Reaffirming the age-old belief that men are incapable of doing more than one thing at once, a new study has shown that men really are worse at multitasking than women, although it does depend on the task. Professor Keith Laws, a psychologist at the University of Hertfordshire, who led the research, and colleagues found that [...]
Arranging seminars and discussion forums to explore how to manage a troublesome boss is common. Diane Stafford of The Kansas City Star gives some common takeaways from those seminars. The first one is that you can’t change your boss, you can only change your own reactions. It’s up to you to figure out the communication [...]
Some call it ‘Hardly Required’, while others prefer ‘Harassing Resource’. What are we talking about? HR = once known as ‘Human Resources’. With changing times, the Human got lost somewhere and the Resources are no more accessible. We’ve all had our rather unpleasant experiences (99% of the time) with this species that every company hires [...]
This RFC describes the details of the domain system and protocol, and assumes that the reader is familiar with the concepts discussed in a companion RFC, "Domain Names - Concepts and Facilities" [RFC-1034]. The domain system is a mixture of functions and data types which are an official protocol and functions and data types which are still experimental. Since the domain system is intentionally extensible, new data types and experimental behavior should always be expected in parts of the system beyond the official protocol. The official protocol parts include standard queries, responses and the Internet class RR data formats (e.g., host addresses). Since the previous RFC set, several definitions have changed, so some previous definitions are obsolete. Experimental or obsolete features are clearly marked in these RFCs, and such information should be used with caution. The reader is especially cautioned not to depend on the values which appear in examples to be current or complete, since their purpose is primarily pedagogical. Distribution of this memo is unlimited. Table of Contents 1. STATUS OF THIS MEMO 1 2. INTRODUCTION 3 2.1. Overview 3 2.2. Common configurations 4 2.3. Conventions 7 2.3.1. Preferred name syntax 7 2.3.2. Data Transmission Order 8 2.3.3. Character Case 9 2.3.4. Size limits 10 3. DOMAIN NAME SPACE AND RR DEFINITIONS 10 3.1. Name space definitions 10 3.2. RR definitions 11 3.2.1. Format 11 3.2.2. TYPE values 12 3.2.3. QTYPE values 12 3.2.4. CLASS values 13 Mockapetris [Page 1]
RFC 1035 Domain Implementation and Specification November 1987 3.2.5. QCLASS values 13 3.3. Standard RRs 13 3.3.1. CNAME RDATA format 14 3.3.2. HINFO RDATA format 14 3.3.3. MB RDATA format (EXPERIMENTAL) 14 3.3.4. MD RDATA format (Obsolete) 15 3.3.5. MF RDATA format (Obsolete) 15 3.3.6. MG RDATA format (EXPERIMENTAL) 16 3.3.7. MINFO RDATA format (EXPERIMENTAL) 16 3.3.8. MR RDATA format (EXPERIMENTAL) 17 3.3.9. MX RDATA format 17 3.3.10. NULL RDATA format (EXPERIMENTAL) 17 3.3.11. NS RDATA format 18 3.3.12. PTR RDATA format 18 3.3.13. SOA RDATA format 19 3.3.14. TXT RDATA format 20 3.4. ARPA Internet specific RRs 20 3.4.1. A RDATA format 20 3.4.2. WKS RDATA format 21 3.5. IN-ADDR.ARPA domain 22 3.6. Defining new types, classes, and special namespaces 24 4. MESSAGES 25 4.1. Format 25 4.1.1. Header section format 26 4.1.2. Question section format 28 4.1.3. Resource record format 29 4.1.4. Message compression 30 4.2. Transport 32 4.2.1. UDP usage 32 4.2.2. TCP usage 32 5. MASTER FILES 33 5.1. Format 33 5.2. Use of master files to define zones 35 5.3. Master file example 36 6. NAME SERVER IMPLEMENTATION 37 6.1. Architecture 37 6.1.1. Control 37 6.1.2. Database 37 6.1.3. Time 39 6.2. Standard query processing 39 6.3. Zone refresh and reload processing 39 6.4. Inverse queries (Optional) 40 6.4.1. The contents of inverse queries and responses 40 6.4.2. Inverse query and response example 41 6.4.3. Inverse query processing 42 Mockapetris [Page 2]
RFC 1035 Domain Implementation and Specification November 1987 6.5. Completion queries and responses 42 7. RESOLVER IMPLEMENTATION 43 7.1. Transforming a user request into a query 43 7.2. Sending the queries 44 7.3. Processing responses 46 7.4. Using the cache 47 8. MAIL SUPPORT 47 8.1. Mail exchange binding 48 8.2. Mailbox binding (Experimental) 48 9. REFERENCES and BIBLIOGRAPHY 50 Index 54
The goal of domain names is to provide a mechanism for naming resources
in such a way that the names are usable in different hosts, networks,
protocol families, internets, and administrative organizations.
