
+                lists use a mailbox X-request for the RMAILBX field of
+                mailing list X, e.g., Msgroup-request for Msgroup.  This
+                field provides a more general mechanism.
+
+
+EMAILBX         A <domain-name> which specifies a mailbox which is to
+                receive error messages related to the mailing list or
+                mailbox specified by the owner of the MINFO RR (similar
+                to the ERRORS-TO: field which has been proposed).  If
+                this domain name names the root, errors should be
+                returned to the sender of the message.
+
+MINFO records cause no additional section processing.  Although these
+records can be associated with a simple mailbox, they are usually used
+with a mailing list.
+
+
+
+
+
+
+
+
+Mockapetris                                                    [Page 16]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+3.3.8. MR RDATA format (EXPERIMENTAL)
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                   NEWNAME                     /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+NEWNAME         A <domain-name> which specifies a mailbox which is the
+                proper rename of the specified mailbox.
+
+MR records cause no additional section processing.  The main use for MR
+is as a forwarding entry for a user who has moved to a different
+mailbox.
+
+3.3.9. MX RDATA format
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                  PREFERENCE                   |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                   EXCHANGE                    /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+PREFERENCE      A 16 bit integer which specifies the preference given to
+                this RR among others at the same owner.  Lower values
+                are preferred.
+
+EXCHANGE        A <domain-name> which specifies a host willing to act as
+                a mail exchange for the owner name.
+
+MX records cause type A additional section processing for the host
+specified by EXCHANGE.  The use of MX RRs is explained in detail in
+[RFC-974].
+
+3.3.10. NULL RDATA format (EXPERIMENTAL)
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                  <anything>                   /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+Anything at all may be in the RDATA field so long as it is 65535 octets
+or less.
+
+
+
+
+Mockapetris                                                    [Page 17]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+NULL records cause no additional section processing.  NULL RRs are not
+allowed in master files.  NULLs are used as placeholders in some
+experimental extensions of the DNS.
+
+3.3.11. NS RDATA format
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                   NSDNAME                     /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+NSDNAME         A <domain-name> which specifies a host which should be
+                authoritative for the specified class and domain.
+
+NS records cause both the usual additional section processing to locate
+a type A record, and, when used in a referral, a special search of the
+zone in which they reside for glue information.
+
+The NS RR states that the named host should be expected to have a zone
+starting at owner name of the specified class.  Note that the class may
+not indicate the protocol family which should be used to communicate
+with the host, although it is typically a strong hint.  For example,
+hosts which are name servers for either Internet (IN) or Hesiod (HS)
+class information are normally queried using IN class protocols.
+
+3.3.12. PTR RDATA format
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                   PTRDNAME                    /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+PTRDNAME        A <domain-name> which points to some location in the
+                domain name space.
+
+PTR records cause no additional section processing.  These RRs are used
+in special domains to point to some other location in the domain space.
+These records are simple data, and don't imply any special processing
+similar to that performed by CNAME, which identifies aliases.  See the
+description of the IN-ADDR.ARPA domain for an example.
+
+
+
+
+
+
+
+
+Mockapetris                                                    [Page 18]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+3.3.13. SOA RDATA format
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                     MNAME                     /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                     RNAME                     /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    SERIAL                     |
+    |                                               |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    REFRESH                    |
+    |                                               |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                     RETRY                     |
+    |                                               |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    EXPIRE                     |
+    |                                               |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    MINIMUM                    |
+    |                                               |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+MNAME           The <domain-name> of the name server that was the
+                original or primary source of data for this zone.
+
+RNAME           A <domain-name> which specifies the mailbox of the
+                person responsible for this zone.
+
+SERIAL          The unsigned 32 bit version number of the original copy
+                of the zone.  Zone transfers preserve this value.  This
+                value wraps and should be compared using sequence space
+                arithmetic.
+
+REFRESH         A 32 bit time interval before the zone should be
+                refreshed.
+
+RETRY           A 32 bit time interval that should elapse before a
+                failed refresh should be retried.
+
+EXPIRE          A 32 bit time value that specifies the upper limit on
+                the time interval that can elapse before the zone is no
+                longer authoritative.
+
+
+
+
+
+Mockapetris                                                    [Page 19]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+MINIMUM         The unsigned 32 bit minimum TTL field that should be
+                exported with any RR from this zone.
+
+SOA records cause no additional section processing.
+
+All times are in units of seconds.
+
+Most of these fields are pertinent only for name server maintenance
+operations.  However, MINIMUM is used in all query operations that
+retrieve RRs from a zone.  Whenever a RR is sent in a response to a
+query, the TTL field is set to the maximum of the TTL field from the RR
+and the MINIMUM field in the appropriate SOA.  Thus MINIMUM is a lower
+bound on the TTL field for all RRs in a zone.  Note that this use of
+MINIMUM should occur when the RRs are copied into the response and not
+when the zone is loaded from a master file or via a zone transfer.  The
+reason for this provison is to allow future dynamic update facilities to
+change the SOA RR with known semantics.
+
+
+3.3.14. TXT RDATA format
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    /                   TXT-DATA                    /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+TXT-DATA        One or more <character-string>s.
+
+TXT RRs are used to hold descriptive text.  The semantics of the text
+depends on the domain where it is found.
+
+3.4. Internet specific RRs
+
+3.4.1. A RDATA format
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    ADDRESS                    |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+ADDRESS         A 32 bit Internet address.
+
+Hosts that have multiple Internet addresses will have multiple A
+records.
+
+
+
+
+
+Mockapetris                                                    [Page 20]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+A records cause no additional section processing.  The RDATA section of
+an A line in a master file is an Internet address expressed as four
+decimal numbers separated by dots without any imbedded spaces (e.g.,
+"10.2.0.52" or "192.0.5.6").
+
+3.4.2. WKS RDATA format
+
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    ADDRESS                    |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |       PROTOCOL        |                       |
+    +--+--+--+--+--+--+--+--+                       |
+    |                                               |
+    /                   <BIT MAP>                   /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+ADDRESS         An 32 bit Internet address
+
+PROTOCOL        An 8 bit IP protocol number
+
+<BIT MAP>       A variable length bit map.  The bit map must be a
+                multiple of 8 bits long.
+
+The WKS record is used to describe the well known services supported by
+a particular protocol on a particular internet address.  The PROTOCOL
+field specifies an IP protocol number, and the bit map has one bit per
+port of the specified protocol.  The first bit corresponds to port 0,
+the second to port 1, etc.  If the bit map does not include a bit for a
+protocol of interest, that bit is assumed zero.  The appropriate values
+and mnemonics for ports and protocols are specified in [RFC-1010].
+
+For example, if PROTOCOL=TCP (6), the 26th bit corresponds to TCP port
+25 (SMTP).  If this bit is set, a SMTP server should be listening on TCP
+port 25; if zero, SMTP service is not supported on the specified
+address.
+
+The purpose of WKS RRs is to provide availability information for
+servers for TCP and UDP.  If a server supports both TCP and UDP, or has
+multiple Internet addresses, then multiple WKS RRs are used.
+
+WKS RRs cause no additional section processing.
+
+In master files, both ports and protocols are expressed using mnemonics
+or decimal numbers.
