+Network Working Group P. Mockapetris
+Request for Comments: 1034 ISI
+Obsoletes: RFCs 882, 883, 973 November 1987
+
+
+ DOMAIN NAMES - CONCEPTS AND FACILITIES
+
+
+
+1. STATUS OF THIS MEMO
+
+This RFC is an introduction to the Domain Name System (DNS), and omits
+many details which can be found in a companion RFC, "Domain Names -
+Implementation and Specification" [RFC-1035]. That RFC assumes that the
+reader is familiar with the concepts discussed in this memo.
+
+A subset of DNS functions and data types constitute an official
+protocol. The official protocol includes standard queries and their
+responses and most of the Internet class data formats (e.g., host
+addresses).
+
+However, the domain system is intentionally extensible. Researchers are
+continuously proposing, implementing and experimenting with new data
+types, query types, classes, functions, etc. Thus while the components
+of the official protocol are expected to stay essentially unchanged and
+operate as a production service, experimental behavior should always be
+expected in extensions beyond the official protocol. 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.
+
+2. INTRODUCTION
+
+This RFC introduces domain style names, their use for Internet mail and
+host address support, and the protocols and servers used to implement
+domain name facilities.
+
+2.1. The history of domain names
+
+The impetus for the development of the domain system was growth in the
+Internet:
+
+ - Host name to address mappings were maintained by the Network
+ Information Center (NIC) in a single file (HOSTS.TXT) which
+ was FTPed by all hosts [RFC-952, RFC-953]. The total network
+
+
+
+Mockapetris [Page 1]
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+
+
+ bandwidth consumed in distributing a new version by this
+ scheme is proportional to the square of the number of hosts in
+ the network, and even when multiple levels of FTP are used,
+ the outgoing FTP load on the NIC host is considerable.
+ Explosive growth in the number of hosts didn't bode well for
+ the future.
+
+ - The network population was also changing in character. The
+ timeshared hosts that made up the original ARPANET were being
+ replaced with local networks of workstations. Local
+ organizations were administering their own names and
+ addresses, but had to wait for the NIC to change HOSTS.TXT to
+ make changes visible to the Internet at large. Organizations
+ also wanted some local structure on the name space.
+
+ - The applications on the Internet were getting more
+ sophisticated and creating a need for general purpose name
+ service.
+
+
+The result was several ideas about name spaces and their management
+[IEN-116, RFC-799, RFC-819, RFC-830]. The proposals varied, but a
+common thread was the idea of a hierarchical name space, with the
+hierarchy roughly corresponding to organizational structure, and names
+using "." as the character to mark the boundary between hierarchy
+levels. A design using a distributed database and generalized resources
+was described in [RFC-882, RFC-883]. Based on experience with several
+implementations, the system evolved into the scheme described in this
+memo.
+
+The terms "domain" or "domain name" are used in many contexts beyond the
+DNS described here. Very often, the term domain name is used to refer
+to a name with structure indicated by dots, but no relation to the DNS.
+This is particularly true in mail addressing [Quarterman 86].
+
+2.2. DNS design goals
+
+The design goals of the DNS influence its structure. They are:
+
+ - The primary goal is a consistent name space which will be used
+ for referring to resources. In order to avoid the problems
+ caused by ad hoc encodings, names should not be required to
+ contain network identifiers, addresses, routes, or similar
+ information as part of the name.
+
+ - The sheer size of the database and frequency of updates
+ suggest that it must be maintained in a distributed manner,
+ with local caching to improve performance. Approaches that
+
+
+
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+
+
+ attempt to collect a consistent copy of the entire database
+ will become more and more expensive and difficult, and hence
+ should be avoided. The same principle holds for the structure
+ of the name space, and in particular mechanisms for creating
+ and deleting names; these should also be distributed.
+
+ - Where there tradeoffs between the cost of acquiring data, the
+ speed of updates, and the accuracy of caches, the source of
+ the data should control the tradeoff.
+
+ - The costs of implementing such a facility dictate that it be
+ generally useful, and not restricted to a single application.
+ We should be able to use names to retrieve host addresses,
+ mailbox data, and other as yet undetermined information. All
+ data associated with a name is tagged with a type, and queries
+ can be limited to a single type.
+
+ - Because we want the name space to be useful in dissimilar
+ networks and applications, we provide the ability to use the
+ same name space with different protocol families or
+ management. For example, host address formats differ between
+ protocols, though all protocols have the notion of address.
+ The DNS tags all data with a class as well as the type, so
+ that we can allow parallel use of different formats for data
+ of type address.
+
+ - We want name server transactions to be independent of the
+ communications system that carries them. Some systems may
+ wish to use datagrams for queries and responses, and only
+ establish virtual circuits for transactions that need the
+ reliability (e.g., database updates, long transactions); other
+ systems will use virtual circuits exclusively.
+
+ - The system should be useful across a wide spectrum of host
+ capabilities. Both personal computers and large timeshared
+ hosts should be able to use the system, though perhaps in
+ different ways.
+
+2.3. Assumptions about usage
+
+The organization of the domain system derives from some assumptions
+about the needs and usage patterns of its user community and is designed
+to avoid many of the the complicated problems found in general purpose
+database systems.
+
+The assumptions are:
+
+ - The size of the total database will initially be proportional
+
+
+
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+
+
+ to the number of hosts using the system, but will eventually
+ grow to be proportional to the number of users on those hosts
+ as mailboxes and other information are added to the domain
+ system.
+
+ - Most of the data in the system will change very slowly (e.g.,
+ mailbox bindings, host addresses), but that the system should
+ be able to deal with subsets that change more rapidly (on the
+ order of seconds or minutes).
+
+ - The administrative boundaries used to distribute
+ responsibility for the database will usually correspond to
+ organizations that have one or more hosts. Each organization
+ that has responsibility for a particular set of domains will
+ provide redundant name servers, either on the organization's
+ own hosts or other hosts that the organization arranges to
+ use.
+
+ - Clients of the domain system should be able to identify
+ trusted name servers they prefer to use before accepting
+ referrals to name servers outside of this "trusted" set.
+
+ - Access to information is more critical than instantaneous
+ updates or guarantees of consistency. Hence the update
+ process allows updates to percolate out through the users of
+ the domain system rather than guaranteeing that all copies are
+ simultaneously updated. When updates are unavailable due to
+ network or host failure, the usual course is to believe old
+ information while continuing efforts to update it. The
+ general model is that copies are distributed with timeouts for
+ refreshing. The distributor sets the timeout value and the
+ recipient of the distribution is responsible for performing
+ the refresh. In special situations, very short intervals can
+ be specified, or the owner can prohibit copies.
+
+ - In any system that has a distributed database, a particular
+ name server may be presented with a query that can only be
+ answered by some other server. The two general approaches to
+ dealing with this problem are "recursive", in which the first
+ server pursues the query for the client at another server, and
+ "iterative", in which the server refers the client to another
+ server and lets the client pursue the query. Both approaches
+ have advantages and disadvantages, but the iterative approach
+ is preferred for the datagram style of access. The domain
+ system requires implementation of the iterative approach, but
+ allows the recursive approach as an option.
+
+
+
+
+
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+
+
+The domain system assumes that all data originates in master files
+scattered through the hosts that use the domain system. These master
+files are updated by local system administrators. Master files are text
+files that are read by a local name server, and hence become available
+through the name servers to users of the domain system. The user
+programs access name servers through standard programs called resolvers.
+
+The standard format of master files allows them to be exchanged between
+hosts (via FTP, mail, or some other mechanism); this facility is useful
+when an organization wants a domain, but doesn't want to support a name
+server. The organization can maintain the master files locally using a
+text editor, transfer them to a foreign host which runs a name server,
+and then arrange with the system administrator of the name server to get
+the files loaded.
+
+Each host's name servers and resolvers are configured by a local system
+administrator [RFC-1033]. For a name server, this configuration data
+includes the identity of local master files and instructions on which
+non-local master files are to be loaded from foreign servers. The name
+server uses the master files or copies to load its zones. For
+resolvers, the configuration data identifies the name servers which
+should be the primary sources of information.
+
+The domain system defines procedures for accessing the data and for
+referrals to other name servers. The domain system also defines
+procedures for caching retrieved data and for periodic refreshing of
+data defined by the system administrator.
+
+The system administrators provide:
+
+ - The definition of zone boundaries.
+
+ - Master files of data.
+
+ - Updates to master files.
+
+ - Statements of the refresh policies desired.
+
+The domain system provides:
+
+ - Standard formats for resource data.
+
+ - Standard methods for querying the database.
+
+ - Standard methods for name servers to refresh local data from
+ foreign name servers.
+
+
+
+
+
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+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+2.4. Elements of the DNS
+
+The DNS has three major components:
+
+ - The DOMAIN NAME SPACE and RESOURCE RECORDS, which are
+ specifications for a tree structured name space and data
+ associated with the names. Conceptually, each node and leaf
+ of the domain name space tree names a set of information, and
+ query operations are attempts to extract specific types of
+ information from a particular set. A query names the domain
+ name of interest and describes the type of resource
+ information that is desired. For example, the Internet
+ uses some of its domain names to identify hosts; queries for
+ address resources return Internet host addresses.
+
+ - NAME SERVERS are server programs which hold information about
+ the domain tree's structure and set information. A name
+ server may cache structure or set information about any part
+ of the domain tree, but in general a particular name server
+ has complete information about a subset of the domain space,
+ and pointers to other name servers that can be used to lead to
+ information from any part of the domain tree. Name servers
+ know the parts of the domain tree for which they have complete
+ information; a name server is said to be an AUTHORITY for
+ these parts of the name space. Authoritative information is
+ organized into units called ZONEs, and these zones can be
+ automatically distributed to the name servers which provide
+ redundant service for the data in a zone.
+
+ - RESOLVERS are programs that extract information from name
+ servers in response to client requests. Resolvers must be
+ able to access at least one name server and use that name
+ server's information to answer a query directly, or pursue the
+ query using referrals to other name servers. A resolver will
+ typically be a system routine that is directly accessible to
+ user programs; hence no protocol is necessary between the
+ resolver and the user program.
+
+These three components roughly correspond to the three layers or views
+of the domain system:
+
+ - From the user's point of view, the domain system is accessed
+ through a simple procedure or OS call to a local resolver.
+ The domain space consists of a single tree and the user can
+ request information from any section of the tree.
+
+ - From the resolver's point of view, the domain system is
+ composed of an unknown number of name servers. Each name
+
+
+
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+
+
+ server has one or more pieces of the whole domain tree's data,
+ but the resolver views each of these databases as essentially
+ static.
+
+ - From a name server's point of view, the domain system consists
+ of separate sets of local information called zones. The name
+ server has local copies of some of the zones. The name server
+ must periodically refresh its zones from master copies in
+ local files or foreign name servers. The name server must
+ concurrently process queries that arrive from resolvers.
+
+In the interests of performance, implementations may couple these
+functions. For example, a resolver on the same machine as a name server
+might share a database consisting of the the zones managed by the name
+server and the cache managed by the resolver.
+
+3. DOMAIN NAME SPACE and RESOURCE RECORDS
+
+3.1. Name space specifications and terminology
+
+The domain name space is a tree structure. Each node and leaf on the
+tree corresponds to a resource set (which may be empty). The domain
+system makes no distinctions between the uses of the interior nodes and
+leaves, and this memo uses the term "node" to refer to both.
+
+Each node has a label, which is zero to 63 octets in length. Brother
+nodes may not have the same label, although the same label can be used
+for nodes which are not brothers. One label is reserved, and that is
+the null (i.e., zero length) label used for the root.
+
+The domain name of a node is the list of the labels on the path from the
+node to the root of the tree. By convention, the labels that compose a
+domain name are printed or read left to right, from the most specific
+(lowest, farthest from the root) to the least specific (highest, closest
+to the root).