From the user's point of view, domain names are useful as arguments to a
local agent, called a resolver, which retrieves information associated
with the domain name. Thus a user might ask for the host address or
mail information associated with a particular domain name. To enable
the user to request a particular type of information, an appropriate
query type is passed to the resolver with the domain name. To the user,
the domain tree is a single information space; the resolver is
responsible for hiding the distribution of data among name servers from
the user.
From the resolver's point of view, the database that makes up the domain
space is distributed among various name servers. Different parts of the
domain space are stored in different name servers, although a particular
data item will be stored redundantly in two or more name servers. The
resolver starts with knowledge of at least one name server. When the
resolver processes a user query it asks a known name server for the
information; in return, the resolver either receives the desired
information or a referral to another name server. Using these
referrals, resolvers learn the identities and contents of other name
servers. Resolvers are responsible for dealing with the distribution of
the domain space and dealing with the effects of name server failure by
consulting redundant databases in other servers.
Name servers manage two kinds of data. The first kind of data held in
sets called zones; each zone is the complete database for a particular
"pruned" subtree of the domain space. This data is called
authoritative. A name server periodically checks to make sure that its
zones are up to date, and if not, obtains a new copy of updated zones
Mockapetris [Page 3]
RFC 1035 Domain Implementation and Specification November 1987 from master files stored locally or in another name server. The second kind of data is cached data which was acquired by a local resolver. This data may be incomplete, but improves the performance of the retrieval process when non-local data is repeatedly accessed. Cached data is eventually discarded by a timeout mechanism. This functional structure isolates the problems of user interface, failure recovery, and distribution in the resolvers and isolates the database update and refresh problems in the name servers.
depending on whether the host runs programs that retrieve information
from the domain system, name servers that answer queries from other
hosts, or various combinations of both functions. The simplest, and
perhaps most typical, configuration is shown below:
Local Host | Foreign
|
+---------+ +----------+ | +--------+
| | user queries | |queries | | |
| User |-------------->| |---------|->|Foreign |
| Program | | Resolver | | | Name |
| |<--------------| |<--------|--| Server |
| | user responses| |responses| | |
+---------+ +----------+ | +--------+
| A |
cache additions | | references |
V | |
+----------+ |
| cache | |
+----------+ |
User programs interact with the domain name space through resolvers; the
format of user queries and user responses is specific to the host and
its operating system. User queries will typically be operating system
calls, and the resolver and its cache will be part of the host operating
system. Less capable hosts may choose to implement the resolver as a
subroutine to be linked in with every program that needs its services.
Resolvers answer user queries with information they acquire via queries
to foreign name servers and the local cache.
Note that the resolver may have to make several queries to several
different foreign name servers to answer a particular user query, and
hence the resolution of a user query may involve several network
accesses and an arbitrary amount of time. The queries to foreign name
servers and the corresponding responses have a standard format described
Mockapetris [Page 4]
RFC 1035 Domain Implementation and Specification November 1987 in this memo, and may be datagrams. Depending on its capabilities, a name server could be a stand alone program on a dedicated machine or a process or processes on a large timeshared host. A simple configuration might be: Local Host | Foreign | +---------+ | / /| | +---------+ | +----------+ | +--------+ | | | | |responses| | | | | | | Name |---------|->|Foreign | | Master |-------------->| Server | | |Resolver| | files | | | |<--------|--| | | |/ | | queries | +--------+ +---------+ +----------+ | Here a primary name server acquires information about one or more zones by reading master files from its local file system, and answers queries about those zones that arrive from foreign resolvers. The DNS requires that all zones be redundantly supported by more than one name server. Designated secondary servers can acquire zones and check for updates from the primary server using the zone transfer protocol of the DNS. This configuration is shown below: Local Host | Foreign | +---------+ | / /| | +---------+ | +----------+ | +--------+ | | | | |responses| | | | | | | Name |---------|->|Foreign | | Master |-------------->| Server | | |Resolver| | files | | | |<--------|--| | | |/ | | queries | +--------+ +---------+ +----------+ | A |maintenance | +--------+ | +------------|->| | | queries | |Foreign | | | | Name | +------------------|--| Server | maintenance responses | +--------+ In this configuration, the name server periodically establishes a virtual circuit to a foreign name server to acquire a copy of a zone or to check that an existing copy has not changed. The messages sent for Mockapetris [Page 5]