+
+
+
+
+Mockapetris                                                    [Page 21]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+3.5. IN-ADDR.ARPA domain
+
+The Internet uses a special domain to support gateway location and
+Internet address to host mapping.  Other classes may employ a similar
+strategy in other domains.  The intent of this domain is to provide a
+guaranteed method to perform host address to host name mapping, and to
+facilitate queries to locate all gateways on a particular network in the
+Internet.
+
+Note that both of these services are similar to functions that could be
+performed by inverse queries; the difference is that this part of the
+domain name space is structured according to address, and hence can
+guarantee that the appropriate data can be located without an exhaustive
+search of the domain space.
+
+The domain begins at IN-ADDR.ARPA and has a substructure which follows
+the Internet addressing structure.
+
+Domain names in the IN-ADDR.ARPA domain are defined to have up to four
+labels in addition to the IN-ADDR.ARPA suffix.  Each label represents
+one octet of an Internet address, and is expressed as a character string
+for a decimal value in the range 0-255 (with leading zeros omitted
+except in the case of a zero octet which is represented by a single
+zero).
+
+Host addresses are represented by domain names that have all four labels
+specified.  Thus data for Internet address 10.2.0.52 is located at
+domain name 52.0.2.10.IN-ADDR.ARPA.  The reversal, though awkward to
+read, allows zones to be delegated which are exactly one network of
+address space.  For example, 10.IN-ADDR.ARPA can be a zone containing
+data for the ARPANET, while 26.IN-ADDR.ARPA can be a separate zone for
+MILNET.  Address nodes are used to hold pointers to primary host names
+in the normal domain space.
+
+Network numbers correspond to some non-terminal nodes at various depths
+in the IN-ADDR.ARPA domain, since Internet network numbers are either 1,
+2, or 3 octets.  Network nodes are used to hold pointers to the primary
+host names of gateways attached to that network.  Since a gateway is, by
+definition, on more than one network, it will typically have two or more
+network nodes which point at it.  Gateways will also have host level
+pointers at their fully qualified addresses.
+
+Both the gateway pointers at network nodes and the normal host pointers
+at full address nodes use the PTR RR to point back to the primary domain
+names of the corresponding hosts.
+
+For example, the IN-ADDR.ARPA domain will contain information about the
+ISI gateway between net 10 and 26, an MIT gateway from net 10 to MIT's
+
+
+
+Mockapetris                                                    [Page 22]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+net 18, and hosts A.ISI.EDU and MULTICS.MIT.EDU.  Assuming that ISI
+gateway has addresses 10.2.0.22 and 26.0.0.103, and a name MILNET-
+GW.ISI.EDU, and the MIT gateway has addresses 10.0.0.77 and 18.10.0.4
+and a name GW.LCS.MIT.EDU, the domain database would contain:
+
+    10.IN-ADDR.ARPA.           PTR MILNET-GW.ISI.EDU.
+    10.IN-ADDR.ARPA.           PTR GW.LCS.MIT.EDU.
+    18.IN-ADDR.ARPA.           PTR GW.LCS.MIT.EDU.
+    26.IN-ADDR.ARPA.           PTR MILNET-GW.ISI.EDU.
+    22.0.2.10.IN-ADDR.ARPA.    PTR MILNET-GW.ISI.EDU.
+    103.0.0.26.IN-ADDR.ARPA.   PTR MILNET-GW.ISI.EDU.
+    77.0.0.10.IN-ADDR.ARPA.    PTR GW.LCS.MIT.EDU.
+    4.0.10.18.IN-ADDR.ARPA.    PTR GW.LCS.MIT.EDU.
+    103.0.3.26.IN-ADDR.ARPA.   PTR A.ISI.EDU.
+    6.0.0.10.IN-ADDR.ARPA.     PTR MULTICS.MIT.EDU.
+
+Thus a program which wanted to locate gateways on net 10 would originate
+a query of the form QTYPE=PTR, QCLASS=IN, QNAME=10.IN-ADDR.ARPA.  It
+would receive two RRs in response:
+
+    10.IN-ADDR.ARPA.           PTR MILNET-GW.ISI.EDU.
+    10.IN-ADDR.ARPA.           PTR GW.LCS.MIT.EDU.
+
+The program could then originate QTYPE=A, QCLASS=IN queries for MILNET-
+GW.ISI.EDU. and GW.LCS.MIT.EDU. to discover the Internet addresses of
+these gateways.
+
+A resolver which wanted to find the host name corresponding to Internet
+host address 10.0.0.6 would pursue a query of the form QTYPE=PTR,
+QCLASS=IN, QNAME=6.0.0.10.IN-ADDR.ARPA, and would receive:
+
+    6.0.0.10.IN-ADDR.ARPA.     PTR MULTICS.MIT.EDU.
+
+Several cautions apply to the use of these services:
+   - Since the IN-ADDR.ARPA special domain and the normal domain
+     for a particular host or gateway will be in different zones,
+     the possibility exists that that the data may be inconsistent.
+
+   - Gateways will often have two names in separate domains, only
+     one of which can be primary.
+
+   - Systems that use the domain database to initialize their
+     routing tables must start with enough gateway information to
+     guarantee that they can access the appropriate name server.
+
+   - The gateway data only reflects the existence of a gateway in a
+     manner equivalent to the current HOSTS.TXT file.  It doesn't
+     replace the dynamic availability information from GGP or EGP.
+
+
+
+Mockapetris                                                    [Page 23]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+3.6. Defining new types, classes, and special namespaces
+
+The previously defined types and classes are the ones in use as of the
+date of this memo.  New definitions should be expected.  This section
+makes some recommendations to designers considering additions to the
+existing facilities.  The mailing list NAMEDROPPERS@SRI-NIC.ARPA is the
+forum where general discussion of design issues takes place.
+
+In general, a new type is appropriate when new information is to be
+added to the database about an existing object, or we need new data
+formats for some totally new object.  Designers should attempt to define
+types and their RDATA formats that are generally applicable to all
+classes, and which avoid duplication of information.  New classes are
+appropriate when the DNS is to be used for a new protocol, etc which
+requires new class-specific data formats, or when a copy of the existing
+name space is desired, but a separate management domain is necessary.
+
+New types and classes need mnemonics for master files; the format of the
+master files requires that the mnemonics for type and class be disjoint.
+
+TYPE and CLASS values must be a proper subset of QTYPEs and QCLASSes
+respectively.
+
+The present system uses multiple RRs to represent multiple values of a
+type rather than storing multiple values in the RDATA section of a
+single RR.  This is less efficient for most applications, but does keep
+RRs shorter.  The multiple RRs assumption is incorporated in some
+experimental work on dynamic update methods.
+
+The present system attempts to minimize the duplication of data in the
+database in order to insure consistency.  Thus, in order to find the
+address of the host for a mail exchange, you map the mail domain name to
+a host name, then the host name to addresses, rather than a direct
+mapping to host address.  This approach is preferred because it avoids
+the opportunity for inconsistency.
+
+In defining a new type of data, multiple RR types should not be used to
+create an ordering between entries or express different formats for
+equivalent bindings, instead this information should be carried in the
+body of the RR and a single type used.  This policy avoids problems with
+caching multiple types and defining QTYPEs to match multiple types.
