Username / Password : Request For Comments

RFC Number : 882

Title : Domain names: Concepts and facilities.

Network Working Group P. Mockapetris
Request for Comments: 882 ISI
November 1983


| |
| This RFC introduces domain style names, their use |
| for ARPA Internet mail and host address support, |
| and the protocols and servers used to implement |
| domain name facilities. |
| |
| This memo describes the conceptual framework of the |
| domain system and some uses, but it omits many |
| uses, fields, and implementation details. A |
| complete specification of formats, timeouts, etc. |
| is presented in RFC 883, 'Domain Names - |
| Implementation and Specification'. That RFC |
| assumes that the reader is familiar with the |
| concepts discussed in this memo. |
| |


The need for domain names

As applications grow to span multiple hosts, then networks, and
finally internets, these applications must also span multiple
administrative boundaries and related methods of operation
(protocols, data formats, etc). The number of resources (for
example mailboxes), the number of locations for resources, and the
diversity of such an environment cause formidable problems when we
wish to create consistent methods for referencing particular
resources that are similar but scattered throughout the

The ARPA Internet illustrates the size-related problems; it is a
large system and is likely to grow much larger. The need to have
a mapping between host names (e.g., USC-ISIF) and ARPA Internet
addresses (e.g., is beginning to stress the existing
mechanisms. Currently hosts in the ARPA Internet are registered
with the Network Information Center (NIC) and listed in a global
table (available as the file HOSTS.TXT on the SRI-NIC
host) [1]. The size of this table, and especially the frequency
of updates to the table are near the limit of manageability. What
is needed is a distributed database that performs the same
function, and hence avoids the problems caused by a centralized

The problem for computer mail is more severe. While mail system
implementers long ago recognized the impossibility of centralizing

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mailbox names, they have also created an increasingly large and
irregular set of methods for identifying the location of a
mailbox. Some of these methods involve the use of routes and
forwarding hosts as part of the mail destination address, and
consequently force the mail user to know multiple address formats,
the capabilities of various forwarders, and ad hoc tricks for
passing address specifications through intermediaries.

These problems have common characteristics that suggest the nature
of any solution:

The basic need is for 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 contain
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 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.

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.

Because we want the name space to be useful in dissimilar
networks, it is unlikely that all users of domain names will be
able to agree on the set of resources or resource information
that names will be used to retrieve. Hence names refer to a
set of resources, and queries contain resource identifiers.
The only standard types of information that we expect to see
throughout the name space is structuring information for the
name space itself, and resources that are described using
domain names and no nonstandard data.

We also want the name server transactions to be independent of
the communications system that carries them. Some systems may
wish to use datagrams for simple 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.

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Elements of the solution

The proposed solution has three major components:

The DOMAIN NAME SPACE, which is a specification for a tree
structured name space. 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 ARPA Internet uses some of
its domain names to identify hosts; queries for address
resources return ARPA Internet host addresses. However, to
preserve the generality of the domain mechanism, domain names
are not required to have a one-to-one correspondence with host
names, host addresses, or any other type of information.

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; these parts are called ZONEs; a name server is an
AUTHORITY for these parts of the name space.

RESOLVERS are programs that extract information from name
servers in response to user 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 simple procedure or OS calls to resolvers. 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
server has one or more pieces of the whole domain tree's data,

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but the resolver views each of these databases as essentially

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 using
the local zones.

In the interests of performance, these layers blur a bit. For
example, resolvers on the same machine as a name server may share
a database and may also introduce foreign information for use in
later queries. This cached information is treated differently
from the authoritative data in zones.

Database model

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
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

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 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

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updates or guarantees of consistency. Hence the update process
allows updates to percolate out though 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.

Some users will wish to access the database via datagrams;
others will prefer virtual circuits. The domain system is
designed so that simple queries and responses can use either
style, although refreshing operations need the reliability of
virtual circuits. The same overall message format is used for
all communication. The domain system does not assume any
special properties of the communications system, and hence
could be used with any datagram or virtual circuit protocol.

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. The optional
recursive style is discussed in [14], and omitted from further
discussion in this memo.

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 to users of the domain system. A standard
format for these files is given in [14].

The standard format allows these files 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.