+
+Internally, programs that manipulate domain names should represent them
+as sequences of labels, where each label is a length octet followed by
+an octet string. Because all domain names end at the root, which has a
+null string for a label, these internal representations can use a length
+byte of zero to terminate a domain name.
+
+By convention, domain names can be stored with arbitrary case, but
+domain name comparisons for all present domain functions are done in a
+case-insensitive manner, assuming an ASCII character set, and a high
+order zero bit. This means that you are free to create a node with
+label "A" or a node with label "a", but not both as brothers; you could
+refer to either using "a" or "A". When you receive a domain name or
+
+
+
+Mockapetris [Page 7]
+\f
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+
+
+label, you should preserve its case. The rationale for this choice is
+that we may someday need to add full binary domain names for new
+services; existing services would not be changed.
+
+When a user needs to type a domain name, the length of each label is
+omitted and the labels are separated by dots ("."). Since a complete
+domain name ends with the root label, this leads to a printed form which
+ends in a dot. We use this property to distinguish between:
+
+ - a character string which represents a complete domain name
+ (often called "absolute"). For example, "poneria.ISI.EDU."
+
+ - a character string that represents the starting labels of a
+ domain name which is incomplete, and should be completed by
+ local software using knowledge of the local domain (often
+ called "relative"). For example, "poneria" used in the
+ ISI.EDU domain.
+
+Relative names are either taken relative to a well known origin, or to a
+list of domains used as a search list. Relative names appear mostly at
+the user interface, where their interpretation varies from
+implementation to implementation, and in master files, where they are
+relative to a single origin domain name. The most common interpretation
+uses the root "." as either the single origin or as one of the members
+of the search list, so a multi-label relative name is often one where
+the trailing dot has been omitted to save typing.
+
+To simplify implementations, the total number of octets that represent a
+domain name (i.e., the sum of all label octets and label lengths) is
+limited to 255.
+
+A domain is identified by a domain name, and consists of that part of
+the domain name space that is at or below the domain name which
+specifies the domain. A domain is a subdomain of another domain if it
+is contained within that domain. This relationship can be tested by
+seeing if the subdomain's name ends with the containing domain's name.
+For example, A.B.C.D is a subdomain of B.C.D, C.D, D, and " ".
+
+3.2. Administrative guidelines on use
+
+As a matter of policy, the DNS technical specifications do not mandate a
+particular tree structure or rules for selecting labels; its goal is to
+be as general as possible, so that it can be used to build arbitrary
+applications. In particular, the system was designed so that the name
+space did not have to be organized along the lines of network
+boundaries, name servers, etc. The rationale for this is not that the
+name space should have no implied semantics, but rather that the choice
+of implied semantics should be left open to be used for the problem at
+
+
+
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+
+
+hand, and that different parts of the tree can have different implied
+semantics. For example, the IN-ADDR.ARPA domain is organized and
+distributed by network and host address because its role is to translate
+from network or host numbers to names; NetBIOS domains [RFC-1001, RFC-
+1002] are flat because that is appropriate for that application.
+
+However, there are some guidelines that apply to the "normal" parts of
+the name space used for hosts, mailboxes, etc., that will make the name
+space more uniform, provide for growth, and minimize problems as
+software is converted from the older host table. The political
+decisions about the top levels of the tree originated in RFC-920.
+Current policy for the top levels is discussed in [RFC-1032]. MILNET
+conversion issues are covered in [RFC-1031].
+
+Lower domains which will eventually be broken into multiple zones should
+provide branching at the top of the domain so that the eventual
+decomposition can be done without renaming. Node labels which use
+special characters, leading digits, etc., are likely to break older
+software which depends on more restrictive choices.
+
+3.3. Technical guidelines on use
+
+Before the DNS can be used to hold naming information for some kind of
+object, two needs must be met:
+
+ - A convention for mapping between object names and domain
+ names. This describes how information about an object is
+ accessed.
+
+ - RR types and data formats for describing the object.
+
+These rules can be quite simple or fairly complex. Very often, the
+designer must take into account existing formats and plan for upward
+compatibility for existing usage. Multiple mappings or levels of
+mapping may be required.
+
+For hosts, the mapping depends on the existing syntax for host names
+which is a subset of the usual text representation for domain names,
+together with RR formats for describing host addresses, etc. Because we
+need a reliable inverse mapping from address to host name, a special
+mapping for addresses into the IN-ADDR.ARPA domain is also defined.
+
+For mailboxes, the mapping is slightly more complex. The usual mail
+address <local-part>@<mail-domain> is mapped into a domain name by
+converting <local-part> into a single label (regardles of dots it
+contains), converting <mail-domain> into a domain name using the usual
+text format for domain names (dots denote label breaks), and
+concatenating the two to form a single domain name. Thus the mailbox
+
+
+
+Mockapetris [Page 9]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+HOSTMASTER@SRI-NIC.ARPA is represented as a domain name by
+HOSTMASTER.SRI-NIC.ARPA. An appreciation for the reasons behind this
+design also must take into account the scheme for mail exchanges [RFC-
+974].
+
+The typical user is not concerned with defining these rules, but should
+understand that they usually are the result of numerous compromises
+between desires for upward compatibility with old usage, interactions
+between different object definitions, and the inevitable urge to add new
+features when defining the rules. The way the DNS is used to support
+some object is often more crucial than the restrictions inherent in the
+DNS.
+
+3.4. Example name space
+
+The following figure shows a part of the current domain name space, and
+is used in many examples in this RFC. Note that the tree is a very
+small subset of the actual name space.
+
+ |
+ |
+ +---------------------+------------------+
+ | | |
+ MIL EDU ARPA
+ | | |
+ | | |
+ +-----+-----+ | +------+-----+-----+
+ | | | | | | |
+ BRL NOSC DARPA | IN-ADDR SRI-NIC ACC
+ |
+ +--------+------------------+---------------+--------+
+ | | | | |
+ UCI MIT | UDEL YALE
+ | ISI
+ | |
+ +---+---+ |
+ | | |
+ LCS ACHILLES +--+-----+-----+--------+
+ | | | | | |
+ XX A C VAXA VENERA Mockapetris
+
+In this example, the root domain has three immediate subdomains: MIL,
+EDU, and ARPA. The LCS.MIT.EDU domain has one immediate subdomain named
+XX.LCS.MIT.EDU. All of the leaves are also domains.
+
+3.5. Preferred name syntax
+
+The DNS specifications attempt to be as general as possible in the rules
+
+
+
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+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+for constructing domain names. The idea is that the name of any
+existing object can be expressed as a domain name with minimal changes.
+However, when assigning a domain name for an object, the prudent user
+will select a name which satisfies both the rules of the domain system
+and any existing rules for the object, whether these rules are published
+or implied by existing programs.
+
+For example, when naming a mail domain, the user should satisfy both the
+rules of this memo and those in RFC-822. When creating a new host name,
+the old rules for HOSTS.TXT should be followed. This avoids problems
+when old software is converted to use domain names.
+
+The following syntax will result in fewer problems with many
+applications that use domain names (e.g., mail, TELNET).
+
+<domain> ::= <subdomain> | " "
+
+<subdomain> ::= <label> | <subdomain> "." <label>
+
+<label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
+
+<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
+
+<let-dig-hyp> ::= <let-dig> | "-"
+
+<let-dig> ::= <letter> | <digit>
+
+<letter> ::= any one of the 52 alphabetic characters A through Z in
+upper case and a through z in lower case
+
+<digit> ::= any one of the ten digits 0 through 9
+
+Note that while upper and lower case letters are allowed in domain
+names, no significance is attached to the case. That is, two names with
+the same spelling but different case are to be treated as if identical.
+
+The labels must follow the rules for ARPANET host names. They must
+start with a letter, end with a letter or digit, and have as interior
+characters only letters, digits, and hyphen. There are also some
+restrictions on the length. Labels must be 63 characters or less.
+
+For example, the following strings identify hosts in the Internet:
+
+A.ISI.EDU XX.LCS.MIT.EDU SRI-NIC.ARPA
+
+3.6. Resource Records
+
+A domain name identifies a node. Each node has a set of resource
+
+
+
+Mockapetris [Page 11]
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+
+
+information, which may be empty. The set of resource information
+associated with a particular name is composed of separate resource
+records (RRs). The order of RRs in a set is not significant, and need
+not be preserved by name servers, resolvers, or other parts of the DNS.
+
+When we talk about a specific RR, we assume it has the following:
+
+owner which is the domain name where the RR is found.
+
+type which is an encoded 16 bit value that specifies the type
+ of the resource in this resource record. Types refer to
+ abstract resources.
+
+ This memo uses the following types:
+
+ A a host address
+
+ CNAME identifies the canonical name of an
+ alias
+
+ HINFO identifies the CPU and OS used by a host
+
+ MX identifies a mail exchange for the
+ domain. See [RFC-974 for details.
+
+ NS
+ the authoritative name server for the domain
+
+ PTR
+ a pointer to another part of the domain name space
+
+ SOA
+ identifies the start of a zone of authority]
+
+class which is an encoded 16 bit value which identifies a
+ protocol family or instance of a protocol.
+
+ This memo uses the following classes:
+
+ IN the Internet system
+
+ CH the Chaos system
+
+TTL which is the time to live of the RR. This field is a 32
+ bit integer in units of seconds, an is primarily used by
+ resolvers when they cache RRs. The TTL describes how
+ long a RR can be cached before it should be discarded.
+
+
+
+
+Mockapetris [Page 12]
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+
+
+RDATA which is the type and sometimes class dependent data
+ which describes the resource:
+
+ A For the IN class, a 32 bit IP address
+
+ For the CH class, a domain name followed
+ by a 16 bit octal Chaos address.
+
+ CNAME a domain name.
+
+ MX a 16 bit preference value (lower is
+ better) followed by a host name willing
+ to act as a mail exchange for the owner
+ domain.
+
+ NS a host name.
+
+ PTR a domain name.
+
+ SOA several fields.
+
+The owner name is often implicit, rather than forming an integral part
+of the RR. For example, many name servers internally form tree or hash
+structures for the name space, and chain RRs off nodes. The remaining
+RR parts are the fixed header (type, class, TTL) which is consistent for
+all RRs, and a variable part (RDATA) that fits the needs of the resource
+being described.
+
+The meaning of the TTL field is a time limit on how long an RR can be
+kept in a cache. This limit does not apply to authoritative data in
+zones; it is also timed out, but by the refreshing policies for the
+zone. The TTL is assigned by the administrator for the zone where the
+data originates. While short TTLs can be used to minimize caching, and
+a zero TTL prohibits caching, the realities of Internet performance
+suggest that these times should be on the order of days for the typical
+host. If a change can be anticipated, the TTL can be reduced prior to
+the change to minimize inconsistency during the change, and then
+increased back to its former value following the change.
+
+The data in the RDATA section of RRs is carried as a combination of
+binary strings and domain names. The domain names are frequently used
+as "pointers" to other data in the DNS.
+
+3.6.1. Textual expression of RRs
+
+RRs are represented in binary form in the packets of the DNS protocol,
+and are usually represented in highly encoded form when stored in a name
+server or resolver. In this memo, we adopt a style similar to that used
+
+
+
+Mockapetris [Page 13]
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+
+
+in master files in order to show the contents of RRs. In this format,
+most RRs are shown on a single line, although continuation lines are
+possible using parentheses.
+
+The start of the line gives the owner of the RR. If a line begins with
+a blank, then the owner is assumed to be the same as that of the
+previous RR. Blank lines are often included for readability.
+
+Following the owner, we list the TTL, type, and class of the RR. Class
+and type use the mnemonics defined above, and TTL is an integer before
+the type field. In order to avoid ambiguity in parsing, type and class
+mnemonics are disjoint, TTLs are integers, and the type mnemonic is
+always last. The IN class and TTL values are often omitted from examples
+in the interests of clarity.