+
+For example, the original form of mail exchange binding used two RR
+types one to represent a "closer" exchange (MD) and one to represent a
+"less close" exchange (MF).  The difficulty is that the presence of one
+RR type in a cache doesn't convey any information about the other
+because the query which acquired the cached information might have used
+a QTYPE of MF, MD, or MAILA (which matched both).  The redesigned
+
+
+
+Mockapetris                                                    [Page 24]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+service used a single type (MX) with a "preference" value in the RDATA
+section which can order different RRs.  However, if any MX RRs are found
+in the cache, then all should be there.
+
+4. MESSAGES
+
+4.1. Format
+
+All communications inside of the domain protocol are carried in a single
+format called a message.  The top level format of message is divided
+into 5 sections (some of which are empty in certain cases) shown below:
+
+    +---------------------+
+    |        Header       |
+    +---------------------+
+    |       Question      | the question for the name server
+    +---------------------+
+    |        Answer       | RRs answering the question
+    +---------------------+
+    |      Authority      | RRs pointing toward an authority
+    +---------------------+
+    |      Additional     | RRs holding additional information
+    +---------------------+
+
+The header section is always present.  The header includes fields that
+specify which of the remaining sections are present, and also specify
+whether the message is a query or a response, a standard query or some
+other opcode, etc.
+
+The names of the sections after the header are derived from their use in
+standard queries.  The question section contains fields that describe a
+question to a name server.  These fields are a query type (QTYPE), a
+query class (QCLASS), and a query domain name (QNAME).  The last three
+sections have the same format: a possibly empty list of concatenated
+resource records (RRs).  The answer section contains RRs that answer the
+question; the authority section contains RRs that point toward an
+authoritative name server; the additional records section contains RRs
+which relate to the query, but are not strictly answers for the
+question.
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris                                                    [Page 25]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+4.1.1. Header section format
+
+The header contains the following fields:
+
+                                    1  1  1  1  1  1
+      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                      ID                       |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |QR|   Opcode  |AA|TC|RD|RA|   Z    |   RCODE   |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    QDCOUNT                    |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    ANCOUNT                    |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    NSCOUNT                    |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                    ARCOUNT                    |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+ID              A 16 bit identifier assigned by the program that
+                generates any kind of query.  This identifier is copied
+                the corresponding reply and can be used by the requester
+                to match up replies to outstanding queries.
+
+QR              A one bit field that specifies whether this message is a
+                query (0), or a response (1).
+
+OPCODE          A four bit field that specifies kind of query in this
+                message.  This value is set by the originator of a query
+                and copied into the response.  The values are:
+
+                0               a standard query (QUERY)
+
+                1               an inverse query (IQUERY)
+
+                2               a server status request (STATUS)
+
+                3-15            reserved for future use
+
+AA              Authoritative Answer - this bit is valid in responses,
+                and specifies that the responding name server is an
+                authority for the domain name in question section.
+
+                Note that the contents of the answer section may have
+                multiple owner names because of aliases.  The AA bit
+
+
+
+Mockapetris                                                    [Page 26]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+                corresponds to the name which matches the query name, or
+                the first owner name in the answer section.
+
+TC              TrunCation - specifies that this message was truncated
+                due to length greater than that permitted on the
+                transmission channel.
+
+RD              Recursion Desired - this bit may be set in a query and
+                is copied into the response.  If RD is set, it directs
+                the name server to pursue the query recursively.
+                Recursive query support is optional.
+
+RA              Recursion Available - this be is set or cleared in a
+                response, and denotes whether recursive query support is
+                available in the name server.
+
+Z               Reserved for future use.  Must be zero in all queries
+                and responses.
+
+RCODE           Response code - this 4 bit field is set as part of
+                responses.  The values have the following
+                interpretation:
+
+                0               No error condition
+
+                1               Format error - The name server was
+                                unable to interpret the query.
+
+                2               Server failure - The name server was
+                                unable to process this query due to a
+                                problem with the name server.
+
+                3               Name Error - Meaningful only for
+                                responses from an authoritative name
+                                server, this code signifies that the
+                                domain name referenced in the query does
+                                not exist.
+
+                4               Not Implemented - The name server does
+                                not support the requested kind of query.
+
+                5               Refused - The name server refuses to
+                                perform the specified operation for
+                                policy reasons.  For example, a name
+                                server may not wish to provide the
+                                information to the particular requester,
+                                or a name server may not wish to perform
+                                a particular operation (e.g., zone
+
+
+
+Mockapetris                                                    [Page 27]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+                                transfer) for particular data.
+
+                6-15            Reserved for future use.
+
+QDCOUNT         an unsigned 16 bit integer specifying the number of
+                entries in the question section.
+
+ANCOUNT         an unsigned 16 bit integer specifying the number of
+                resource records in the answer section.
+
+NSCOUNT         an unsigned 16 bit integer specifying the number of name
+                server resource records in the authority records
+                section.
+
+ARCOUNT         an unsigned 16 bit integer specifying the number of
+                resource records in the additional records section.
+
+4.1.2. Question section format
+
+The question section is used to carry the "question" in most queries,
+i.e., the parameters that define what is being asked.  The section
+contains QDCOUNT (usually 1) entries, each of the following format:
+
+                                    1  1  1  1  1  1
+      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                                               |
+    /                     QNAME                     /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                     QTYPE                     |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                     QCLASS                    |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+QNAME           a domain name represented as a sequence of labels, where
+                each label consists of a length octet followed by that
+                number of octets.  The domain name terminates with the
+                zero length octet for the null label of the root.  Note
+                that this field may be an odd number of octets; no
+                padding is used.
+
+QTYPE           a two octet code which specifies the type of the query.
+                The values for this field include all codes valid for a
+                TYPE field, together with some more general codes which
+                can match more than one type of RR.
+
+
+
+Mockapetris                                                    [Page 28]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+QCLASS          a two octet code that specifies the class of the query.
+                For example, the QCLASS field is IN for the Internet.
+
+4.1.3. Resource record format
+
+The answer, authority, and additional sections all share the same
+format: a variable number of resource records, where the number of
+records is specified in the corresponding count field in the header.
+Each resource record has the following format:
+                                    1  1  1  1  1  1
+      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                                               |
+    /                                               /
+    /                      NAME                     /
+    |                                               |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                      TYPE                     |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                     CLASS                     |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                      TTL                      |
+    |                                               |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    |                   RDLENGTH                    |
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
+    /                     RDATA                     /
+    /                                               /
+    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+NAME            a domain name to which this resource record pertains.
+
+TYPE            two octets containing one of the RR type codes.  This
+                field specifies the meaning of the data in the RDATA
+                field.
+
+CLASS           two octets which specify the class of the data in the
+                RDATA field.
+
+TTL             a 32 bit unsigned integer that specifies the time
+                interval (in seconds) that the resource record may be
+                cached before it should be discarded.  Zero values are
+                interpreted to mean that the RR can only be used for the
+                transaction in progress, and should not be cached.
+
+
+
+
+
+Mockapetris                                                    [Page 29]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+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
+
+
+
+Mockapetris                                                    [Page 30]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+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
+
+
+
+Mockapetris                                                    [Page 31]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+defined by a single octet of zeros at 92; the root domain name has no
+labels.