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Each host's name servers and resolvers are configured by a local
system administrator. 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


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). Each
node and leaf has an associated label. Labels are NOT guaranteed
to be unique, with the exception of the root node, which has a
null label. The domain name of a node or leaf is the path from
the root of the tree to the node or leaf. By convention, the
labels that compose a domain name are read left to right, from the
most specific (lowest) to the least specific (highest).

Internally, programs that manipulate domain names 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

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representations can use a length byte of zero to terminate a
domain name. When domain names are printed, labels in a path are
separated by dots ('.'). The root label and its associated dot
are omitted from printed domain names, but the root can be named
by a null domain name (' ' in this memo).

To simplify implementations, the total number of octets that
represent label octets and label lengths is limited to 255. Thus
a printed domain name can be up to 254 characters.

A special label is defined that matches any other label. This
label is the asterisk or '*'. An asterisk matches a single label.
Thus *.ARPA matches FOO.ARPA, but does not match FOO.BAR.ARPA.
The asterisk is mainly used to create default resource records at
the boundary between protocol families, and requires prudence in
its use.

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 has the containing
domain's name as the right part of its name. For example, A.B.C.D
is a subdomain of B.C.D, C.D, D, and ' '.

This tree structure is intended to parallel the administrative
organization and delegation of authority. Potentially, each node
or leaf on the tree can create new subdomains ad infinitum. In
practice, this delegation can be limited by the administrator of
the name servers that manage the domain space and resource data.

The following figure shows an example of a domain name space.

| | |
| |
+-----+-----+ |
| | |

In this example, the root domain has three immediate subdomains:
COLORS, FLAVORS, and TRUTH. The FLAVORS domain has one immediate
subdomain named NATURAL.FLAVORS. All of the leaves are also

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domains. This domain tree has the names ' '(the root), COLORS,
STRAWBERRY.NATURAL.FLAVORS, and TRUTH. If we wished to add a new
domain of ARTIFICIAL under FLAVORS, FLAVORS would typically be the
administrative entity that would decide; if we wished to create
would typically be the appropriate administrative entity.

Resource set information

A domain name identifies a set of resource information. The set
of resource information associated with a particular name is
composed of separate resource records (RRs).

Each resource record has the following major components:

The domain name which identifies resource set that holds this
record, and hence the 'owner' of the information. For example,
a RR that specifies a host address has a domain name the
specifies the host having that address. Thus F.ISI.ARPA might
be the owner of a RR which specified an address field of Since name servers typically store their resource
information in tree structures paralleling the organization of
the domain space, this information can usually be stored
implicitly in the database; however it is always included in
each resource record carried in a message.

Other information used to manage the RR, such as length fields,
timeouts, etc. This information is omitted in much of this
memo, but is discussed in [14].

A resource type field that specifies the type of the resource
in this resource record. Types refer to abstract resources
such as host addresses or mail delivery agents. The type field
is two octets long and uses an encoding that is standard
throughout the domain name system.

A class field identifies the format of the resource data, such
as the ARPA Internet format (IN) or the Computer Science
Network format (CSNET), for certain RR types (such as address
data). Note that while the class may separate different
protocol families, networks, etc. it does not do so in all
cases. For example, the IN class uses 32 bit IP addresses
exclusively, but the CSNET class uses 32 bit IP addresses, X.25
addresses, and phone numbers. Thus the class field should be
used as a guide for interpreting the resource data. The class
field is two octets long and uses an encoding that is standard
throughout the domain name system.

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Resource data that describes the resource. The format of this
data can be determined given the type and class fields, but
always starts with a two octet length field that allows a name
server or resolver to determine the boundaries of the resource
data in any transaction, even if it cannot 'understand' the
resource data itself. Thus name servers and resolvers can hold
and pass on records which they cannot interpret. The format of
the internal data is restricted only by the maximum length of
65535 octets; for example the host address record might specify
a fixed 32 bit number for one class, and a variable length list
of addresses in another class.

While the class field in effect partitions the resource data in
the domain name system into separate parallel sections according
to class, services can span class boundaries if they use
compatible resource data formats. For example, the domain name
system uses compatible formats for structure information, and the
mail data decouples mail agent identification from details of how
to contact the agent (e.g. host addresses).