+
+The resource data or RDATA section of the RR are given using knowledge
+of the typical representation for the data.
+
+For example, we might show the RRs carried in a message as:
+
+ ISI.EDU. MX 10 VENERA.ISI.EDU.
+ MX 10 VAXA.ISI.EDU.
+ VENERA.ISI.EDU. A 128.9.0.32
+ A 10.1.0.52
+ VAXA.ISI.EDU. A 10.2.0.27
+ A 128.9.0.33
+
+The MX RRs have an RDATA section which consists of a 16 bit number
+followed by a domain name. The address RRs use a standard IP address
+format to contain a 32 bit internet address.
+
+This example shows six RRs, with two RRs at each of three domain names.
+
+Similarly we might see:
+
+ XX.LCS.MIT.EDU. IN A 10.0.0.44
+ CH A MIT.EDU. 2420
+
+This example shows two addresses for XX.LCS.MIT.EDU, each of a different
+class.
+
+3.6.2. Aliases and canonical names
+
+In existing systems, hosts and other resources often have several names
+that identify the same resource. For example, the names C.ISI.EDU and
+USC-ISIC.ARPA both identify the same host. Similarly, in the case of
+mailboxes, many organizations provide many names that actually go to the
+same mailbox; for example Mockapetris@C.ISI.EDU, Mockapetris@B.ISI.EDU,
+
+
+
+Mockapetris [Page 14]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+and PVM@ISI.EDU all go to the same mailbox (although the mechanism
+behind this is somewhat complicated).
+
+Most of these systems have a notion that one of the equivalent set of
+names is the canonical or primary name and all others are aliases.
+
+The domain system provides such a feature using the canonical name
+(CNAME) RR. A CNAME RR identifies its owner name as an alias, and
+specifies the corresponding canonical name in the RDATA section of the
+RR. If a CNAME RR is present at a node, no other data should be
+present; this ensures that the data for a canonical name and its aliases
+cannot be different. This rule also insures that a cached CNAME can be
+used without checking with an authoritative server for other RR types.
+
+CNAME RRs cause special action in DNS software. When a name server
+fails to find a desired RR in the resource set associated with the
+domain name, it checks to see if the resource set consists of a CNAME
+record with a matching class. If so, the name server includes the CNAME
+record in the response and restarts the query at the domain name
+specified in the data field of the CNAME record. The one exception to
+this rule is that queries which match the CNAME type are not restarted.
+
+For example, suppose a name server was processing a query with for USC-
+ISIC.ARPA, asking for type A information, and had the following resource
+records:
+
+ USC-ISIC.ARPA IN CNAME C.ISI.EDU
+
+ C.ISI.EDU IN A 10.0.0.52
+
+Both of these RRs would be returned in the response to the type A query,
+while a type CNAME or * query should return just the CNAME.
+
+Domain names in RRs which point at another name should always point at
+the primary name and not the alias. This avoids extra indirections in
+accessing information. For example, the address to name RR for the
+above host should be:
+
+ 52.0.0.10.IN-ADDR.ARPA IN PTR C.ISI.EDU
+
+rather than pointing at USC-ISIC.ARPA. Of course, by the robustness
+principle, domain software should not fail when presented with CNAME
+chains or loops; CNAME chains should be followed and CNAME loops
+signalled as an error.
+
+3.7. Queries
+
+Queries are messages which may be sent to a name server to provoke a
+
+
+
+Mockapetris [Page 15]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+response. In the Internet, queries are carried in UDP datagrams or over
+TCP connections. The response by the name server either answers the
+question posed in the query, refers the requester to another set of name
+servers, or signals some error condition.
+
+In general, the user does not generate queries directly, but instead
+makes a request to a resolver which in turn sends one or more queries to
+name servers and deals with the error conditions and referrals that may
+result. Of course, the possible questions which can be asked in a query
+does shape the kind of service a resolver can provide.
+
+DNS queries and responses are carried in a standard message format. The
+message format has a header containing a number of fixed fields which
+are always present, and four sections which carry query parameters and
+RRs.
+
+The most important field in the header is a four bit field called an
+opcode which separates different queries. Of the possible 16 values,
+one (standard query) is part of the official protocol, two (inverse
+query and status query) are options, one (completion) is obsolete, and
+the rest are unassigned.
+
+The four sections are:
+
+Question Carries the query name and other query parameters.
+
+Answer Carries RRs which directly answer the query.
+
+Authority Carries RRs which describe other authoritative servers.
+ May optionally carry the SOA RR for the authoritative
+ data in the answer section.
+
+Additional Carries RRs which may be helpful in using the RRs in the
+ other sections.
+
+Note that the content, but not the format, of these sections varies with
+header opcode.
+
+3.7.1. Standard queries
+
+A standard query specifies a target domain name (QNAME), query type
+(QTYPE), and query class (QCLASS) and asks for RRs which match. This
+type of query makes up such a vast majority of DNS queries that we use
+the term "query" to mean standard query unless otherwise specified. The
+QTYPE and QCLASS fields are each 16 bits long, and are a superset of
+defined types and classes.
+
+
+
+
+
+Mockapetris [Page 16]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+The QTYPE field may contain:
+
+<any type> matches just that type. (e.g., A, PTR).
+
+AXFR special zone transfer QTYPE.
+
+MAILB matches all mail box related RRs (e.g. MB and MG).
+
+* matches all RR types.
+
+The QCLASS field may contain:
+
+<any class> matches just that class (e.g., IN, CH).
+
+* matches aLL RR classes.
+
+Using the query domain name, QTYPE, and QCLASS, the name server looks
+for matching RRs. In addition to relevant records, the name server may
+return RRs that point toward a name server that has the desired
+information or RRs that are expected to be useful in interpreting the
+relevant RRs. For example, a name server that doesn't have the
+requested information may know a name server that does; a name server
+that returns a domain name in a relevant RR may also return the RR that
+binds that domain name to an address.
+
+For example, a mailer tying to send mail to Mockapetris@ISI.EDU might
+ask the resolver for mail information about ISI.EDU, resulting in a
+query for QNAME=ISI.EDU, QTYPE=MX, QCLASS=IN. The response's answer
+section would be:
+
+ ISI.EDU. MX 10 VENERA.ISI.EDU.
+ MX 10 VAXA.ISI.EDU.
+
+while the additional section might be:
+
+ VAXA.ISI.EDU. A 10.2.0.27
+ A 128.9.0.33
+ VENERA.ISI.EDU. A 10.1.0.52
+ A 128.9.0.32
+
+Because the server assumes that if the requester wants mail exchange
+information, it will probably want the addresses of the mail exchanges
+soon afterward.
+
+Note that the QCLASS=* construct requires special interpretation
+regarding authority. Since a particular name server may not know all of
+the classes available in the domain system, it can never know if it is
+authoritative for all classes. Hence responses to QCLASS=* queries can
+
+
+
+Mockapetris [Page 17]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+never be authoritative.
+
+3.7.2. Inverse queries (Optional)
+
+Name servers may also support inverse queries that map a particular
+resource to a domain name or domain names that have that resource. For
+example, while a standard query might map a domain name to a SOA RR, the
+corresponding inverse query might map the SOA RR back to the domain
+name.
+
+Implementation of this service is optional in a name server, but all
+name servers must at least be able to understand an inverse query
+message and return a not-implemented error response.
+
+The domain system cannot guarantee the completeness or uniqueness of
+inverse queries because the domain system is organized by domain name
+rather than by host address or any other resource type. Inverse queries
+are primarily useful for debugging and database maintenance activities.
+
+Inverse queries may not return the proper TTL, and do not indicate cases
+where the identified RR is one of a set (for example, one address for a
+host having multiple addresses). Therefore, the RRs returned in inverse
+queries should never be cached.
+
+Inverse queries are NOT an acceptable method for mapping host addresses
+to host names; use the IN-ADDR.ARPA domain instead.
+
+A detailed discussion of inverse queries is contained in [RFC-1035].
+
+3.8. Status queries (Experimental)
+
+To be defined.
+
+3.9. Completion queries (Obsolete)
+
+The optional completion services described in RFCs 882 and 883 have been
+deleted. Redesigned services may become available in the future, or the
+opcodes may be reclaimed for other use.
+
+4. NAME SERVERS
+
+4.1. Introduction
+
+Name servers are the repositories of information that make up the domain
+database. The database is divided up into sections called zones, which
+are distributed among the name servers. While name servers can have
+several optional functions and sources of data, the essential task of a
+name server is to answer queries using data in its zones. By design,
+
+
+
+Mockapetris [Page 18]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+name servers can answer queries in a simple manner; the response can
+always be generated using only local data, and either contains the
+answer to the question or a referral to other name servers "closer" to
+the desired information.
+
+A given zone will be available from several name servers to insure its
+availability in spite of host or communication link failure. By
+administrative fiat, we require every zone to be available on at least
+two servers, and many zones have more redundancy than that.
+
+A given name server will typically support one or more zones, but this
+gives it authoritative information about only a small section of the
+domain tree. It may also have some cached non-authoritative data about
+other parts of the tree. The name server marks its responses to queries
+so that the requester can tell whether the response comes from
+authoritative data or not.
+
+4.2. How the database is divided into zones
+
+The domain database is partitioned in two ways: by class, and by "cuts"
+made in the name space between nodes.
+
+The class partition is simple. The database for any class is organized,
+delegated, and maintained separately from all other classes. Since, by
+convention, the name spaces are the same for all classes, the separate
+classes can be thought of as an array of parallel namespace trees. Note
+that the data attached to nodes will be different for these different
+parallel classes. The most common reasons for creating a new class are
+the necessity for a new data format for existing types or a desire for a
+separately managed version of the existing name space.
+
+Within a class, "cuts" in the name space can be made between any two
+adjacent nodes. After all cuts are made, each group of connected name
+space is a separate zone. The zone is said to be authoritative for all
+names in the connected region. Note that the "cuts" in the name space
+may be in different places for different classes, the name servers may
+be different, etc.
+
+These rules mean that every zone has at least one node, and hence domain
+name, for which it is authoritative, and all of the nodes in a
+particular zone are connected. Given, the tree structure, every zone
+has a highest node which is closer to the root than any other node in
+the zone. The name of this node is often used to identify the zone.
+
+It would be possible, though not particularly useful, to partition the
+name space so that each domain name was in a separate zone or so that
+all nodes were in a single zone. Instead, the database is partitioned
+at points where a particular organization wants to take over control of
+
+
+
+Mockapetris [Page 19]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+a subtree. Once an organization controls its own zone it can
+unilaterally change the data in the zone, grow new tree sections
+connected to the zone, delete existing nodes, or delegate new subzones
+under its zone.
+
+If the organization has substructure, it may want to make further
+internal partitions to achieve nested delegations of name space control.
+In some cases, such divisions are made purely to make database
+maintenance more convenient.
+
+4.2.1. Technical considerations
+
+The data that describes a zone has four major parts:
+
+ - Authoritative data for all nodes within the zone.
+
+ - Data that defines the top node of the zone (can be thought of
+ as part of the authoritative data).
+
+ - Data that describes delegated subzones, i.e., cuts around the
+ bottom of the zone.
+
+ - Data that allows access to name servers for subzones
+ (sometimes called "glue" data).
+
+All of this data is expressed in the form of RRs, so a zone can be
+completely described in terms of a set of RRs. Whole zones can be
+transferred between name servers by transferring the RRs, either carried
+in a series of messages or by FTPing a master file which is a textual
+representation.
+
+The authoritative data for a zone is simply all of the RRs attached to
+all of the nodes from the top node of the zone down to leaf nodes or
+nodes above cuts around the bottom edge of the zone.
+
+Though logically part of the authoritative data, the RRs that describe
+the top node of the zone are especially important to the zone's
+management. These RRs are of two types: name server RRs that list, one
+per RR, all of the servers for the zone, and a single SOA RR that
+describes zone management parameters.