+
+4.2. Transport
+
+The DNS assumes that messages will be transmitted as datagrams or in a
+byte stream carried by a virtual circuit.  While virtual circuits can be
+used for any DNS activity, datagrams are preferred for queries due to
+their lower overhead and better performance.  Zone refresh activities
+must use virtual circuits because of the need for reliable transfer.
+
+The Internet supports name server access using TCP [RFC-793] on server
+port 53 (decimal) as well as datagram access using UDP [RFC-768] on UDP
+port 53 (decimal).
+
+4.2.1. UDP usage
+
+Messages sent using UDP user server port 53 (decimal).
+
+Messages carried by UDP are restricted to 512 bytes (not counting the IP
+or UDP headers).  Longer messages are truncated and the TC bit is set in
+the header.
+
+UDP is not acceptable for zone transfers, but is the recommended method
+for standard queries in the Internet.  Queries sent using UDP may be
+lost, and hence a retransmission strategy is required.  Queries or their
+responses may be reordered by the network, or by processing in name
+servers, so resolvers should not depend on them being returned in order.
+
+The optimal UDP retransmission policy will vary with performance of the
+Internet and the needs of the client, but the following are recommended:
+
+   - The client should try other servers and server addresses
+     before repeating a query to a specific address of a server.
+
+   - The retransmission interval should be based on prior
+     statistics if possible.  Too aggressive retransmission can
+     easily slow responses for the community at large.  Depending
+     on how well connected the client is to its expected servers,
+     the minimum retransmission interval should be 2-5 seconds.
+
+More suggestions on server selection and retransmission policy can be
+found in the resolver section of this memo.
+
+4.2.2. TCP usage
+
+Messages sent over TCP connections use server port 53 (decimal).  The
+message is prefixed with a two byte length field which gives the message
+
+
+
+Mockapetris                                                    [Page 32]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+length, excluding the two byte length field.  This length field allows
+the low-level processing to assemble a complete message before beginning
+to parse it.
+
+Several connection management policies are recommended:
+
+   - The server should not block other activities waiting for TCP
+     data.
+
+   - The server should support multiple connections.
+
+   - The server should assume that the client will initiate
+     connection closing, and should delay closing its end of the
+     connection until all outstanding client requests have been
+     satisfied.
+
+   - If the server needs to close a dormant connection to reclaim
+     resources, it should wait until the connection has been idle
+     for a period on the order of two minutes.  In particular, the
+     server should allow the SOA and AXFR request sequence (which
+     begins a refresh operation) to be made on a single connection.
+     Since the server would be unable to answer queries anyway, a
+     unilateral close or reset may be used instead of a graceful
+     close.
+
+5. MASTER FILES
+
+Master files are text files that contain RRs in text form.  Since the
+contents of a zone can be expressed in the form of a list of RRs a
+master file is most often used to define a zone, though it can be used
+to list a cache's contents.  Hence, this section first discusses the
+format of RRs in a master file, and then the special considerations when
+a master file is used to create a zone in some name server.
+
+5.1. Format
+
+The format of these files is a sequence of entries.  Entries are
+predominantly line-oriented, though parentheses can be used to continue
+a list of items across a line boundary, and text literals can contain
+CRLF within the text.  Any combination of tabs and spaces act as a
+delimiter between the separate items that make up an entry.  The end of
+any line in the master file can end with a comment.  The comment starts
+with a ";" (semicolon).
+
+The following entries are defined:
+
+    <blank>[<comment>]
+
+
+
+
+Mockapetris                                                    [Page 33]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+    $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.
+
+
+
+
+
+Mockapetris                                                    [Page 34]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+<character-string> is expressed in one or two ways: as a contiguous set
+of characters without interior spaces, or as a string beginning with a "
+and ending with a ".  Inside a " delimited string any character can
+occur, except for a " itself, which must be quoted using \ (back slash).
+
+Because these files are text files several special encodings are
+necessary to allow arbitrary data to be loaded.  In particular:
+
+                of the root.
+
+@               A free standing @ is used to denote the current origin.
+
+\X              where X is any character other than a digit (0-9), is
+                used to quote that character so that its special meaning
+                does not apply.  For example, "\." can be used to place
+                a dot character in a label.
+
+\DDD            where each D is a digit is the octet corresponding to
+                the decimal number described by DDD.  The resulting
+                octet is assumed to be text and is not checked for
+                special meaning.
+
+( )             Parentheses are used to group data that crosses a line
+                boundary.  In effect, line terminations are not
+                recognized within parentheses.
+
+;               Semicolon is used to start a comment; the remainder of
+                the line is ignored.
+
+5.2. Use of master files to define zones
+
+When a master file is used to load a zone, the operation should be
+suppressed if any errors are encountered in the master file.  The
+rationale for this is that a single error can have widespread
+consequences.  For example, suppose that the RRs defining a delegation
+have syntax errors; then the server will return authoritative name
+errors for all names in the subzone (except in the case where the
+subzone is also present on the server).
+
+Several other validity checks that should be performed in addition to
+insuring that the file is syntactically correct:
+
+   1. All RRs in the file should have the same class.
+
+   2. Exactly one SOA RR should be present at the top of the zone.
+
+   3. If delegations are present and glue information is required,
+      it should be present.
+
+
+
+Mockapetris                                                    [Page 35]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+   4. Information present outside of the authoritative nodes in the
+      zone should be glue information, rather than the result of an
+      origin or similar error.
+
+5.3. Master file example
+
+The following is an example file which might be used to define the
+ISI.EDU zone.and is loaded with an origin of ISI.EDU:
+
+@   IN  SOA     VENERA      Action\.domains (
+                                 20     ; SERIAL
+                                 7200   ; REFRESH
+                                 600    ; RETRY
+                                 3600000; EXPIRE
+                                 60)    ; MINIMUM
+
+        NS      A.ISI.EDU.
+        NS      VENERA
+        NS      VAXA
+        MX      10      VENERA
+        MX      20      VAXA
+
+A       A       26.3.0.103
+
+VENERA  A       10.1.0.52
+        A       128.9.0.32
+
+VAXA    A       10.2.0.27
+        A       128.9.0.33
+
+
+$INCLUDE <SUBSYS>ISI-MAILBOXES.TXT
+
+Where the file <SUBSYS>ISI-MAILBOXES.TXT is:
+
+    MOE     MB      A.ISI.EDU.
+    LARRY   MB      A.ISI.EDU.
+    CURLEY  MB      A.ISI.EDU.
+    STOOGES MG      MOE
+            MG      LARRY
+            MG      CURLEY
+
+Note the use of the \ character in the SOA RR to specify the responsible
+person mailbox "Action.domains@E.ISI.EDU".
+
+
+
+
+
+
+
+Mockapetris                                                    [Page 36]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+6. NAME SERVER IMPLEMENTATION
+
+6.1. Architecture
+
+The optimal structure for the name server will depend on the host
+operating system and whether the name server is integrated with resolver
+operations, either by supporting recursive service, or by sharing its
+database with a resolver.  This section discusses implementation
+considerations for a name server which shares a database with a
+resolver, but most of these concerns are present in any name server.