This memo uses the following types in its examples:

A - the host address associated with the domain name

MF - identifies a mail forwarder for the domain

MD - identifies a mail destination for the domain

NS - the authoritative name server for the domain

SOA - identifies the start of a zone of authority

CNAME - identifies the canonical name of an alias

This memo uses the following classes in its examples:

IN - the ARPA Internet system

CS - the CSNET system

The first type of resource record holds a host name to host
address binding. Its fields are:

| | A | | information |

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The content of the class specific information varies according to
the value in the CLASS field; for the ARPA Internet, it is the 32
bit ARPA Internet address of the host, for the CSNET it might be
the phone number of the host. For example, F.ISI.ARPA might have
two A records of the form:

|F.ISI.ARPA| A | IN | |
|F.ISI.ARPA| A | CS | 213-822-2112 |

Note that the data formats for the A type are class dependent, and
the Internet address and phone number formats shown above are for
purposes of illustration only. The actual data formats are
specified in [14]. For example, CS class data for type A records
might actually be a list of Internet addresses, phone numbers and
TELENET addresses.

The mail forwarder (MF) and mail delivery (MD) records have the
following format:

| | MD/MF | | |

The field is a domain name of the host that will
handle mail; note that this domain name may be completely
different from the domain name which names the resource record.
For example, F.ISI.ARPA might have two records of the form:


These records mean that mail for F.ISI.ARPA can either be
delivered to the host F.ISI.ARPA or forwarded to B.ISI.ARPA, which
will accept responsibility for its eventual delivery. In
principle, an additional name lookup is required to map the domain
name of the host to the appropriate address, in practice this
information is usually returned in the response to the mail query.

The SOA and NS types of resource records are used to define limits

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of authority. The domain name given by the owner field of a SOA
record is the start of a zone; the domain name given by the owner
field of a NS record identifies a point in the name space where
authority has been delegated, and hence marks the zone boundary.
Except in the case where a name server delegates authority to
itself, the SOA identifies the top limit of authority, and NS
records define the first name outside of a zone. These resource
records have a standard format for all of the name space:

| | SOA | | |

| | NS | | |

The SOA record marks the start of a zone when it is present in a
database; the NS record both marks the end of a zone started by an
SOA (if a higher SOA is present) and also points to a name server
that has a copy of the zone specified by the NS record.

The in the SOA record specifies the original
source of the information in the zone and other information used
by name servers to organize their activities. SOA records are
never cached (otherwise they would create false zones); they can
only be created in special name server maintenance operations.

The NS record says that a name server which is authoritative for
records of the given CLASS can be found at .


Queries to a name server must include a domain name which
identifies the target resource set (QNAME), and the type and class
of desired resource records. The type and class fields in a query
can include any of the corresponding type and class fields that
are defined for resource records; in addition, the query type
(QTYPE) and query class (QCLASS) fields may contain special values
that match more than one of the corresponding fields in RRs.

For example, the QTYPE field may contain:

MAILA - matches all mail agent RRs (e.g. MD and MF).

* - matches any RR type.

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The QCLASS field may contain:

* - matches any RR class.

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.

Note that the QCLASS=* construct requires special interpretation
regarding authority. Since a 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 never be authoritative.

Example space

For purposes of exposition, the following name space is used for
the remainder of this memo:

| | |
| | |
+-----+-----+ | +-----+-----+
| | | | | |
| | | | |
| |
+---+---+ +---+---+
| | | | |

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Name servers store a distributed database consisting of the
structure of the domain name space, the resource sets associated
with domain names, and other information used to coordinate
actions between name servers.

In general, a name server will be an authority for all or part of
a particular domain. The region covered by this authority is
called a zone. Name servers may be responsible for no
authoritative data, and hence have no zones, or may have several
zones. When a name server has multiple zones, the zones may have
no common borders or zones may be contiguous.

While administrators should not construct overlapping zones, and
name servers must defend against overlapping zones, overlapping is
regarded as a non-fatal flaw in the database. Hence the measures
taken to protect against it are omitted for the remainder of this
memo. A detailed discussion can be found in [14].

When presented with a query for a domain name over which it has
authority, a name server returns the desired resource information
or an indication that the query refers to a domain name or
resource that does not exist. If a name server is presented with
a query for a domain name that is not within its authority, it may
have the desired information, but it will also return a response
that points toward an authoritative name server. If a name server
is not an authority for a query, it can never return a negative

There is no requirement that a name server for a domain reside in
a host which has a name in the same domain, although this will
usually be the case. There is also no restriction on the number
of name servers that can have authority over a particular domain;
most domains will have redundant authoritative name servers. The
assumption is that different authoritative copies are identical,
even though inconsistencies are possible as updates are made.