+
+The RRs that describe cuts around the bottom of the zone are NS RRs that
+name the servers for the subzones. Since the cuts are between nodes,
+these RRs are NOT part of the authoritative data of the zone, and should
+be exactly the same as the corresponding RRs in the top node of the
+subzone. Since name servers are always associated with zone boundaries,
+NS RRs are only found at nodes which are the top node of some zone. In
+the data that makes up a zone, NS RRs are found at the top node of the
+
+
+
+Mockapetris [Page 20]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+zone (and are authoritative) and at cuts around the bottom of the zone
+(where they are not authoritative), but never in between.
+
+One of the goals of the zone structure is that any zone have all the
+data required to set up communications with the name servers for any
+subzones. That is, parent zones have all the information needed to
+access servers for their children zones. The NS RRs that name the
+servers for subzones are often not enough for this task since they name
+the servers, but do not give their addresses. In particular, if the
+name of the name server is itself in the subzone, we could be faced with
+the situation where the NS RRs tell us that in order to learn a name
+server's address, we should contact the server using the address we wish
+to learn. To fix this problem, a zone contains "glue" RRs which are not
+part of the authoritative data, and are address RRs for the servers.
+These RRs are only necessary if the name server's name is "below" the
+cut, and are only used as part of a referral response.
+
+4.2.2. Administrative considerations
+
+When some organization wants to control its own domain, the first step
+is to identify the proper parent zone, and get the parent zone's owners
+to agree to the delegation of control. While there are no particular
+technical constraints dealing with where in the tree this can be done,
+there are some administrative groupings discussed in [RFC-1032] which
+deal with top level organization, and middle level zones are free to
+create their own rules. For example, one university might choose to use
+a single zone, while another might choose to organize by subzones
+dedicated to individual departments or schools. [RFC-1033] catalogs
+available DNS software an discusses administration procedures.
+
+Once the proper name for the new subzone is selected, the new owners
+should be required to demonstrate redundant name server support. Note
+that there is no requirement that the servers for a zone reside in a
+host which has a name in that domain. In many cases, a zone will be
+more accessible to the internet at large if its servers are widely
+distributed rather than being within the physical facilities controlled
+by the same organization that manages the zone. For example, in the
+current DNS, one of the name servers for the United Kingdom, or UK
+domain, is found in the US. This allows US hosts to get UK data without
+using limited transatlantic bandwidth.
+
+As the last installation step, the delegation NS RRs and glue RRs
+necessary to make the delegation effective should be added to the parent
+zone. The administrators of both zones should insure that the NS and
+glue RRs which mark both sides of the cut are consistent and remain so.
+
+4.3. Name server internals
+
+
+
+
+Mockapetris [Page 21]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+4.3.1. Queries and responses
+
+The principal activity of name servers is to answer standard queries.
+Both the query and its response are carried in a standard message format
+which is described in [RFC-1035]. The query contains a QTYPE, QCLASS,
+and QNAME, which describe the types and classes of desired information
+and the name of interest.
+
+The way that the name server answers the query depends upon whether it
+is operating in recursive mode or not:
+
+ - The simplest mode for the server is non-recursive, since it
+ can answer queries using only local information: the response
+ contains an error, the answer, or a referral to some other
+ server "closer" to the answer. All name servers must
+ implement non-recursive queries.
+
+ - The simplest mode for the client is recursive, since in this
+ mode the name server acts in the role of a resolver and
+ returns either an error or the answer, but never referrals.
+ This service is optional in a name server, and the name server
+ may also choose to restrict the clients which can use
+ recursive mode.
+
+Recursive service is helpful in several situations:
+
+ - a relatively simple requester that lacks the ability to use
+ anything other than a direct answer to the question.
+
+ - a request that needs to cross protocol or other boundaries and
+ can be sent to a server which can act as intermediary.
+
+ - a network where we want to concentrate the cache rather than
+ having a separate cache for each client.
+
+Non-recursive service is appropriate if the requester is capable of
+pursuing referrals and interested in information which will aid future
+requests.
+
+The use of recursive mode is limited to cases where both the client and
+the name server agree to its use. The agreement is negotiated through
+the use of two bits in query and response messages:
+
+ - The recursion available, or RA bit, is set or cleared by a
+ name server in all responses. The bit is true if the name
+ server is willing to provide recursive service for the client,
+ regardless of whether the client requested recursive service.
+ That is, RA signals availability rather than use.
+
+
+
+Mockapetris [Page 22]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+ - Queries contain a bit called recursion desired or RD. This
+ bit specifies specifies whether the requester wants recursive
+ service for this query. Clients may request recursive service
+ from any name server, though they should depend upon receiving
+ it only from servers which have previously sent an RA, or
+ servers which have agreed to provide service through private
+ agreement or some other means outside of the DNS protocol.
+
+The recursive mode occurs when a query with RD set arrives at a server
+which is willing to provide recursive service; the client can verify
+that recursive mode was used by checking that both RA and RD are set in
+the reply. Note that the name server should never perform recursive
+service unless asked via RD, since this interferes with trouble shooting
+of name servers and their databases.
+
+If recursive service is requested and available, the recursive response
+to a query will be one of the following:
+
+ - The answer to the query, possibly preface by one or more CNAME
+ RRs that specify aliases encountered on the way to an answer.
+
+ - A name error indicating that the name does not exist. This
+ may include CNAME RRs that indicate that the original query
+ name was an alias for a name which does not exist.
+
+ - A temporary error indication.
+
+If recursive service is not requested or is not available, the non-
+recursive response will be one of the following:
+
+ - An authoritative name error indicating that the name does not
+ exist.
+
+ - A temporary error indication.
+
+ - Some combination of:
+
+ RRs that answer the question, together with an indication
+ whether the data comes from a zone or is cached.
+
+ A referral to name servers which have zones which are closer
+ ancestors to the name than the server sending the reply.
+
+ - RRs that the name server thinks will prove useful to the
+ requester.
+
+
+
+
+
+
+Mockapetris [Page 23]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+4.3.2. Algorithm
+
+The actual algorithm used by the name server will depend on the local OS
+and data structures used to store RRs. The following algorithm assumes
+that the RRs are organized in several tree structures, one for each
+zone, and another for the cache:
+
+ 1. Set or clear the value of recursion available in the response
+ depending on whether the name server is willing to provide
+ recursive service. If recursive service is available and
+ requested via the RD bit in the query, go to step 5,
+ otherwise step 2.
+
+ 2. Search the available zones for the zone which is the nearest
+ ancestor to QNAME. If such a zone is found, go to step 3,
+ otherwise step 4.
+
+ 3. Start matching down, label by label, in the zone. The
+ matching process can terminate several ways:
+
+ a. If the whole of QNAME is matched, we have found the
+ node.
+
+ If the data at the node is a CNAME, and QTYPE doesn't
+ match CNAME, copy the CNAME RR into the answer section
+ of the response, change QNAME to the canonical name in
+ the CNAME RR, and go back to step 1.
+
+ Otherwise, copy all RRs which match QTYPE into the
+ answer section and go to step 6.
+
+ b. If a match would take us out of the authoritative data,
+ we have a referral. This happens when we encounter a
+ node with NS RRs marking cuts along the bottom of a
+ zone.
+
+ Copy the NS RRs for the subzone into the authority
+ section of the reply. Put whatever addresses are
+ available into the additional section, using glue RRs
+ if the addresses are not available from authoritative
+ data or the cache. Go to step 4.
+
+ c. If at some label, a match is impossible (i.e., the
+ corresponding label does not exist), look to see if a
+ the "*" label exists.
+
+ If the "*" label does not exist, check whether the name
+ we are looking for is the original QNAME in the query
+
+
+
+Mockapetris [Page 24]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+ or a name we have followed due to a CNAME. If the name
+ is original, set an authoritative name error in the
+ response and exit. Otherwise just exit.
+
+ If the "*" label does exist, match RRs at that node
+ against QTYPE. If any match, copy them into the answer
+ section, but set the owner of the RR to be QNAME, and
+ not the node with the "*" label. Go to step 6.
+
+ 4. Start matching down in the cache. If QNAME is found in the
+ cache, copy all RRs attached to it that match QTYPE into the
+ answer section. If there was no delegation from
+ authoritative data, look for the best one from the cache, and
+ put it in the authority section. Go to step 6.
+
+ 5. Using the local resolver or a copy of its algorithm (see
+ resolver section of this memo) to answer the query. Store
+ the results, including any intermediate CNAMEs, in the answer
+ section of the response.
+
+ 6. Using local data only, attempt to add other RRs which may be
+ useful to the additional section of the query. Exit.
+
+4.3.3. Wildcards
+
+In the previous algorithm, special treatment was given to RRs with owner
+names starting with the label "*". Such RRs are called wildcards.
+Wildcard RRs can be thought of as instructions for synthesizing RRs.
+When the appropriate conditions are met, the name server creates RRs
+with an owner name equal to the query name and contents taken from the
+wildcard RRs.
+
+This facility is most often used to create a zone which will be used to
+forward mail from the Internet to some other mail system. The general
+idea is that any name in that zone which is presented to server in a
+query will be assumed to exist, with certain properties, unless explicit
+evidence exists to the contrary. Note that the use of the term zone
+here, instead of domain, is intentional; such defaults do not propagate
+across zone boundaries, although a subzone may choose to achieve that
+appearance by setting up similar defaults.
+
+The contents of the wildcard RRs follows the usual rules and formats for
+RRs. The wildcards in the zone have an owner name that controls the
+query names they will match. The owner name of the wildcard RRs is of
+the form "*.<anydomain>", where <anydomain> is any domain name.
+<anydomain> should not contain other * labels, and should be in the
+authoritative data of the zone. The wildcards potentially apply to
+descendants of <anydomain>, but not to <anydomain> itself. Another way
+
+
+
+Mockapetris [Page 25]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+to look at this is that the "*" label always matches at least one whole
+label and sometimes more, but always whole labels.
+
+Wildcard RRs do not apply:
+
+ - When the query is in another zone. That is, delegation cancels
+ the wildcard defaults.
+
+ - When the query name or a name between the wildcard domain and
+ the query name is know to exist. For example, if a wildcard
+ RR has an owner name of "*.X", and the zone also contains RRs
+ attached to B.X, the wildcards would apply to queries for name
+ Z.X (presuming there is no explicit information for Z.X), but
+ not to B.X, A.B.X, or X.
+
+A * label appearing in a query name has no special effect, but can be
+used to test for wildcards in an authoritative zone; such a query is the
+only way to get a response containing RRs with an owner name with * in
+it. The result of such a query should not be cached.
+
+Note that the contents of the wildcard RRs are not modified when used to
+synthesize RRs.
+
+To illustrate the use of wildcard RRs, suppose a large company with a
+large, non-IP/TCP, network wanted to create a mail gateway. If the
+company was called X.COM, and IP/TCP capable gateway machine was called
+A.X.COM, the following RRs might be entered into the COM zone:
+
+ X.COM MX 10 A.X.COM
+
+ *.X.COM MX 10 A.X.COM
+
+ A.X.COM A 1.2.3.4
+ A.X.COM MX 10 A.X.COM
+
+ *.A.X.COM MX 10 A.X.COM
+
+This would cause any MX query for any domain name ending in X.COM to
+return an MX RR pointing at A.X.COM. Two wildcard RRs are required
+since the effect of the wildcard at *.X.COM is inhibited in the A.X.COM
+subtree by the explicit data for A.X.COM. Note also that the explicit
+MX data at X.COM and A.X.COM is required, and that none of the RRs above
+would match a query name of XX.COM.