+
+6.1.1. Control
+
+A name server must employ multiple concurrent activities, whether they
+are implemented as separate tasks in the host's OS or multiplexing
+inside a single name server program.  It is simply not acceptable for a
+name server to block the service of UDP requests while it waits for TCP
+data for refreshing or query activities.  Similarly, a name server
+should not attempt to provide recursive service without processing such
+requests in parallel, though it may choose to serialize requests from a
+single client, or to regard identical requests from the same client as
+duplicates.  A name server should not substantially delay requests while
+it reloads a zone from master files or while it incorporates a newly
+refreshed zone into its database.
+
+6.1.2. Database
+
+While name server implementations are free to use any internal data
+structures they choose, the suggested structure consists of three major
+parts:
+
+   - A "catalog" data structure which lists the zones available to
+     this server, and a "pointer" to the zone data structure.  The
+     main purpose of this structure is to find the nearest ancestor
+     zone, if any, for arriving standard queries.
+
+   - Separate data structures for each of the zones held by the
+     name server.
+
+   - A data structure for cached data. (or perhaps separate caches
+     for different classes)
+
+All of these data structures can be implemented an identical tree
+structure format, with different data chained off the nodes in different
+parts: in the catalog the data is pointers to zones, while in the zone
+and cache data structures, the data will be RRs.  In designing the tree
+framework the designer should recognize that query processing will need
+to traverse the tree using case-insensitive label comparisons; and that
+
+
+
+Mockapetris                                                    [Page 37]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+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
+
+
+
+Mockapetris                                                    [Page 38]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+includes the catalog structure (including the state of refreshing for
+each zone) and the zone data itself.
+
+6.1.3. Time
+
+Both the TTL data for RRs and the timing data for refreshing activities
+depends on 32 bit timers in units of seconds.  Inside the database,
+refresh timers and TTLs for cached data conceptually "count down", while
+data in the zone stays with constant TTLs.
+
+A recommended implementation strategy is to store time in two ways:  as
+a relative increment and as an absolute time.  One way to do this is to
+use positive 32 bit numbers for one type and negative numbers for the
+other.  The RRs in zones use relative times; the refresh timers and
+cache data use absolute times.  Absolute numbers are taken with respect
+to some known origin and converted to relative values when placed in the
+response to a query.  When an absolute TTL is negative after conversion
+to relative, then the data is expired and should be ignored.
+
+6.2. Standard query processing
+
+The major algorithm for standard query processing is presented in
+[RFC-1034].
+
+When processing queries with QCLASS=*, or some other QCLASS which
+matches multiple classes, the response should never be authoritative
+unless the server can guarantee that the response covers all classes.
+
+When composing a response, RRs which are to be inserted in the
+additional section, but duplicate RRs in the answer or authority
+sections, may be omitted from the additional section.
+
+When a response is so long that truncation is required, the truncation
+should start at the end of the response and work forward in the
+datagram.  Thus if there is any data for the authority section, the
+answer section is guaranteed to be unique.
+
+The MINIMUM value in the SOA should be used to set a floor on the TTL of
+data distributed from a zone.  This floor function should be done when
+the data is copied into a response.  This will allow future dynamic
+update protocols to change the SOA MINIMUM field without ambiguous
+semantics.
+
+6.3. Zone refresh and reload processing
+
+In spite of a server's best efforts, it may be unable to load zone data
+from a master file due to syntax errors, etc., or be unable to refresh a
+zone within the its expiration parameter.  In this case, the name server
+
+
+
+Mockapetris                                                    [Page 39]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+should answer queries as if it were not supposed to possess the zone.
+
+If a master is sending a zone out via AXFR, and a new version is created
+during the transfer, the master should continue to send the old version
+if possible.  In any case, it should never send part of one version and
+part of another.  If completion is not possible, the master should reset
+the connection on which the zone transfer is taking place.
+
+6.4. Inverse queries (Optional)
+
+Inverse queries are an optional part of the DNS.  Name servers are not
+required to support any form of inverse queries.  If a name server
+receives an inverse query that it does not support, it returns an error
+response with the "Not Implemented" error set in the header.  While
+inverse query support is optional, all name servers must be at least
+able to return the error response.
+
+6.4.1. The contents of inverse queries and responses          Inverse
+queries reverse the mappings performed by standard query operations;
+while a standard query maps a domain name to a resource, an inverse
+query maps a resource to a domain name.  For example, a standard query
+might bind a domain name to a host address; the corresponding inverse
+query binds the host address to a domain name.
+
+Inverse queries take the form of a single RR in the answer section of
+the message, with an empty question section.  The owner name of the
+query RR and its TTL are not significant.  The response carries
+questions in the question section which identify all names possessing
+the query RR WHICH THE NAME SERVER KNOWS.  Since no name server knows
+about all of the domain name space, the response can never be assumed to
+be complete.  Thus inverse queries are primarily useful for database
+management and debugging activities.  Inverse queries are NOT an
+acceptable method of mapping host addresses to host names; use the IN-
+ADDR.ARPA domain instead.
+
+Where possible, name servers should provide case-insensitive comparisons
+for inverse queries.  Thus an inverse query asking for an MX RR of
+"Venera.isi.edu" should get the same response as a query for
+"VENERA.ISI.EDU"; an inverse query for HINFO RR "IBM-PC UNIX" should
+produce the same result as an inverse query for "IBM-pc unix".  However,
+this cannot be guaranteed because name servers may possess RRs that
+contain character strings but the name server does not know that the
+data is character.
+
+When a name server processes an inverse query, it either returns:
+
+   1. zero, one, or multiple domain names for the specified
+      resource as QNAMEs in the question section
+
+
+
+Mockapetris                                                    [Page 40]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+   2. an error code indicating that the name server doesn't support
+      inverse mapping of the specified resource type.
+
+When the response to an inverse query contains one or more QNAMEs, the
+owner name and TTL of the RR in the answer section which defines the
+inverse query is modified to exactly match an RR found at the first
+QNAME.
+
+RRs returned in the inverse queries cannot be cached using the same
+mechanism as is used for the replies to standard queries.  One reason
+for this is that a name might have multiple RRs of the same type, and
+only one would appear.  For example, an inverse query for a single
+address of a multiply homed host might create the impression that only
+one address existed.
+
+6.4.2. Inverse query and response example          The overall structure
+of an inverse query for retrieving the domain name that corresponds to
+Internet address 10.1.0.52 is shown below:
+
+                         +-----------------------------------------+
+           Header        |          OPCODE=IQUERY, ID=997          |
+                         +-----------------------------------------+
+          Question       |                 <empty>                 |
+                         +-----------------------------------------+
+           Answer        |        <anyname> A IN 10.1.0.52         |
+                         +-----------------------------------------+
+          Authority      |                 <empty>                 |
+                         +-----------------------------------------+
+         Additional      |                 <empty>                 |
+                         +-----------------------------------------+
+
+This query asks for a question whose answer is the Internet style
+address 10.1.0.52.  Since the owner name is not known, any domain name
+can be used as a placeholder (and is ignored).  A single octet of zero,
+signifying the root, is usually used because it minimizes the length of
+the message.  The TTL of the RR is not significant.  The response to
+this query might be:
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris                                                    [Page 41]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+                         +-----------------------------------------+
+           Header        |         OPCODE=RESPONSE, ID=997         |
+                         +-----------------------------------------+
+          Question       |QTYPE=A, QCLASS=IN, QNAME=VENERA.ISI.EDU |
+                         +-----------------------------------------+
+           Answer        |  VENERA.ISI.EDU  A IN 10.1.0.52         |
+                         +-----------------------------------------+
+          Authority      |                 <empty>                 |
+                         +-----------------------------------------+
+         Additional      |                 <empty>                 |
+                         +-----------------------------------------+
+
+Note that the QTYPE in a response to an inverse query is the same as the
+TYPE field in the answer section of the inverse query.  Responses to
+inverse queries may contain multiple questions when the inverse is not
+unique.  If the question section in the response is not empty, then the
+RR in the answer section is modified to correspond to be an exact copy
+of an RR at the first QNAME.