Name server functions are designed to allow for very simple
implementations of name servers. The simplest name server has a
static set of information and uses datagrams to receive queries
and return responses.

More sophisticated name server implementations can improve the
performance of their clients by caching information from other
domains. Although this information can be acquired in a number of
ways, the normal method is to store the information acquired by a

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resolver when the resolver consults other name servers. In a
sophisticated host, the resolver and name server will coordinate
their actions and use a shared database. This cooperation
requires the incorporation of a time-to-live (TTL) field in all
cached resource records. Caching is discussed in the resolver
section of this memo; this section is devoted to the actions of a
name servers that don't cache.

In order to free simple name servers of the requirement of
managing these timeouts, simple name servers should only contain
resource records that are expected to remain constant over very
long periods or resource records for which the name server is an
authority. In the following discussion, the TTL field is assumed
to be stored in the resource record but is omitted in descriptions
of databases and responses in the interest of clarity.

Authority and administrative control of domains

Although we want to have the potential of delegating the
privileges of name space management at every node, we don't want
such delegation to be required.

Hence we introduce the concept of authority. Authority is vested
in name servers. A name server has authority over all of its
domain until it delegates authority for a subdomain to some other
name server.

Any administrative entity that wishes to establish its own domain
must provide a name server, and have that server accepted by the
parent name server (i.e. the name server that has authority over
the place in the domain name space that will hold the new domain).
While the principles of authority allow acceptance to be at the
discretion of parent name servers, the following criteria are used
by the root, and are recommended to all name servers because they
are responsible for their children's actions:

1. It must register with the parent administrator of domains.

2. It must identify a responsible person.

3. In must provide redundant name servers.

The domain name must be registered with the administrator to avoid
name conflicts and to make the domain related information
available to other domains. The central administrator may have
further requirements, and a domain is not registered until the
central administrator agrees that all requirements are met.

There must be a responsible person associated with each domain to

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be a contact point for questions about the domain, to verify and
update the domain related information, and to resolve any problems
(e.g., protocol violations) with hosts in the domain.

The domain must provide redundant (i.e., two or more) name servers
to provide the name to address resolution service. These name
servers must be accessible from outside the domain (as well as
inside) and must resolve names for at least all the hosts in the

Once the central administrator is satisfied, he will communicate
the existence to the appropriate administrators of other domains
so that they can incorporate NS records for the new name server
into their databases.

Name server logic

The processing steps that a name server performs in responding to
a query are conceptually simple, although implementations may have
internal databases that are quite complex.

For purposes of explanation, we assume that the query consists of
a type QTYPE, a class QCLASS, and a domain name QNAME; we assume
that the name server stores its RRs in sets where each set has all
of the RRs for a particular domain. Note that this database
structure and the following algorithms are meant to illustrate one
possible implementation, rather than a specification of how all
servers must be implemented.

The following notation is used:

ord(DOMAIN-NAME) returns the number of labels in DOMAIN-NAME.

findset(DOMAIN-NAME) returns a pointer to the set of stored RRs
for DOMAIN-NAME, or NULL if there is no such

set(POINTER) refers to a set located previously by
findset, where POINTER is the value returned
by findset.

relevant(QTYPE,TYPE) returns true if a RR of the specified TYPE is
relevant to the specified QTYPE. For
example, relevant(MAILA,MF) is true and
relevant(MAILA,NS) is false.

right(NAME,NUMBER) returns a domain name that is the rightmost
NUMBER labels in the string NAME.

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copy(RR) copies the resource record specified by RR
into the response.