+
+4.3.4. Negative response caching (Optional)
+
+The DNS provides an optional service which allows name servers to
+distribute, and resolvers to cache, negative results with TTLs. For
+
+
+
+Mockapetris [Page 26]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+example, a name server can distribute a TTL along with a name error
+indication, and a resolver receiving such information is allowed to
+assume that the name does not exist during the TTL period without
+consulting authoritative data. Similarly, a resolver can make a query
+with a QTYPE which matches multiple types, and cache the fact that some
+of the types are not present.
+
+This feature can be particularly important in a system which implements
+naming shorthands that use search lists beacuse a popular shorthand,
+which happens to require a suffix toward the end of the search list,
+will generate multiple name errors whenever it is used.
+
+The method is that a name server may add an SOA RR to the additional
+section of a response when that response is authoritative. The SOA must
+be that of the zone which was the source of the authoritative data in
+the answer section, or name error if applicable. The MINIMUM field of
+the SOA controls the length of time that the negative result may be
+cached.
+
+Note that in some circumstances, the answer section may contain multiple
+owner names. In this case, the SOA mechanism should only be used for
+the data which matches QNAME, which is the only authoritative data in
+this section.
+
+Name servers and resolvers should never attempt to add SOAs to the
+additional section of a non-authoritative response, or attempt to infer
+results which are not directly stated in an authoritative response.
+There are several reasons for this, including: cached information isn't
+usually enough to match up RRs and their zone names, SOA RRs may be
+cached due to direct SOA queries, and name servers are not required to
+output the SOAs in the authority section.
+
+This feature is optional, although a refined version is expected to
+become part of the standard protocol in the future. Name servers are
+not required to add the SOA RRs in all authoritative responses, nor are
+resolvers required to cache negative results. Both are recommended.
+All resolvers and recursive name servers are required to at least be
+able to ignore the SOA RR when it is present in a response.
+
+Some experiments have also been proposed which will use this feature.
+The idea is that if cached data is known to come from a particular zone,
+and if an authoritative copy of the zone's SOA is obtained, and if the
+zone's SERIAL has not changed since the data was cached, then the TTL of
+the cached data can be reset to the zone MINIMUM value if it is smaller.
+This usage is mentioned for planning purposes only, and is not
+recommended as yet.
+
+
+
+
+
+Mockapetris [Page 27]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+4.3.5. Zone maintenance and transfers
+
+Part of the job of a zone administrator is to maintain the zones at all
+of the name servers which are authoritative for the zone. When the
+inevitable changes are made, they must be distributed to all of the name
+servers. While this distribution can be accomplished using FTP or some
+other ad hoc procedure, the preferred method is the zone transfer part
+of the DNS protocol.
+
+The general model of automatic zone transfer or refreshing is that one
+of the name servers is the master or primary for the zone. Changes are
+coordinated at the primary, typically by editing a master file for the
+zone. After editing, the administrator signals the master server to
+load the new zone. The other non-master or secondary servers for the
+zone periodically check for changes (at a selectable interval) and
+obtain new zone copies when changes have been made.
+
+To detect changes, secondaries just check the SERIAL field of the SOA
+for the zone. In addition to whatever other changes are made, the
+SERIAL field in the SOA of the zone is always advanced whenever any
+change is made to the zone. The advancing can be a simple increment, or
+could be based on the write date and time of the master file, etc. The
+purpose is to make it possible to determine which of two copies of a
+zone is more recent by comparing serial numbers. Serial number advances
+and comparisons use sequence space arithmetic, so there is a theoretic
+limit on how fast a zone can be updated, basically that old copies must
+die out before the serial number covers half of its 32 bit range. In
+practice, the only concern is that the compare operation deals properly
+with comparisons around the boundary between the most positive and most
+negative 32 bit numbers.
+
+The periodic polling of the secondary servers is controlled by
+parameters in the SOA RR for the zone, which set the minimum acceptable
+polling intervals. The parameters are called REFRESH, RETRY, and
+EXPIRE. Whenever a new zone is loaded in a secondary, the secondary
+waits REFRESH seconds before checking with the primary for a new serial.
+If this check cannot be completed, new checks are started every RETRY
+seconds. The check is a simple query to the primary for the SOA RR of
+the zone. If the serial field in the secondary's zone copy is equal to
+the serial returned by the primary, then no changes have occurred, and
+the REFRESH interval wait is restarted. If the secondary finds it
+impossible to perform a serial check for the EXPIRE interval, it must
+assume that its copy of the zone is obsolete an discard it.
+
+When the poll shows that the zone has changed, then the secondary server
+must request a zone transfer via an AXFR request for the zone. The AXFR
+may cause an error, such as refused, but normally is answered by a
+sequence of response messages. The first and last messages must contain
+
+
+
+Mockapetris [Page 28]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+the data for the top authoritative node of the zone. Intermediate
+messages carry all of the other RRs from the zone, including both
+authoritative and non-authoritative RRs. The stream of messages allows
+the secondary to construct a copy of the zone. Because accuracy is
+essential, TCP or some other reliable protocol must be used for AXFR
+requests.
+
+Each secondary server is required to perform the following operations
+against the master, but may also optionally perform these operations
+against other secondary servers. This strategy can improve the transfer
+process when the primary is unavailable due to host downtime or network
+problems, or when a secondary server has better network access to an
+"intermediate" secondary than to the primary.
+
+5. RESOLVERS
+
+5.1. Introduction
+
+Resolvers are programs that interface user programs to domain name
+servers. In the simplest case, a resolver receives a request from a
+user program (e.g., mail programs, TELNET, FTP) in the form of a
+subroutine call, system call etc., and returns the desired information
+in a form compatible with the local host's data formats.
+
+The resolver is located on the same machine as the program that requests
+the resolver's services, but it may need to consult name servers on
+other hosts. Because a resolver may need to consult several name
+servers, or may have the requested information in a local cache, the
+amount of time that a resolver will take to complete can vary quite a
+bit, from milliseconds to several seconds.
+
+A very important goal of the resolver is to eliminate network delay and
+name server load from most requests by answering them from its cache of
+prior results. It follows that caches which are shared by multiple
+processes, users, machines, etc., are more efficient than non-shared
+caches.
+
+5.2. Client-resolver interface
+
+5.2.1. Typical functions
+
+The client interface to the resolver is influenced by the local host's
+conventions, but the typical resolver-client interface has three
+functions:
+
+ 1. Host name to host address translation.
+
+ This function is often defined to mimic a previous HOSTS.TXT
+
+
+
+Mockapetris [Page 29]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+ based function. Given a character string, the caller wants
+ one or more 32 bit IP addresses. Under the DNS, it
+ translates into a request for type A RRs. Since the DNS does
+ not preserve the order of RRs, this function may choose to
+ sort the returned addresses or select the "best" address if
+ the service returns only one choice to the client. Note that
+ a multiple address return is recommended, but a single
+ address may be the only way to emulate prior HOSTS.TXT
+ services.
+
+ 2. Host address to host name translation
+
+ This function will often follow the form of previous
+ functions. Given a 32 bit IP address, the caller wants a
+ character string. The octets of the IP address are reversed,
+ used as name components, and suffixed with "IN-ADDR.ARPA". A
+ type PTR query is used to get the RR with the primary name of
+ the host. For example, a request for the host name
+ corresponding to IP address 1.2.3.4 looks for PTR RRs for
+ domain name "4.3.2.1.IN-ADDR.ARPA".
+
+ 3. General lookup function
+
+ This function retrieves arbitrary information from the DNS,
+ and has no counterpart in previous systems. The caller
+ supplies a QNAME, QTYPE, and QCLASS, and wants all of the
+ matching RRs. This function will often use the DNS format
+ for all RR data instead of the local host's, and returns all
+ RR content (e.g., TTL) instead of a processed form with local
+ quoting conventions.
+
+When the resolver performs the indicated function, it usually has one of
+the following results to pass back to the client:
+
+ - One or more RRs giving the requested data.
+
+ In this case the resolver returns the answer in the
+ appropriate format.
+
+ - A name error (NE).
+
+ This happens when the referenced name does not exist. For
+ example, a user may have mistyped a host name.
+
+ - A data not found error.
+
+ This happens when the referenced name exists, but data of the
+ appropriate type does not. For example, a host address
+
+
+
+Mockapetris [Page 30]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+ function applied to a mailbox name would return this error
+ since the name exists, but no address RR is present.
+
+It is important to note that the functions for translating between host
+names and addresses may combine the "name error" and "data not found"
+error conditions into a single type of error return, but the general
+function should not. One reason for this is that applications may ask
+first for one type of information about a name followed by a second
+request to the same name for some other type of information; if the two
+errors are combined, then useless queries may slow the application.
+
+5.2.2. Aliases
+
+While attempting to resolve a particular request, the resolver may find
+that the name in question is an alias. For example, the resolver might
+find that the name given for host name to address translation is an
+alias when it finds the CNAME RR. If possible, the alias condition
+should be signalled back from the resolver to the client.
+
+In most cases a resolver simply restarts the query at the new name when
+it encounters a CNAME. However, when performing the general function,
+the resolver should not pursue aliases when the CNAME RR matches the
+query type. This allows queries which ask whether an alias is present.
+For example, if the query type is CNAME, the user is interested in the
+CNAME RR itself, and not the RRs at the name it points to.
+
+Several special conditions can occur with aliases. Multiple levels of
+aliases should be avoided due to their lack of efficiency, but should
+not be signalled as an error. Alias loops and aliases which point to
+non-existent names should be caught and an error condition passed back
+to the client.
+
+5.2.3. Temporary failures
+
+In a less than perfect world, all resolvers will occasionally be unable
+to resolve a particular request. This condition can be caused by a
+resolver which becomes separated from the rest of the network due to a
+link failure or gateway problem, or less often by coincident failure or
+unavailability of all servers for a particular domain.
+
+It is essential that this sort of condition should not be signalled as a
+name or data not present error to applications. This sort of behavior
+is annoying to humans, and can wreak havoc when mail systems use the
+DNS.
+
+While in some cases it is possible to deal with such a temporary problem
+by blocking the request indefinitely, this is usually not a good choice,
+particularly when the client is a server process that could move on to
+
+
+
+Mockapetris [Page 31]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+other tasks. The recommended solution is to always have temporary
+failure as one of the possible results of a resolver function, even
+though this may make emulation of existing HOSTS.TXT functions more
+difficult.
+
+5.3. Resolver internals
+
+Every resolver implementation uses slightly different algorithms, and
+typically spends much more logic dealing with errors of various sorts
+than typical occurances. This section outlines a recommended basic
+strategy for resolver operation, but leaves details to [RFC-1035].
+
+5.3.1. Stub resolvers
+
+One option for implementing a resolver is to move the resolution
+function out of the local machine and into a name server which supports
+recursive queries. This can provide an easy method of providing domain
+service in a PC which lacks the resources to perform the resolver
+function, or can centralize the cache for a whole local network or
+organization.
+
+All that the remaining stub needs is a list of name server addresses
+that will perform the recursive requests. This type of resolver
+presumably needs the information in a configuration file, since it
+probably lacks the sophistication to locate it in the domain database.
+The user also needs to verify that the listed servers will perform the
+recursive service; a name server is free to refuse to perform recursive
+services for any or all clients. The user should consult the local
+system administrator to find name servers willing to perform the
+service.
+
+This type of service suffers from some drawbacks. Since the recursive
+requests may take an arbitrary amount of time to perform, the stub may
+have difficulty optimizing retransmission intervals to deal with both
+lost UDP packets and dead servers; the name server can be easily
+overloaded by too zealous a stub if it interprets retransmissions as new
+requests. Use of TCP may be an answer, but TCP may well place burdens
+on the host's capabilities which are similar to those of a real
+resolver.
+
+5.3.2. Resources
+
+In addition to its own resources, the resolver may also have shared
+access to zones maintained by a local name server. This gives the
+resolver the advantage of more rapid access, but the resolver must be
+careful to never let cached information override zone data. In this
+discussion the term "local information" is meant to mean the union of
+the cache and such shared zones, with the understanding that
+
+
+
+Mockapetris [Page 32]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+authoritative data is always used in preference to cached data when both
+are present.