+
+6.4.3. Inverse query processing
+
+Name servers that support inverse queries can support these operations
+through exhaustive searches of their databases, but this becomes
+impractical as the size of the database increases.  An alternative
+approach is to invert the database according to the search key.
+
+For name servers that support multiple zones and a large amount of data,
+the recommended approach is separate inversions for each zone.  When a
+particular zone is changed during a refresh, only its inversions need to
+be redone.
+
+Support for transfer of this type of inversion may be included in future
+versions of the domain system, but is not supported in this version.
+
+6.5. Completion queries and responses
+
+The optional completion services described in RFC-882 and RFC-883 have
+been deleted.  Redesigned services may become available in the future.
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris                                                    [Page 42]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+7. RESOLVER IMPLEMENTATION
+
+The top levels of the recommended resolver algorithm are discussed in
+[RFC-1034].  This section discusses implementation details assuming the
+database structure suggested in the name server implementation section
+of this memo.
+
+7.1. Transforming a user request into a query
+
+The first step a resolver takes is to transform the client's request,
+stated in a format suitable to the local OS, into a search specification
+for RRs at a specific name which match a specific QTYPE and QCLASS.
+Where possible, the QTYPE and QCLASS should correspond to a single type
+and a single class, because this makes the use of cached data much
+simpler.  The reason for this is that the presence of data of one type
+in a cache doesn't confirm the existence or non-existence of data of
+other types, hence the only way to be sure is to consult an
+authoritative source.  If QCLASS=* is used, then authoritative answers
+won't be available.
+
+Since a resolver must be able to multiplex multiple requests if it is to
+perform its function efficiently, each pending request is usually
+represented in some block of state information.  This state block will
+typically contain:
+
+   - A timestamp indicating the time the request began.
+     The timestamp is used to decide whether RRs in the database
+     can be used or are out of date.  This timestamp uses the
+     absolute time format previously discussed for RR storage in
+     zones and caches.  Note that when an RRs TTL indicates a
+     relative time, the RR must be timely, since it is part of a
+     zone.  When the RR has an absolute time, it is part of a
+     cache, and the TTL of the RR is compared against the timestamp
+     for the start of the request.
+
+     Note that using the timestamp is superior to using a current
+     time, since it allows RRs with TTLs of zero to be entered in
+     the cache in the usual manner, but still used by the current
+     request, even after intervals of many seconds due to system
+     load, query retransmission timeouts, etc.
+
+   - Some sort of parameters to limit the amount of work which will
+     be performed for this request.
+
+     The amount of work which a resolver will do in response to a
+     client request must be limited to guard against errors in the
+     database, such as circular CNAME references, and operational
+     problems, such as network partition which prevents the
+
+
+
+Mockapetris                                                    [Page 43]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+     resolver from accessing the name servers it needs.  While
+     local limits on the number of times a resolver will retransmit
+     a particular query to a particular name server address are
+     essential, the resolver should have a global per-request
+     counter to limit work on a single request.  The counter should
+     be set to some initial value and decremented whenever the
+     resolver performs any action (retransmission timeout,
+     retransmission, etc.)  If the counter passes zero, the request
+     is terminated with a temporary error.
+
+     Note that if the resolver structure allows one request to
+     start others in parallel, such as when the need to access a
+     name server for one request causes a parallel resolve for the
+     name server's addresses, the spawned request should be started
+     with a lower counter.  This prevents circular references in
+     the database from starting a chain reaction of resolver
+     activity.
+
+   - The SLIST data structure discussed in [RFC-1034].
+
+     This structure keeps track of the state of a request if it
+     must wait for answers from foreign name servers.
+
+7.2. Sending the queries
+
+As described in [RFC-1034], the basic task of the resolver is to
+formulate a query which will answer the client's request and direct that
+query to name servers which can provide the information.  The resolver
+will usually only have very strong hints about which servers to ask, in
+the form of NS RRs, and may have to revise the query, in response to
+CNAMEs, or revise the set of name servers the resolver is asking, in
+response to delegation responses which point the resolver to name
+servers closer to the desired information.  In addition to the
+information requested by the client, the resolver may have to call upon
+its own services to determine the address of name servers it wishes to
+contact.
+
+In any case, the model used in this memo assumes that the resolver is
+multiplexing attention between multiple requests, some from the client,
+and some internally generated.  Each request is represented by some
+state information, and the desired behavior is that the resolver
+transmit queries to name servers in a way that maximizes the probability
+that the request is answered, minimizes the time that the request takes,
+and avoids excessive transmissions.  The key algorithm uses the state
+information of the request to select the next name server address to
+query, and also computes a timeout which will cause the next action
+should a response not arrive.  The next action will usually be a
+transmission to some other server, but may be a temporary error to the
+
+
+
+Mockapetris                                                    [Page 44]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+client.
+
+The resolver always starts with a list of server names to query (SLIST).
+This list will be all NS RRs which correspond to the nearest ancestor
+zone that the resolver knows about.  To avoid startup problems, the
+resolver should have a set of default servers which it will ask should
+it have no current NS RRs which are appropriate.  The resolver then adds
+to SLIST all of the known addresses for the name servers, and may start
+parallel requests to acquire the addresses of the servers when the
+resolver has the name, but no addresses, for the name servers.
+
+To complete initialization of SLIST, the resolver attaches whatever
+history information it has to the each address in SLIST.  This will
+usually consist of some sort of weighted averages for the response time
+of the address, and the batting average of the address (i.e., how often
+the address responded at all to the request).  Note that this
+information should be kept on a per address basis, rather than on a per
+name server basis, because the response time and batting average of a
+particular server may vary considerably from address to address.  Note
+also that this information is actually specific to a resolver address /
+server address pair, so a resolver with multiple addresses may wish to
+keep separate histories for each of its addresses.  Part of this step
+must deal with addresses which have no such history; in this case an
+expected round trip time of 5-10 seconds should be the worst case, with
+lower estimates for the same local network, etc.
+
+Note that whenever a delegation is followed, the resolver algorithm
+reinitializes SLIST.
+
+The information establishes a partial ranking of the available name
+server addresses.  Each time an address is chosen and the state should
+be altered to prevent its selection again until all other addresses have
+been tried.  The timeout for each transmission should be 50-100% greater
+than the average predicted value to allow for variance in response.
+
+Some fine points:
+
+   - The resolver may encounter a situation where no addresses are
+     available for any of the name servers named in SLIST, and
+     where the servers in the list are precisely those which would
+     normally be used to look up their own addresses.  This
+     situation typically occurs when the glue address RRs have a
+     smaller TTL than the NS RRs marking delegation, or when the
+     resolver caches the result of a NS search.  The resolver
+     should detect this condition and restart the search at the
+     next ancestor zone, or alternatively at the root.