The name server code could be represented as the following
sequence of steps:

{ find out whether the database makes this server
authoritative for the domain name specified by QNAME }

for i:=0 to ord(QNAME) { sequence through all nodes in QNAME }
do begin
if ptr<>NULL
then { there is domain data for this domain name }
for all RRs in set(ptr)
do if type(RR)=NS and class(RR)=QCLASS
then begin
for all RRs in set(ptr)
do if type(RR)=SOA and class(RR)=QCLASS
then auth:=true

{ copy out authority search results }

if auth
then { if authority check for domain found }
if ptr=null
then return(Name error)
else { if not authority, copy NS RRs }
for all RRs in set(nsptr)
do if (type(RR)=NS and class(RR)=QCLASS)
then copy(RR);

{ Copy all RRs that answer the question }

for all RRs in set(ptr)
do if class(RR)=QCLASS and relevant(QTYPE,type(RR))
then copy(RR);

The first section of the code (delimited by the for loop over all

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of the subnodes of QNAME) discovers whether the name server is
authoritative for the domain specified by QNAME. It sequences
through all containing domains of QNAME, starting at the root. If
it encounters a SOA it knows that the name server is authoritative
unless it finds a lower NS RR which delegates authority. If the
name server is authoritative, it sets auth=true; if the name
server is not authoritative, it sets NSptr to point to the set
which contains the NS RR closest to the domain specified by QNAME.

The second section of the code reflects the result of the
authority search into the response. If the name server is
authoritative, the code checks to see that the domain specified by
QNAME exists; if not, a name error is returned. If the name
server is not authoritative, the code copies the RRs for a closer
name server into the response.

The last section of the code copies all relevant RRs into the

Note that this code is not meant as an actual implementation and
is incomplete in several aspects. For example, it doesn't deal
with providing additional information, wildcards, QCLASS=*, or
with overlapping zones. The first two of these issues are dealt
with in the following discussions, the remaining issues are
discussed in [14].

Additional information

When a resolver returns information to a user program, the
returned information will often lead to a second query. For
example, if a mailer asks a resolver for the appropriate mail
agent for a particular domain name, the name server queried by the
resolver returns a domain name that identifies the agent. In
general, we would expect that the mailer would then request the
domain name to address binding for the mail agent, and a new name
server query would result.

To avoid this duplication of effort, name servers return
additional information with a response which satisfies the
anticipated query. This information is kept in a separate section
of the response. Name servers are required to complete the
appropriate additional information if such information is
available, but the requestor should not depend on the presence of
the information since the name server may not have it. If the
resolver caches the additional information, it can respond to the
second query without an additional network transaction.

The appropriate information is defined in [14], but generally

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Domain Names - Concepts and Facilities

consists of host to address bindings for domain names in returned

Aliases and canonical names

In existing systems, hosts and other resources often have several
names that identify the same resource. For example, under current
ARPA Internet naming support, USC-ISIF and ISIF 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@ISIF, Mockapetris@ISIB, etc., 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 name and all others are aliases.

The domain system provides a similar feature using the canonical
name (CNAME) RR. When a name server fails to find a desired RR in
a set associated with some domain name, it checks to see if the
resource set contains a CNAME record with a matching class. If
so, the name server includes the CNAME record in the response, and
continues the query at the domain name specified in the data field
of the CNAME record.

Suppose a name server was processing a query with QNAME=ISIF.ARPA,
QTYPE=A, and QCLASS=IN, and had the following resource records:


Both of these RRs would be returned in the response.

In the above example, because ISIF.ARPA has no RRs other than the
CNAME RR, the resources associated with ISIF.ARPA will appear to
be exactly those associated with F.ISI.ARPA for the IN CLASS.
Since the CNAME is effective only when the search fails, a CNAME
can also be used to construct defaults. For example, suppose the
name server had the following set of RRs:


Using this database, type A queries for XXXX.ARPA would return the
queries to XXXX.ARPA would return the XXXX.ARPA MF RR without any
information from F.ISI.ARPA. This structure might be used to send

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Domain Names - Concepts and Facilities

mail addressed to XXXX.ARPA to A.ISI.ARPA and to direct TELNET for


In certain cases, an administrator may wish to associate default
resource information for all or part of a domain. For example,
the CSNET domain administrator may wish to establish IN class mail
forwarding for all hosts in the CSNET domain without IN
capability. In such a case, the domain system provides a special
label '*' that matches any other label. Note that '*' matches
only a single label, and not zero or more than one label. Note
also that the '*' is distinct from the '*' values for QCLASS and

The semantics of '*' depend upon whether it appears in a query
domain name (QNAME) or in a RR in a database.

When an '*' is used in a QNAME, it can only match a '*' in a
resource record.