+
+The following resolver algorithm assumes that all functions have been
+converted to a general lookup function, and uses the following data
+structures to represent the state of a request in progress in the
+resolver:
+
+SNAME the domain name we are searching for.
+
+STYPE the QTYPE of the search request.
+
+SCLASS the QCLASS of the search request.
+
+SLIST a structure which describes the name servers and the
+ zone which the resolver is currently trying to query.
+ This structure keeps track of the resolver's current
+ best guess about which name servers hold the desired
+ information; it is updated when arriving information
+ changes the guess. This structure includes the
+ equivalent of a zone name, the known name servers for
+ the zone, the known addresses for the name servers, and
+ history information which can be used to suggest which
+ server is likely to be the best one to try next. The
+ zone name equivalent is a match count of the number of
+ labels from the root down which SNAME has in common with
+ the zone being queried; this is used as a measure of how
+ "close" the resolver is to SNAME.
+
+SBELT a "safety belt" structure of the same form as SLIST,
+ which is initialized from a configuration file, and
+ lists servers which should be used when the resolver
+ doesn't have any local information to guide name server
+ selection. The match count will be -1 to indicate that
+ no labels are known to match.
+
+CACHE A structure which stores the results from previous
+ responses. Since resolvers are responsible for
+ discarding old RRs whose TTL has expired, most
+ implementations convert the interval specified in
+ arriving RRs to some sort of absolute time when the RR
+ is stored in the cache. Instead of counting the TTLs
+ down individually, the resolver just ignores or discards
+ old RRs when it runs across them in the course of a
+ search, or discards them during periodic sweeps to
+ reclaim the memory consumed by old RRs.
+
+
+
+
+
+Mockapetris [Page 33]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+5.3.3. Algorithm
+
+The top level algorithm has four steps:
+
+ 1. See if the answer is in local information, and if so return
+ it to the client.
+
+ 2. Find the best servers to ask.
+
+ 3. Send them queries until one returns a response.
+
+ 4. Analyze the response, either:
+
+ a. if the response answers the question or contains a name
+ error, cache the data as well as returning it back to
+ the client.
+
+ b. if the response contains a better delegation to other
+ servers, cache the delegation information, and go to
+ step 2.
+
+ c. if the response shows a CNAME and that is not the
+ answer itself, cache the CNAME, change the SNAME to the
+ canonical name in the CNAME RR and go to step 1.
+
+ d. if the response shows a servers failure or other
+ bizarre contents, delete the server from the SLIST and
+ go back to step 3.
+
+Step 1 searches the cache for the desired data. If the data is in the
+cache, it is assumed to be good enough for normal use. Some resolvers
+have an option at the user interface which will force the resolver to
+ignore the cached data and consult with an authoritative server. This
+is not recommended as the default. If the resolver has direct access to
+a name server's zones, it should check to see if the desired data is
+present in authoritative form, and if so, use the authoritative data in
+preference to cached data.
+
+Step 2 looks for a name server to ask for the required data. The
+general strategy is to look for locally-available name server RRs,
+starting at SNAME, then the parent domain name of SNAME, the
+grandparent, and so on toward the root. Thus if SNAME were
+Mockapetris.ISI.EDU, this step would look for NS RRs for
+Mockapetris.ISI.EDU, then ISI.EDU, then EDU, and then . (the root).
+These NS RRs list the names of hosts for a zone at or above SNAME. Copy
+the names into SLIST. Set up their addresses using local data. It may
+be the case that the addresses are not available. The resolver has many
+choices here; the best is to start parallel resolver processes looking
+
+
+
+Mockapetris [Page 34]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+for the addresses while continuing onward with the addresses which are
+available. Obviously, the design choices and options are complicated
+and a function of the local host's capabilities. The recommended
+priorities for the resolver designer are:
+
+ 1. Bound the amount of work (packets sent, parallel processes
+ started) so that a request can't get into an infinite loop or
+ start off a chain reaction of requests or queries with other
+ implementations EVEN IF SOMEONE HAS INCORRECTLY CONFIGURED
+ SOME DATA.
+
+ 2. Get back an answer if at all possible.
+
+ 3. Avoid unnecessary transmissions.
+
+ 4. Get the answer as quickly as possible.
+
+If the search for NS RRs fails, then the resolver initializes SLIST from
+the safety belt SBELT. The basic idea is that when the resolver has no
+idea what servers to ask, it should use information from a configuration
+file that lists several servers which are expected to be helpful.
+Although there are special situations, the usual choice is two of the
+root servers and two of the servers for the host's domain. The reason
+for two of each is for redundancy. The root servers will provide
+eventual access to all of the domain space. The two local servers will
+allow the resolver to continue to resolve local names if the local
+network becomes isolated from the internet due to gateway or link
+failure.
+
+In addition to the names and addresses of the servers, the SLIST data
+structure can be sorted to use the best servers first, and to insure
+that all addresses of all servers are used in a round-robin manner. The
+sorting can be a simple function of preferring addresses on the local
+network over others, or may involve statistics from past events, such as
+previous response times and batting averages.
+
+Step 3 sends out queries until a response is received. The strategy is
+to cycle around all of the addresses for all of the servers with a
+timeout between each transmission. In practice it is important to use
+all addresses of a multihomed host, and too aggressive a retransmission
+policy actually slows response when used by multiple resolvers
+contending for the same name server and even occasionally for a single
+resolver. SLIST typically contains data values to control the timeouts
+and keep track of previous transmissions.
+
+Step 4 involves analyzing responses. The resolver should be highly
+paranoid in its parsing of responses. It should also check that the
+response matches the query it sent using the ID field in the response.
+
+
+
+Mockapetris [Page 35]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+The ideal answer is one from a server authoritative for the query which
+either gives the required data or a name error. The data is passed back
+to the user and entered in the cache for future use if its TTL is
+greater than zero.
+
+If the response shows a delegation, the resolver should check to see
+that the delegation is "closer" to the answer than the servers in SLIST
+are. This can be done by comparing the match count in SLIST with that
+computed from SNAME and the NS RRs in the delegation. If not, the reply
+is bogus and should be ignored. If the delegation is valid the NS
+delegation RRs and any address RRs for the servers should be cached.
+The name servers are entered in the SLIST, and the search is restarted.
+
+If the response contains a CNAME, the search is restarted at the CNAME
+unless the response has the data for the canonical name or if the CNAME
+is the answer itself.
+
+Details and implementation hints can be found in [RFC-1035].
+
+6. A SCENARIO
+
+In our sample domain space, suppose we wanted separate administrative
+control for the root, MIL, EDU, MIT.EDU and ISI.EDU zones. We might
+allocate name servers as follows:
+
+
+ |(C.ISI.EDU,SRI-NIC.ARPA
+ | A.ISI.EDU)
+ +---------------------+------------------+
+ | | |
+ MIL EDU ARPA
+ |(SRI-NIC.ARPA, |(SRI-NIC.ARPA, |
+ | A.ISI.EDU | C.ISI.EDU) |
+ +-----+-----+ | +------+-----+-----+
+ | | | | | | |
+ BRL NOSC DARPA | IN-ADDR SRI-NIC ACC
+ |
+ +--------+------------------+---------------+--------+
+ | | | | |
+ UCI MIT | UDEL YALE
+ |(XX.LCS.MIT.EDU, ISI
+ |ACHILLES.MIT.EDU) |(VAXA.ISI.EDU,VENERA.ISI.EDU,
+ +---+---+ | A.ISI.EDU)
+ | | |
+ LCS ACHILLES +--+-----+-----+--------+
+ | | | | | |
+ XX A C VAXA VENERA Mockapetris
+
+
+
+
+Mockapetris [Page 36]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+In this example, the authoritative name server is shown in parentheses
+at the point in the domain tree at which is assumes control.
+
+Thus the root name servers are on C.ISI.EDU, SRI-NIC.ARPA, and
+A.ISI.EDU. The MIL domain is served by SRI-NIC.ARPA and A.ISI.EDU. The
+EDU domain is served by SRI-NIC.ARPA. and C.ISI.EDU. Note that servers
+may have zones which are contiguous or disjoint. In this scenario,
+C.ISI.EDU has contiguous zones at the root and EDU domains. A.ISI.EDU
+has contiguous zones at the root and MIL domains, but also has a non-
+contiguous zone at ISI.EDU.
+
+6.1. C.ISI.EDU name server
+
+C.ISI.EDU is a name server for the root, MIL, and EDU domains of the IN
+class, and would have zones for these domains. The zone data for the
+root domain might be:
+
+ . IN SOA SRI-NIC.ARPA. HOSTMASTER.SRI-NIC.ARPA. (
+ 870611 ;serial
+ 1800 ;refresh every 30 min
+ 300 ;retry every 5 min
+ 604800 ;expire after a week
+ 86400) ;minimum of a day
+ NS A.ISI.EDU.
+ NS C.ISI.EDU.
+ NS SRI-NIC.ARPA.
+
+ MIL. 86400 NS SRI-NIC.ARPA.
+ 86400 NS A.ISI.EDU.
+
+ EDU. 86400 NS SRI-NIC.ARPA.
+ 86400 NS C.ISI.EDU.
+
+ SRI-NIC.ARPA. A 26.0.0.73
+ A 10.0.0.51
+ MX 0 SRI-NIC.ARPA.
+ HINFO DEC-2060 TOPS20
+
+ ACC.ARPA. A 26.6.0.65
+ HINFO PDP-11/70 UNIX
+ MX 10 ACC.ARPA.
+
+ USC-ISIC.ARPA. CNAME C.ISI.EDU.
+
+ 73.0.0.26.IN-ADDR.ARPA. PTR SRI-NIC.ARPA.
+ 65.0.6.26.IN-ADDR.ARPA. PTR ACC.ARPA.
+ 51.0.0.10.IN-ADDR.ARPA. PTR SRI-NIC.ARPA.
+ 52.0.0.10.IN-ADDR.ARPA. PTR C.ISI.EDU.
+
+
+
+Mockapetris [Page 37]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+ 103.0.3.26.IN-ADDR.ARPA. PTR A.ISI.EDU.
+
+ A.ISI.EDU. 86400 A 26.3.0.103
+ C.ISI.EDU. 86400 A 10.0.0.52
+
+This data is represented as it would be in a master file. Most RRs are
+single line entries; the sole exception here is the SOA RR, which uses
+"(" to start a multi-line RR and ")" to show the end of a multi-line RR.
+Since the class of all RRs in a zone must be the same, only the first RR
+in a zone need specify the class. When a name server loads a zone, it
+forces the TTL of all authoritative RRs to be at least the MINIMUM field
+of the SOA, here 86400 seconds, or one day. The NS RRs marking
+delegation of the MIL and EDU domains, together with the glue RRs for
+the servers host addresses, are not part of the authoritative data in
+the zone, and hence have explicit TTLs.
+
+Four RRs are attached to the root node: the SOA which describes the root
+zone and the 3 NS RRs which list the name servers for the root. The
+data in the SOA RR describes the management of the zone. The zone data
+is maintained on host SRI-NIC.ARPA, and the responsible party for the
+zone is HOSTMASTER@SRI-NIC.ARPA. A key item in the SOA is the 86400
+second minimum TTL, which means that all authoritative data in the zone
+has at least that TTL, although higher values may be explicitly
+specified.
+
+The NS RRs for the MIL and EDU domains mark the boundary between the
+root zone and the MIL and EDU zones. Note that in this example, the
+lower zones happen to be supported by name servers which also support
+the root zone.