+
+
+
+
+
+Mockapetris                                                    [Page 45]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+   - 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.
+
+
+
+
+Mockapetris                                                    [Page 46]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+7.4. Using the cache
+
+In general, we expect a resolver to cache all data which it receives in
+responses since it may be useful in answering future client requests.
+However, there are several types of data which should not be cached:
+
+   - When several RRs of the same type are available for a
+     particular owner name, the resolver should either cache them
+     all or none at all.  When a response is truncated, and a
+     resolver doesn't know whether it has a complete set, it should
+     not cache a possibly partial set of RRs.
+
+   - Cached data should never be used in preference to
+     authoritative data, so if caching would cause this to happen
+     the data should not be cached.
+
+   - The results of an inverse query should not be cached.
+
+   - The results of standard queries where the QNAME contains "*"
+     labels if the data might be used to construct wildcards.  The
+     reason is that the cache does not necessarily contain existing
+     RRs or zone boundary information which is necessary to
+     restrict the application of the wildcard RRs.
+
+   - RR data in responses of dubious reliability.  When a resolver
+     receives unsolicited responses or RR data other than that
+     requested, it should discard it without caching it.  The basic
+     implication is that all sanity checks on a packet should be
+     performed before any of it is cached.
+
+In a similar vein, when a resolver has a set of RRs for some name in a
+response, and wants to cache the RRs, it should check its cache for
+already existing RRs.  Depending on the circumstances, either the data
+in the response or the cache is preferred, but the two should never be
+combined.  If the data in the response is from authoritative data in the
+answer section, it is always preferred.
+
+8. MAIL SUPPORT
+
+The domain system defines a standard for mapping mailboxes into domain
+names, and two methods for using the mailbox information to derive mail
+routing information.  The first method is called mail exchange binding
+and the other method is mailbox binding.  The mailbox encoding standard
+and mail exchange binding are part of the DNS official protocol, and are
+the recommended method for mail routing in the Internet.  Mailbox
+binding is an experimental feature which is still under development and
+subject to change.
+
+
+
+
+Mockapetris                                                    [Page 47]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+The mailbox encoding standard assumes a mailbox name of the form
+"<local-part>@<mail-domain>".  While the syntax allowed in each of these
+sections varies substantially between the various mail internets, the
+preferred syntax for the ARPA Internet is given in [RFC-822].
+
+The DNS encodes the <local-part> as a single label, and encodes the
+<mail-domain> as a domain name.  The single label from the <local-part>
+is prefaced to the domain name from <mail-domain> to form the domain
+name corresponding to the mailbox.  Thus the mailbox HOSTMASTER@SRI-
+NIC.ARPA is mapped into the domain name HOSTMASTER.SRI-NIC.ARPA.  If the
+<local-part> contains dots or other special characters, its
+representation in a master file will require the use of backslash
+quoting to ensure that the domain name is properly encoded.  For
+example, the mailbox Action.domains@ISI.EDU would be represented as
+Action\.domains.ISI.EDU.
+
+8.1. Mail exchange binding
+
+Mail exchange binding uses the <mail-domain> part of a mailbox
+specification to determine where mail should be sent.  The <local-part>
+is not even consulted.  [RFC-974] specifies this method in detail, and
+should be consulted before attempting to use mail exchange support.
+
+One of the advantages of this method is that it decouples mail
+destination naming from the hosts used to support mail service, at the
+cost of another layer of indirection in the lookup function.  However,
+the addition layer should eliminate the need for complicated "%", "!",
+etc encodings in <local-part>.
+
+The essence of the method is that the <mail-domain> is used as a domain
+name to locate type MX RRs which list hosts willing to accept mail for
+<mail-domain>, together with preference values which rank the hosts
+according to an order specified by the administrators for <mail-domain>.
+
+In this memo, the <mail-domain> ISI.EDU is used in examples, together
+with the hosts VENERA.ISI.EDU and VAXA.ISI.EDU as mail exchanges for
+ISI.EDU.  If a mailer had a message for Mockapetris@ISI.EDU, it would
+route it by looking up MX RRs for ISI.EDU.  The MX RRs at ISI.EDU name
+VENERA.ISI.EDU and VAXA.ISI.EDU, and type A queries can find the host
+addresses.
+
+8.2. Mailbox binding (Experimental)
+
+In mailbox binding, the mailer uses the entire mail destination
+specification to construct a domain name.  The encoded domain name for
+the mailbox is used as the QNAME field in a QTYPE=MAILB query.
+
+Several outcomes are possible for this query:
+
+
+
+Mockapetris                                                    [Page 48]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+   1. The query can return a name error indicating that the mailbox
+      does not exist as a domain name.
+
+      In the long term, this would indicate that the specified
+      mailbox doesn't exist.  However, until the use of mailbox
+      binding is universal, this error condition should be
+      interpreted to mean that the organization identified by the
+      global part does not support mailbox binding.  The
+      appropriate procedure is to revert to exchange binding at
+      this point.
+
+   2. The query can return a Mail Rename (MR) RR.
+
+      The MR RR carries new mailbox specification in its RDATA
+      field.  The mailer should replace the old mailbox with the
+      new one and retry the operation.
+
+   3. The query can return a MB RR.
+
+      The MB RR carries a domain name for a host in its RDATA
+      field.  The mailer should deliver the message to that host
+      via whatever protocol is applicable, e.g., b,SMTP.
+
+   4. The query can return one or more Mail Group (MG) RRs.
+
+      This condition means that the mailbox was actually a mailing
+      list or mail group, rather than a single mailbox.  Each MG RR
+      has a RDATA field that identifies a mailbox that is a member
+      of the group.  The mailer should deliver a copy of the
+      message to each member.
+
+   5. The query can return a MB RR as well as one or more MG RRs.
+
+      This condition means the the mailbox was actually a mailing
+      list.  The mailer can either deliver the message to the host
+      specified by the MB RR, which will in turn do the delivery to
+      all members, or the mailer can use the MG RRs to do the
+      expansion itself.
+
+In any of these cases, the response may include a Mail Information
+(MINFO) RR.  This RR is usually associated with a mail group, but is
+legal with a MB.  The MINFO RR identifies two mailboxes.  One of these
+identifies a responsible person for the original mailbox name.  This
+mailbox should be used for requests to be added to a mail group, etc.
+The second mailbox name in the MINFO RR identifies a mailbox that should
+receive error messages for mail failures.  This is particularly
+appropriate for mailing lists when errors in member names should be
+reported to a person other than the one who sends a message to the list.
+
+
+
+Mockapetris                                                    [Page 49]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+New fields may be added to this RR in the future.
+
+
+9. REFERENCES and BIBLIOGRAPHY
+
+[Dyer 87]       S. Dyer, F. Hsu, "Hesiod", Project Athena
+                Technical Plan - Name Service, April 1987, version 1.9.
+
+                Describes the fundamentals of the Hesiod name service.
+
+[IEN-116]       J. Postel, "Internet Name Server", IEN-116,
+                USC/Information Sciences Institute, August 1979.
+
+                A name service obsoleted by the Domain Name System, but
+                still in use.