When '*' appears in a RR in a database, it can never override
an existing exact match. For example, if a name server
received a query for the domain UDEL.CSNET, and had appropriate
RRs for both UDEL.CSNET and *.CSNET, the UDEL.CSNET RRs would
be used and the *.CSNET RRs would be ignored. If a query to
the same database specified FOO.CSNET, the *.CSNET RR would be
used, but the corresponding labels from the QNAME would replace
the '*'. Thus the FOO.CSNET query would match the *.CSNET RR
and return a RR for FOO.CSNET rather than *.CSNET.

RRs containing '*' labels are copied exactly when zones are
transfered via name server maintenance operations.

These semantics are easily implemented by having the name server
first search for an exact match for a query, and then replacing
the leftmost label with a '*' and trying again, repeating the
process until all labels became '*' or the search succeeded.

TYPE=* in RRs is prohibited. If it were to be allowed, the
requestor would have no way of interpreting the data in the RR
because this data is type dependent.

CLASS=* is also prohibited. Similar effects can be achieved using
QCLASS=*, and allowing both QCLASS=* and CLASS=* leads to
complexities without apparent benefit.

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Domain Names - Concepts and Facilities

A scenario

In our sample domain space, suppose we wanted separate
administrative control for the root, DDN, ARPA, CSNET, MIT and ISI
domains. We might allocate name servers as follows:

| | |
+-----+-----+ |(A.ISI.ARPA)+-----+-----+
| | | | | |
| | | | |
+---+---+ +---+---+
| | | | |

In this example the authoritative name server is shown in
parentheses at the point in the domain tree at which is assumes

Thus the root name servers are on B.ISI.ARPA and UDEL.CSNET, the
DDN name server is on JCS.DDN, the CSNET domain server is on

In an actual system, all domains should have redundant name
servers, but in this example only the ARPA domain has redundant
name servers happen to be not redundant because they handle
different classes.) The F.ISI.ARPA name server has authority over
the ARPA domain, but delegates authority over the MIT.ARPA domain
to the name server on AI.MIT.ARPA. The A.ISI.ARPA name server
also has authority over the ARPA domain, but delegates both the
ISI.ARPA and MIT.ARPA domains to other name servers.

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Domain Names - Concepts and Facilities

B.ISI.ARPA Name server for ' '

B.ISI.ARPA has the root name server for the IN class. Its
database might contain:

Domain Resource Record


UDEL.CSNET A CS 302-555-0000

The SOA record for the root is necessary so that the name server
knows that it is authoritative for the root domain for class IN.
The contents of the SOA resource record point back to A.ISI.ARPA
and denote that the master data for the zone of authority is
originally from this host. The first three NS records denote
delegation of authority. The NS root entry for the B.ISI.ARPA
name server is necessary so that this name server knows about
itself, and can respond correctly to a query for NS information
about the root (for which it is an authority). The root entry for
class CS denotes that UDEL.CSNET is the authoritative name server
for the CS class root. UDEL.CSNET and UDEL.ARPA may or may not
refer to the same name server; from this information it is
impossible to tell.

If this name server was sent a query specifying QTYPE=MAILA,
QCLASS=IN, QNAME=F.ISI.ARPA, it would begin processing (using the
previous algorithm) by determining that it was not an authority
for F.ISI.ARPA. The test would note that it had authority at ' ',
but would also note that the authority was delegated at ARPA and
never reestablished via another SOA. Thus the response would
return the NS record for the domain ARPA.

Any queries presented to this server with QCLASS=CS would result
in the UDEL.CSNET NS record being returned in the response.

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Domain Names - Concepts and Facilities

F.ISI.ARPA Name server for ARPA and ISI.ARPA

In the same domain space, the F.ISI.ARPA database for the domains
ARPA and ISI.ARPA might be:

Domain Resource Record




For the IN class, the SOA RR for ARPA denotes that this name
server is authoritative for the domain ARPA, and that the master
file for this authority is stored on B.ISI.ARPA. This zone
extends to ISI.ARPA, where the database delegates authority back
to this name server in another zone, and doesn't include the
domain MIT.ARPA, which is served by a name server on AI.MIT.ARPA.

This name server is not authoritative for any data in the CS
class. It has a pointer to the root server for CS data which
could be use to resolve CS class queries.