+
+The master file for the EDU zone might be stated relative to the origin
+EDU. The zone data for the EDU domain might be:
+
+ EDU. IN SOA SRI-NIC.ARPA. HOSTMASTER.SRI-NIC.ARPA. (
+ 870729 ;serial
+ 1800 ;refresh every 30 minutes
+ 300 ;retry every 5 minutes
+ 604800 ;expire after a week
+ 86400 ;minimum of a day
+ )
+ NS SRI-NIC.ARPA.
+ NS C.ISI.EDU.
+
+ UCI 172800 NS ICS.UCI
+ 172800 NS ROME.UCI
+ ICS.UCI 172800 A 192.5.19.1
+ ROME.UCI 172800 A 192.5.19.31
+
+
+
+
+Mockapetris [Page 38]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+ ISI 172800 NS VAXA.ISI
+ 172800 NS A.ISI
+ 172800 NS VENERA.ISI.EDU.
+ VAXA.ISI 172800 A 10.2.0.27
+ 172800 A 128.9.0.33
+ VENERA.ISI.EDU. 172800 A 10.1.0.52
+ 172800 A 128.9.0.32
+ A.ISI 172800 A 26.3.0.103
+
+ UDEL.EDU. 172800 NS LOUIE.UDEL.EDU.
+ 172800 NS UMN-REI-UC.ARPA.
+ LOUIE.UDEL.EDU. 172800 A 10.0.0.96
+ 172800 A 192.5.39.3
+
+ YALE.EDU. 172800 NS YALE.ARPA.
+ YALE.EDU. 172800 NS YALE-BULLDOG.ARPA.
+
+ MIT.EDU. 43200 NS XX.LCS.MIT.EDU.
+ 43200 NS ACHILLES.MIT.EDU.
+ XX.LCS.MIT.EDU. 43200 A 10.0.0.44
+ ACHILLES.MIT.EDU. 43200 A 18.72.0.8
+
+Note the use of relative names here. The owner name for the ISI.EDU. is
+stated using a relative name, as are two of the name server RR contents.
+Relative and absolute domain names may be freely intermixed in a master
+
+6.2. Example standard queries
+
+The following queries and responses illustrate name server behavior.
+Unless otherwise noted, the queries do not have recursion desired (RD)
+in the header. Note that the answers to non-recursive queries do depend
+on the server being asked, but do not depend on the identity of the
+requester.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris [Page 39]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+6.2.1. QNAME=SRI-NIC.ARPA, QTYPE=A
+
+The query would look like:
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY |
+ +---------------------------------------------------+
+ Question | QNAME=SRI-NIC.ARPA., QCLASS=IN, QTYPE=A |
+ +---------------------------------------------------+
+ Answer | <empty> |
+ +---------------------------------------------------+
+ Authority | <empty> |
+ +---------------------------------------------------+
+ Additional | <empty> |
+ +---------------------------------------------------+
+
+The response from C.ISI.EDU would be:
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE, AA |
+ +---------------------------------------------------+
+ Question | QNAME=SRI-NIC.ARPA., QCLASS=IN, QTYPE=A |
+ +---------------------------------------------------+
+ Answer | SRI-NIC.ARPA. 86400 IN A 26.0.0.73 |
+ | 86400 IN A 10.0.0.51 |
+ +---------------------------------------------------+
+ Authority | <empty> |
+ +---------------------------------------------------+
+ Additional | <empty> |
+ +---------------------------------------------------+
+
+The header of the response looks like the header of the query, except
+that the RESPONSE bit is set, indicating that this message is a
+response, not a query, and the Authoritative Answer (AA) bit is set
+indicating that the address RRs in the answer section are from
+authoritative data. The question section of the response matches the
+question section of the query.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris [Page 40]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+If the same query was sent to some other server which was not
+authoritative for SRI-NIC.ARPA, the response might be:
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY,RESPONSE |
+ +---------------------------------------------------+
+ Question | QNAME=SRI-NIC.ARPA., QCLASS=IN, QTYPE=A |
+ +---------------------------------------------------+
+ Answer | SRI-NIC.ARPA. 1777 IN A 10.0.0.51 |
+ | 1777 IN A 26.0.0.73 |
+ +---------------------------------------------------+
+ Authority | <empty> |
+ +---------------------------------------------------+
+ Additional | <empty> |
+ +---------------------------------------------------+
+
+This response is different from the previous one in two ways: the header
+does not have AA set, and the TTLs are different. The inference is that
+the data did not come from a zone, but from a cache. The difference
+between the authoritative TTL and the TTL here is due to aging of the
+data in a cache. The difference in ordering of the RRs in the answer
+section is not significant.
+
+6.2.2. QNAME=SRI-NIC.ARPA, QTYPE=*
+
+A query similar to the previous one, but using a QTYPE of *, would
+receive the following response from C.ISI.EDU:
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE, AA |
+ +---------------------------------------------------+
+ Question | QNAME=SRI-NIC.ARPA., QCLASS=IN, QTYPE=* |
+ +---------------------------------------------------+
+ Answer | SRI-NIC.ARPA. 86400 IN A 26.0.0.73 |
+ | A 10.0.0.51 |
+ | MX 0 SRI-NIC.ARPA. |
+ | HINFO DEC-2060 TOPS20 |
+ +---------------------------------------------------+
+ Authority | <empty> |
+ +---------------------------------------------------+
+ Additional | <empty> |
+ +---------------------------------------------------+
+
+
+
+
+
+
+
+
+
+Mockapetris [Page 41]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+If a similar query was directed to two name servers which are not
+authoritative for SRI-NIC.ARPA, the responses might be:
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE |
+ +---------------------------------------------------+
+ Question | QNAME=SRI-NIC.ARPA., QCLASS=IN, QTYPE=* |
+ +---------------------------------------------------+
+ Answer | SRI-NIC.ARPA. 12345 IN A 26.0.0.73 |
+ | A 10.0.0.51 |
+ +---------------------------------------------------+
+ Authority | <empty> |
+ +---------------------------------------------------+
+ Additional | <empty> |
+ +---------------------------------------------------+
+
+and
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE |
+ +---------------------------------------------------+
+ Question | QNAME=SRI-NIC.ARPA., QCLASS=IN, QTYPE=* |
+ +---------------------------------------------------+
+ Answer | SRI-NIC.ARPA. 1290 IN HINFO DEC-2060 TOPS20 |
+ +---------------------------------------------------+
+ Authority | <empty> |
+ +---------------------------------------------------+
+ Additional | <empty> |
+ +---------------------------------------------------+
+
+Neither of these answers have AA set, so neither response comes from
+authoritative data. The different contents and different TTLs suggest
+that the two servers cached data at different times, and that the first
+server cached the response to a QTYPE=A query and the second cached the
+response to a HINFO query.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris [Page 42]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+6.2.3. QNAME=SRI-NIC.ARPA, QTYPE=MX
+
+This type of query might be result from a mailer trying to look up
+routing information for the mail destination HOSTMASTER@SRI-NIC.ARPA.
+The response from C.ISI.EDU would be:
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE, AA |
+ +---------------------------------------------------+
+ Question | QNAME=SRI-NIC.ARPA., QCLASS=IN, QTYPE=MX |
+ +---------------------------------------------------+
+ Answer | SRI-NIC.ARPA. 86400 IN MX 0 SRI-NIC.ARPA.|
+ +---------------------------------------------------+
+ Authority | <empty> |
+ +---------------------------------------------------+
+ Additional | SRI-NIC.ARPA. 86400 IN A 26.0.0.73 |
+ | A 10.0.0.51 |
+ +---------------------------------------------------+
+
+This response contains the MX RR in the answer section of the response.
+The additional section contains the address RRs because the name server
+at C.ISI.EDU guesses that the requester will need the addresses in order
+to properly use the information carried by the MX.
+
+6.2.4. QNAME=SRI-NIC.ARPA, QTYPE=NS
+
+C.ISI.EDU would reply to this query with:
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE, AA |
+ +---------------------------------------------------+
+ Question | QNAME=SRI-NIC.ARPA., QCLASS=IN, QTYPE=NS |
+ +---------------------------------------------------+
+ Answer | <empty> |
+ +---------------------------------------------------+
+ Authority | <empty> |
+ +---------------------------------------------------+
+ Additional | <empty> |
+ +---------------------------------------------------+
+
+The only difference between the response and the query is the AA and
+RESPONSE bits in the header. The interpretation of this response is
+that the server is authoritative for the name, and the name exists, but
+no RRs of type NS are present there.
+
+6.2.5. QNAME=SIR-NIC.ARPA, QTYPE=A
+
+If a user mistyped a host name, we might see this type of query.
+
+
+
+Mockapetris [Page 43]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+C.ISI.EDU would answer it with:
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE, AA, RCODE=NE |
+ +---------------------------------------------------+
+ Question | QNAME=SIR-NIC.ARPA., QCLASS=IN, QTYPE=A |
+ +---------------------------------------------------+
+ Answer | <empty> |
+ +---------------------------------------------------+
+ Authority | . SOA SRI-NIC.ARPA. HOSTMASTER.SRI-NIC.ARPA. |
+ | 870611 1800 300 604800 86400 |
+ +---------------------------------------------------+
+ Additional | <empty> |
+ +---------------------------------------------------+
+
+This response states that the name does not exist. This condition is
+signalled in the response code (RCODE) section of the header.
+
+The SOA RR in the authority section is the optional negative caching
+information which allows the resolver using this response to assume that
+the name will not exist for the SOA MINIMUM (86400) seconds.
+
+6.2.6. QNAME=BRL.MIL, QTYPE=A
+
+If this query is sent to C.ISI.EDU, the reply would be:
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE |
+ +---------------------------------------------------+
+ Question | QNAME=BRL.MIL, QCLASS=IN, QTYPE=A |
+ +---------------------------------------------------+
+ Answer | <empty> |
+ +---------------------------------------------------+
+ Authority | MIL. 86400 IN NS SRI-NIC.ARPA. |
+ | 86400 NS A.ISI.EDU. |
+ +---------------------------------------------------+
+ Additional | A.ISI.EDU. A 26.3.0.103 |
+ | SRI-NIC.ARPA. A 26.0.0.73 |
+ | A 10.0.0.51 |
+ +---------------------------------------------------+
+
+This response has an empty answer section, but is not authoritative, so
+it is a referral. The name server on C.ISI.EDU, realizing that it is
+not authoritative for the MIL domain, has referred the requester to
+servers on A.ISI.EDU and SRI-NIC.ARPA, which it knows are authoritative
+for the MIL domain.
+
+
+
+
+
+Mockapetris [Page 44]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+6.2.7. QNAME=USC-ISIC.ARPA, QTYPE=A
+
+The response to this query from A.ISI.EDU would be:
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE, AA |
+ +---------------------------------------------------+
+ Question | QNAME=USC-ISIC.ARPA., QCLASS=IN, QTYPE=A |
+ +---------------------------------------------------+
+ Answer | USC-ISIC.ARPA. 86400 IN CNAME C.ISI.EDU. |
+ | C.ISI.EDU. 86400 IN A 10.0.0.52 |
+ +---------------------------------------------------+
+ Authority | <empty> |
+ +---------------------------------------------------+
+ Additional | <empty> |
+ +---------------------------------------------------+
+
+Note that the AA bit in the header guarantees that the data matching
+QNAME is authoritative, but does not say anything about whether the data
+for C.ISI.EDU is authoritative. This complete reply is possible because
+A.ISI.EDU happens to be authoritative for both the ARPA domain where
+USC-ISIC.ARPA is found and the ISI.EDU domain where C.ISI.EDU data is
+found.