+
+[Quarterman 86] J. Quarterman, and J. Hoskins, "Notable Computer Networks",
+                Communications of the ACM, October 1986, volume 29, number
+                10.
+
+[RFC-742]       K. Harrenstien, "NAME/FINGER", RFC-742, Network
+                Information Center, SRI International, December 1977.
+
+[RFC-768]       J. Postel, "User Datagram Protocol", RFC-768,
+                USC/Information Sciences Institute, August 1980.
+
+[RFC-793]       J. Postel, "Transmission Control Protocol", RFC-793,
+                USC/Information Sciences Institute, September 1981.
+
+[RFC-799]       D. Mills, "Internet Name Domains", RFC-799, COMSAT,
+                September 1981.
+
+                Suggests introduction of a hierarchy in place of a flat
+                name space for the Internet.
+
+[RFC-805]       J. Postel, "Computer Mail Meeting Notes", RFC-805,
+                USC/Information Sciences Institute, February 1982.
+
+[RFC-810]       E. Feinler, K. Harrenstien, Z. Su, and V. White, "DOD
+                Internet Host Table Specification", RFC-810, Network
+                Information Center, SRI International, March 1982.
+
+                Obsolete.  See RFC-952.
+
+[RFC-811]       K. Harrenstien, V. White, and E. Feinler, "Hostnames
+                Server", RFC-811, Network Information Center, SRI
+                International, March 1982.
+
+
+
+
+Mockapetris                                                    [Page 50]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+                Obsolete.  See RFC-953.
+
+[RFC-812]       K. Harrenstien, and V. White, "NICNAME/WHOIS", RFC-812,
+                Network Information Center, SRI International, March
+                1982.
+
+[RFC-819]       Z. Su, and J. Postel, "The Domain Naming Convention for
+                Internet User Applications", RFC-819, Network
+                Information Center, SRI International, August 1982.
+
+                Early thoughts on the design of the domain system.
+                Current implementation is completely different.
+
+[RFC-821]       J. Postel, "Simple Mail Transfer Protocol", RFC-821,
+                USC/Information Sciences Institute, August 1980.
+
+[RFC-830]       Z. Su, "A Distributed System for Internet Name Service",
+                RFC-830, Network Information Center, SRI International,
+                October 1982.
+
+                Early thoughts on the design of the domain system.
+                Current implementation is completely different.
+
+[RFC-882]       P. Mockapetris, "Domain names - Concepts and
+                Facilities," RFC-882, USC/Information Sciences
+                Institute, November 1983.
+
+                Superceeded by this memo.
+
+[RFC-883]       P. Mockapetris, "Domain names - Implementation and
+                Specification," RFC-883, USC/Information Sciences
+                Institute, November 1983.
+
+                Superceeded by this memo.
+
+[RFC-920]       J. Postel and J. Reynolds, "Domain Requirements",
+                RFC-920, USC/Information Sciences Institute,
+                October 1984.
+
+                Explains the naming scheme for top level domains.
+
+[RFC-952]       K. Harrenstien, M. Stahl, E. Feinler, "DoD Internet Host
+                Table Specification", RFC-952, SRI, October 1985.
+
+                Specifies the format of HOSTS.TXT, the host/address
+                table replaced by the DNS.
+
+
+
+
+
+Mockapetris                                                    [Page 51]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+[RFC-953]       K. Harrenstien, M. Stahl, E. Feinler, "HOSTNAME Server",
+                RFC-953, SRI, October 1985.
+
+                This RFC contains the official specification of the
+                hostname server protocol, which is obsoleted by the DNS.
+                This TCP based protocol accesses information stored in
+                the RFC-952 format, and is used to obtain copies of the
+                host table.
+
+[RFC-973]       P. Mockapetris, "Domain System Changes and
+                Observations", RFC-973, USC/Information Sciences
+                Institute, January 1986.
+
+                Describes changes to RFC-882 and RFC-883 and reasons for
+                them.
+
+[RFC-974]       C. Partridge, "Mail routing and the domain system",
+                RFC-974, CSNET CIC BBN Labs, January 1986.
+
+                Describes the transition from HOSTS.TXT based mail
+                addressing to the more powerful MX system used with the
+                domain system.
+
+[RFC-1001]      NetBIOS Working Group, "Protocol standard for a NetBIOS
+                service on a TCP/UDP transport: Concepts and Methods",
+                RFC-1001, March 1987.
+
+                This RFC and RFC-1002 are a preliminary design for
+                NETBIOS on top of TCP/IP which proposes to base NetBIOS
+                name service on top of the DNS.
+
+[RFC-1002]      NetBIOS Working Group, "Protocol standard for a NetBIOS
+                service on a TCP/UDP transport: Detailed
+                Specifications", RFC-1002, March 1987.
+
+[RFC-1010]      J. Reynolds, and J. Postel, "Assigned Numbers", RFC-1010,
+                USC/Information Sciences Institute, May 1987.
+
+                Contains socket numbers and mnemonics for host names,
+                operating systems, etc.
+
+[RFC-1031]      W. Lazear, "MILNET Name Domain Transition", RFC-1031,
+                November 1987.
+
+                Describes a plan for converting the MILNET to the DNS.
+
+[RFC-1032]      M. Stahl, "Establishing a Domain - Guidelines for
+                Administrators", RFC-1032, November 1987.
+
+
+
+Mockapetris                                                    [Page 52]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+                Describes the registration policies used by the NIC to
+                administer the top level domains and delegate subzones.
+
+[RFC-1033]      M. Lottor, "Domain Administrators Operations Guide",
+                RFC-1033, November 1987.
+
+                A cookbook for domain administrators.
+
+[Solomon 82]    M. Solomon, L. Landweber, and D. Neuhengen, "The CSNET
+                Name Server", Computer Networks, vol 6, nr 3, July 1982.
+
+                Describes a name service for CSNET which is independent
+                from the DNS and DNS use in the CSNET.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris                                                    [Page 53]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+Index
+
+          *   13
+
+          ;   33, 35
+
+          <character-string>   35
+          <domain-name>   34
+
+          @   35
+
+          \   35
+
+          A   12
+
+          Byte order   8
+
+          CH   13
+          Character case   9
+          CLASS   11
+          CNAME   12
+          Completion   42
+          CS   13
+
+          Hesiod   13
+          HINFO   12
+          HS   13
+
+          IN   13
+          IN-ADDR.ARPA domain   22
+          Inverse queries   40
+
+          Mailbox names   47
+          MB   12
+          MD   12
+          MF   12
+          MG   12
+          MINFO   12
+          MINIMUM   20
+          MR   12
+          MX   12
+
+          NS   12
+          NULL   12
+
+          Port numbers   32
+          Primary server   5
+          PTR   12, 18
+
+
+
+Mockapetris                                                    [Page 54]
+
+RFC 1035        Domain Implementation and Specification    November 1987
+
+
+          QCLASS   13
+          QTYPE   12
+
+          RDATA   12
+          RDLENGTH  11
+
+          Secondary server   5
+          SOA   12
+          Stub resolvers   7
+
+          TCP   32
+          TXT   12
+          TYPE   11
+
+          UDP   32
+
+          WKS   12
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris                                                    [Page 55]
+
-- 
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