Suppose this name server received a query of the form
QNAME=A.ISI.ARPA, QTYPE=A, and QCLASS=IN. The authority search

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Domain Names - Concepts and Facilities

would notice the NS record for ' ', its SOA at ARPA, a delegation
at ISI.ARPA, and the reassumption of authority at ISI.ARPA. Hence
it would know that it was an authority for this query. It would
then find the A record for A.ISI.ARPA, and return a datagram
containing this record.

Another query might be QNAME=B.ISI.ARPA, QTYPE=MAILA, QCLASS=*.
In this case the name server would know that it cannot be
authoritative because of the '*' value of QCLASS, and would look
for records for domain B.ISI.ARPA that match. Assuming that the
name server performs the additional record inclusion mentioned in
the name server algorithm, the returned datagram would include:


name server would discover that AI.MIT.ARPA was the authoritative
name server and return the following:


In this case, the requestor is directed to seek information from
the MIT.ARPA domain name server residing on AI.MIT.ARPA.

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Domain Names - Concepts and Facilities

UDEL.ARPA and UDEL.CSNET name server

In the previous discussion of the sample domain, we stated that
UDEL.CSNET and UDEL.ARPA might be the same name server. In this
example, we assume that this is the case. As such, the name
server is an authority for the root for class CS, and an authority
for the CSNET domain for class IN.

This name server deals with mail forwarding between the ARPA
Internet and CSNET systems. Its RRs illustrate one approach to
solving this problem. The name server has the following resource



UDEL.CSNET A CS 302-555-0000
UCI.CSNET A CS 714-555-0000

Suppose this name server received a query of the form
would discover it was authoritative for the CSNET domain under
class IN, but would find no explicit mail data for UCI.CSNET.
However, using the *.CSNET record, it would construct a reply:


If this name server received a query of the form QNAME=UCI.CSNET,
QTYPE=MAILA, and QCLASS=CS, the name server would return:

UCI.CSNET A CS 714-555-0000

Note that although this scheme allows for forwarding of all mail
addressed as .CSNET, it doesn't help with names that
have more than two components, e.g. A.B.CSNET. Although this
problem could be 'fixed' by a series of MF entries for *.*.CSNET,

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Domain Names - Concepts and Facilities

*.*.*.CSNET, etc, a more tasteful solution would be to introduce a
cleverer pattern matching algorithm in the CSNET name server.

Summary of requirements for name servers

The requirements for a name server are as follows:

1. It must be recognized by its parent.

2. It must have complete resource information for all domain
names for which it is the authority.

3. It must periodically refresh authoritative information from
a master file or name server which holds the master.

4. If it caches information it must also handle TTL management
for that information.

5. It must answer simple queries.

Inverse queries

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 query might map a domain name
to a host address, the corresponding inverse query might map the
address 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 an 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. In general, a resolver or other program that wishes to
guarantee that an inverse query will work must use a name server
that is known to have the appropriate data, or ask all name
servers in a domain of interest.

For example, if a resolver wishes to perform an inverse query for
an arbitrary host on the ARPA Internet, it must consult a set of
name servers sufficient to know that all IN data was considered.
In practice, a single inverse query to a name server that has a
fairly comprehensive database should satisfy the vast majority of
inverse queries.

A detailed discussion of inverse queries is contained in [14].

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Domain Names - Concepts and Facilities

Completion services

Some existing systems provide the ability to complete partial
specifications of arguments. The general principle is that the
user types the first few characters of the argument and then hits
an escape character to prompt the system to complete the rest.
Some completion systems require that the user type enough of the
argument to be unique; others do not.

Other systems allow the user to specify one argument and ask the
system to fill in other arguments. For example, many mail systems
allow the user to specify a username without a host for local mail

The domain system defines name server completion transactions that
perform the analogous service for the domain system.
Implementation of this service is optional in a name server, but
all name servers must at least be able to understand a completion
request and return an error response.

When a resolver wishes to request a completion, it sends a name
server a message that sets QNAME to the partial string, QTYPE to
the type of resource desired, and QCLASS to the desired class.
The completion request also includes a RR for the target domain.
The target domain RR identifies the preferred location of the
resource. In completion requests, QNAME must still have a null
label to terminate the name, but its presence is ignored. Note
that a completion request is not a query, but shares some of the
same field formats.