+
+If the same query was sent to C.ISI.EDU, its response might be the same
+as shown above if it had its own address in its cache, but might also
+be:
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris [Page 45]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE, AA |
+ +---------------------------------------------------+
+ Question | QNAME=USC-ISIC.ARPA., QCLASS=IN, QTYPE=A |
+ +---------------------------------------------------+
+ Answer | USC-ISIC.ARPA. 86400 IN CNAME C.ISI.EDU. |
+ +---------------------------------------------------+
+ Authority | ISI.EDU. 172800 IN NS VAXA.ISI.EDU. |
+ | NS A.ISI.EDU. |
+ | NS VENERA.ISI.EDU. |
+ +---------------------------------------------------+
+ Additional | VAXA.ISI.EDU. 172800 A 10.2.0.27 |
+ | 172800 A 128.9.0.33 |
+ | VENERA.ISI.EDU. 172800 A 10.1.0.52 |
+ | 172800 A 128.9.0.32 |
+ | A.ISI.EDU. 172800 A 26.3.0.103 |
+ +---------------------------------------------------+
+
+This reply contains an authoritative reply for the alias USC-ISIC.ARPA,
+plus a referral to the name servers for ISI.EDU. This sort of reply
+isn't very likely given that the query is for the host name of the name
+server being asked, but would be common for other aliases.
+
+6.2.8. QNAME=USC-ISIC.ARPA, QTYPE=CNAME
+
+If this query is sent to either A.ISI.EDU or C.ISI.EDU, the reply would
+be:
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE, AA |
+ +---------------------------------------------------+
+ Question | QNAME=USC-ISIC.ARPA., QCLASS=IN, QTYPE=A |
+ +---------------------------------------------------+
+ Answer | USC-ISIC.ARPA. 86400 IN CNAME C.ISI.EDU. |
+ +---------------------------------------------------+
+ Authority | <empty> |
+ +---------------------------------------------------+
+ Additional | <empty> |
+ +---------------------------------------------------+
+
+Because QTYPE=CNAME, the CNAME RR itself answers the query, and the name
+server doesn't attempt to look up anything for C.ISI.EDU. (Except
+possibly for the additional section.)
+
+6.3. Example resolution
+
+The following examples illustrate the operations a resolver must perform
+for its client. We assume that the resolver is starting without a
+
+
+
+Mockapetris [Page 46]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+cache, as might be the case after system boot. We further assume that
+the system is not one of the hosts in the data and that the host is
+located somewhere on net 26, and that its safety belt (SBELT) data
+structure has the following information:
+
+ Match count = -1
+ SRI-NIC.ARPA. 26.0.0.73 10.0.0.51
+ A.ISI.EDU. 26.3.0.103
+
+This information specifies servers to try, their addresses, and a match
+count of -1, which says that the servers aren't very close to the
+target. Note that the -1 isn't supposed to be an accurate closeness
+measure, just a value so that later stages of the algorithm will work.
+
+The following examples illustrate the use of a cache, so each example
+assumes that previous requests have completed.
+
+6.3.1. Resolve MX for ISI.EDU.
+
+Suppose the first request to the resolver comes from the local mailer,
+which has mail for PVM@ISI.EDU. The mailer might then ask for type MX
+RRs for the domain name ISI.EDU.
+
+The resolver would look in its cache for MX RRs at ISI.EDU, but the
+empty cache wouldn't be helpful. The resolver would recognize that it
+needed to query foreign servers and try to determine the best servers to
+query. This search would look for NS RRs for the domains ISI.EDU, EDU,
+and the root. These searches of the cache would also fail. As a last
+resort, the resolver would use the information from the SBELT, copying
+it into its SLIST structure.
+
+At this point the resolver would need to pick one of the three available
+addresses to try. Given that the resolver is on net 26, it should
+choose either 26.0.0.73 or 26.3.0.103 as its first choice. It would
+then send off a query of the form:
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris [Page 47]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY |
+ +---------------------------------------------------+
+ Question | QNAME=ISI.EDU., QCLASS=IN, QTYPE=MX |
+ +---------------------------------------------------+
+ Answer | <empty> |
+ +---------------------------------------------------+
+ Authority | <empty> |
+ +---------------------------------------------------+
+ Additional | <empty> |
+ +---------------------------------------------------+
+
+The resolver would then wait for a response to its query or a timeout.
+If the timeout occurs, it would try different servers, then different
+addresses of the same servers, lastly retrying addresses already tried.
+It might eventually receive a reply from SRI-NIC.ARPA:
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE |
+ +---------------------------------------------------+
+ Question | QNAME=ISI.EDU., QCLASS=IN, QTYPE=MX |
+ +---------------------------------------------------+
+ Answer | <empty> |
+ +---------------------------------------------------+
+ Authority | ISI.EDU. 172800 IN NS VAXA.ISI.EDU. |
+ | NS A.ISI.EDU. |
+ | NS VENERA.ISI.EDU.|
+ +---------------------------------------------------+
+ Additional | VAXA.ISI.EDU. 172800 A 10.2.0.27 |
+ | 172800 A 128.9.0.33 |
+ | VENERA.ISI.EDU. 172800 A 10.1.0.52 |
+ | 172800 A 128.9.0.32 |
+ | A.ISI.EDU. 172800 A 26.3.0.103 |
+ +---------------------------------------------------+
+
+The resolver would notice that the information in the response gave a
+closer delegation to ISI.EDU than its existing SLIST (since it matches
+three labels). The resolver would then cache the information in this
+response and use it to set up a new SLIST:
+
+ Match count = 3
+ A.ISI.EDU. 26.3.0.103
+ VAXA.ISI.EDU. 10.2.0.27 128.9.0.33
+ VENERA.ISI.EDU. 10.1.0.52 128.9.0.32
+
+A.ISI.EDU appears on this list as well as the previous one, but that is
+purely coincidental. The resolver would again start transmitting and
+waiting for responses. Eventually it would get an answer:
+
+
+
+Mockapetris [Page 48]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE, AA |
+ +---------------------------------------------------+
+ Question | QNAME=ISI.EDU., QCLASS=IN, QTYPE=MX |
+ +---------------------------------------------------+
+ Answer | ISI.EDU. MX 10 VENERA.ISI.EDU. |
+ | MX 20 VAXA.ISI.EDU. |
+ +---------------------------------------------------+
+ Authority | <empty> |
+ +---------------------------------------------------+
+ Additional | VAXA.ISI.EDU. 172800 A 10.2.0.27 |
+ | 172800 A 128.9.0.33 |
+ | VENERA.ISI.EDU. 172800 A 10.1.0.52 |
+ | 172800 A 128.9.0.32 |
+ +---------------------------------------------------+
+
+The resolver would add this information to its cache, and return the MX
+RRs to its client.
+
+6.3.2. Get the host name for address 26.6.0.65
+
+The resolver would translate this into a request for PTR RRs for
+65.0.6.26.IN-ADDR.ARPA. This information is not in the cache, so the
+resolver would look for foreign servers to ask. No servers would match,
+so it would use SBELT again. (Note that the servers for the ISI.EDU
+domain are in the cache, but ISI.EDU is not an ancestor of
+65.0.6.26.IN-ADDR.ARPA, so the SBELT is used.)
+
+Since this request is within the authoritative data of both servers in
+SBELT, eventually one would return:
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris [Page 49]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+ +---------------------------------------------------+
+ Header | OPCODE=SQUERY, RESPONSE, AA |
+ +---------------------------------------------------+
+ Question | QNAME=65.0.6.26.IN-ADDR.ARPA.,QCLASS=IN,QTYPE=PTR |
+ +---------------------------------------------------+
+ Answer | 65.0.6.26.IN-ADDR.ARPA. PTR ACC.ARPA. |
+ +---------------------------------------------------+
+ Authority | <empty> |
+ +---------------------------------------------------+
+ Additional | <empty> |
+ +---------------------------------------------------+
+
+6.3.3. Get the host address of poneria.ISI.EDU
+
+This request would translate into a type A request for poneria.ISI.EDU.
+The resolver would not find any cached data for this name, but would
+find the NS RRs in the cache for ISI.EDU when it looks for foreign
+servers to ask. Using this data, it would construct a SLIST of the
+form:
+
+ Match count = 3
+
+ A.ISI.EDU. 26.3.0.103
+ VAXA.ISI.EDU. 10.2.0.27 128.9.0.33
+ VENERA.ISI.EDU. 10.1.0.52
+
+A.ISI.EDU is listed first on the assumption that the resolver orders its
+choices by preference, and A.ISI.EDU is on the same network.
+
+One of these servers would answer the query.
+
+7. REFERENCES and BIBLIOGRAPHY
+
+[Dyer 87] Dyer, S., and 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.
+
+
+
+
+
+
+
+
+Mockapetris [Page 50]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+[Quarterman 86] Quarterman, J., 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.
+
+ 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.
+
+
+
+
+Mockapetris [Page 51]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+[RFC-830] Z. Su, "A Distributed System for Internet Name Service",
+ RFC-830, Network Information Center, SRI International,
+ October 1982.
+
+ Early thoughts on the design of the domain system.
+ Current implementation is completely different.
+
+[RFC-882] P. Mockapetris, "Domain names - Concepts and
+ Facilities," RFC-882, USC/Information Sciences
+ Institute, November 1983.
+
+ Superceeded by this memo.
+
+[RFC-883] P. Mockapetris, "Domain names - Implementation and
+ Specification," RFC-883, USC/Information Sciences
+ Institute, November 1983.
+
+ Superceeded by this memo.
+
+[RFC-920] J. Postel and J. Reynolds, "Domain Requirements",
+ RFC-920, USC/Information Sciences Institute
+ October 1984.
+
+ Explains the naming scheme for top level domains.
+
+[RFC-952] K. Harrenstien, M. Stahl, E. Feinler, "DoD Internet Host
+ Table Specification", RFC-952, SRI, October 1985.
+
+ Specifies the format of HOSTS.TXT, the host/address
+ table replaced by the DNS.
+
+[RFC-953] K. Harrenstien, M. Stahl, E. Feinler, "HOSTNAME Server",
+ RFC-953, SRI, October 1985.
+
+ This RFC contains the official specification of the
+ hostname server protocol, which is obsoleted by the DNS.
+ This TCP based protocol accesses information stored in
+ the RFC-952 format, and is used to obtain copies of the
+ host table.
+
+[RFC-973] P. Mockapetris, "Domain System Changes and
+ Observations", RFC-973, USC/Information Sciences
+ Institute, January 1986.
+
+ Describes changes to RFC-882 and RFC-883 and reasons for
+ them. Now obsolete.
+
+
+
+
+
+Mockapetris [Page 52]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+[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. K. Stahl, "Establishing a Domain - Guidelines for
+ Administrators", RFC-1032, November 1987.
+
+ Describes the registration policies used by the NIC to
+ administer the top level domains and delegate subzones.
+
+[RFC-1033] M. K. 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]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+Index
+
+ A 12
+ Absolute names 8
+ Aliases 14, 31
+ Authority 6
+ AXFR 17
+
+ Case of characters 7
+ CH 12
+ CNAME 12, 13, 31
+ Completion queries 18
+
+ Domain name 6, 7
+
+ Glue RRs 20
+
+ HINFO 12
+
+ IN 12
+ Inverse queries 16
+ Iterative 4
+
+ Label 7
+
+ Mailbox names 9
+ MX 12
+
+ Name error 27, 36
+ Name servers 5, 17
+ NE 30
+ Negative caching 44
+ NS 12
+
+ Opcode 16
+
+ PTR 12
+
+ QCLASS 16
+ QTYPE 16
+
+ RDATA 13
+ Recursive 4
+ Recursive service 22
+ Relative names 7
+ Resolvers 6
+ RR 12
+
+
+
+
+Mockapetris [Page 54]
+\f
+RFC 1034 Domain Concepts and Facilities November 1987
+
+
+ Safety belt 33
+ Sections 16
+ SOA 12
+ Standard queries 22
+
+ Status queries 18
+ Stub resolvers 32
+
+ TTL 12, 13
+
+ Wildcards 25
+
+ Zone transfers 28
+ Zones 19
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris [Page 55]
+\f
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