For example, a completion request might contain QTYPE=A, QNAME=B,
QCLASS=IN and a RR for ISI.ARPA. This request asks for completion
for a resource whose name begins with 'B' and is 'close' to
ISI.ARPA. This might be a typical shorthand used in the ISI
community which uses 'B' as a way of referring to B.ISI.ARPA.

The first step in processing a completion request is to look for a
'whole label' match. When the name server receives the request
mentioned above, it looks at all records that are of type A, class
IN, and whose domain name starts (on the left) with the labels of
QNAME, in this case, 'B'. If multiple records match, the name
server selects those whose domain names match (from the right) the
most labels of the preferred domain name. If there are still
multiple candidates, the name server selects the records that have
the shortest (in terms of octets in the name) domain name. If
several records remain, then the name server returns them all.

If no records are found in the previous algorithm, the name server
assumes that the rightmost label in QNAME is not complete, and

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Domain Names - Concepts and Facilities

looks for records that match but require addition of characters to
the rightmost label of QNAME. For example, the previous search
would not match BB.ARPA to B, but this search would. If multiple
hits are found, the same discarding strategy is followed.

A detailed discussion of completion can be found in [14].



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

Because a resolver may need to consult several name servers, the
amount of time that a resolver will take to complete can vary.
This variance is part of the justification for the split between
name servers and resolvers; name servers may use datagrams and
have a response time that is essentially equal to network delay
plus a short service time, while resolvers may take an essentially
indeterminate amount of time.

We expect to see two types of resolvers: simple resolvers that can
chain through multiple name servers when required, and more
complicated resolvers that cache resource records for use in
future queries.

Simple resolvers

A simple resolver needs the following capabilities:

1. It must know how to access a name server, and should know the
authoritative name server for the host that it services.

2. It must know the protocol capabilities for its clients so that
it can set the class fields of the queries it sends to return
information that is useful to its clients. If the resolver
serves a client that has multiple protocol capabilities, it
should be able to support the preferences of the client.

The resolver for a multiple protocol client can either collect
information for all classes using the * class value, or iterate
on the classes supported by the client. Note that in either
case, the resolver must understand the preferences of the host.
For example, the host that supports both CSNET and ARPA

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Domain Names - Concepts and Facilities

Internet protocols might prefer mail delivery (MD) to mail
forwarding (MF), regardless of protocol, or might prefer one
protocol regardless of whether MD or MF is required. Care is
required to prevent loops.

3. The resolver must be capable of chaining through multiple name
servers to get to an authoritative name server for any query.
The resolver should guard against loops in referrals; a simple
policy is to discard referrals that don't match more of the
query name than the referring name server, and also to avoid
querying the same name server twice (This test should be done
using addresses of name servers instead of domain names to
avoid problems when a name server has multiple domain names or
errors are present in aliases).

4. The resolver must be able to try alternate name servers when a
name server doesn't respond.

5. The resolver must be able to communicate different failure
conditions to its client. These failure conditions include
unknown domain name, unknown resource for a know domain name,
and inability to access any of the authoritative name servers
for a domain.

6. If the resolver uses datagrams for queries, it must recover
from lost and duplicate datagrams.

Resolvers with cache management

Caching provides a tool for improving the performance of name
service, but also is a potential source of incorrect results. For
example, a database might cache information that is later changed
in the authoritative name servers. While this problem can't be
eliminated without eliminating caching, it can be reduced to an
infrequent problem through the use of timeouts.

When name servers return resource records, each record has an
associated time-to-live (TTL) field. This field is expressed in
seconds, and has 16 bits of significance.

When a resolver caches a returned resource record it must also
remember the TTL field. The resolver must discard the record when
the equivalent amount of time has passed. If the resolver shares
a database with a name server, it must decrement the TTL field of
imported records periodically rather than simply deleting the
record. This strategy is necessary to avoid exporting a resource
record whose TTL field doesn't reflect the amount of time that the
resource record has been cached. Of course, the resolver should

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Domain Names - Concepts and Facilities

not decrement the TTL fields of records for which the associated
name server is an authority.

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Domain Names - Concepts and Facilities

Appendix 1 - Domain Name Syntax Specification

The preferred syntax of domain names is given by the following BNF
rules. Adherence to this syntax will result in fewer problems with
many applications that use domain names (e.g., mail, TELNET). Note
that some applications described in [14] use domain names containing
binary information and hence do not follow this syntax.

::= | ' '


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