RFC 1035 Domain Implementation and Specification November 1987 these maintenance activities follow the same form as queries and responses, but the message sequences are somewhat different. The information flow in a host that supports all aspects of the domain name system is shown below: Local Host | Foreign | +---------+ +----------+ | +--------+ | | user queries | |queries | | | | User |-------------->| |---------|->|Foreign | | Program | | Resolver | | | Name | | |<--------------| |<--------|--| Server | | | user responses| |responses| | | +---------+ +----------+ | +--------+ | A | cache additions | | references | V | | +----------+ | | Shared | | | database | | +----------+ | A | | +---------+ refreshes | | references | / /| | V | +---------+ | +----------+ | +--------+ | | | | |responses| | | | | | | Name |---------|->|Foreign | | Master |-------------->| Server | | |Resolver| | files | | | |<--------|--| | | |/ | | queries | +--------+ +---------+ +----------+ | A |maintenance | +--------+ | +------------|->| | | queries | |Foreign | | | | Name | +------------------|--| Server | maintenance responses | +--------+ The shared database holds domain space data for the local name server and resolver. The contents of the shared database will typically be a mixture of authoritative data maintained by the periodic refresh operations of the name server and cached data from previous resolver requests. The structure of the domain data and the necessity for synchronization between name servers and resolvers imply the general characteristics of this database, but the actual format is up to the local implementor. Mockapetris [Page 6]
RFC 1035 Domain Implementation and Specification November 1987 Information flow can also be tailored so that a group of hosts act together to optimize activities. Sometimes this is done to offload less capable hosts so that they do not have to implement a full resolver. This can be appropriate for PCs or hosts which want to minimize the amount of new network code which is required. This scheme can also allow a group of hosts can share a small number of caches rather than maintaining a large number of separate caches, on the premise that the centralized caches will have a higher hit ratio. In either case, resolvers are replaced with stub resolvers which act as front ends to resolvers located in a recursive server in one or more name servers known to perform that service: Local Hosts | Foreign | +---------+ | | | responses | | Stub |<--------------------+ | | Resolver| | | | |----------------+ | | +---------+ recursive | | | queries | | | V | | +---------+ recursive +----------+ | +--------+ | | queries | |queries | | | | Stub |-------------->| Recursive|---------|->|Foreign | | Resolver| | Server | | | Name | | |<--------------| |<--------|--| Server | +---------+ responses | |responses| | | +----------+ | +--------+ | Central | | | cache | | +----------+ | In any case, note that domain components are always replicated for reliability whenever possible.
The domain system has several conventions dealing with low-level, but fundamental, issues. While the implementor is free to violate these conventions WITHIN HIS OWN SYSTEM, he must observe these conventions in ALL behavior observed from other hosts.
The DNS specifications attempt to be as general as possible in the rules
for constructing domain names. The idea is that the name of any
existing object can be expressed as a domain name with minimal changes.
Mockapetris [Page 7]
RFC 1035 Domain Implementation and Specification November 1987 However, when assigning a domain name for an object, the prudent user will select a name which satisfies both the rules of the domain system and any existing rules for the object, whether these rules are published or implied by existing programs. For example, when naming a mail domain, the user should satisfy both the rules of this memo and those in RFC-822. When creating a new host name, the old rules for HOSTS.TXT should be followed. This avoids problems when old software is converted to use domain names. The following syntax will result in fewer problems with many applications that use domain names (e.g., mail, TELNET). <domain> ::= <subdomain> | " " <subdomain> ::= <label> | <subdomain> "." <label> <label> ::= <letter> [ [ <ldh-str> ] <let-dig> ] <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str> <let-dig-hyp> ::= <let-dig> | "-" <let-dig> ::= <letter> | <digit> <letter> ::= any one of the 52 alphabetic characters A through Z in upper case and a through z in lower case <digit> ::= any one of the ten digits 0 through 9 Note that while upper and lower case letters are allowed in domain names, no significance is attached to the case. That is, two names with the same spelling but different case are to be treated as if identical. The labels must follow the rules for ARPANET host names. They must start with a letter, end with a letter or digit, and have as interior characters only letters, digits, and hyphen. There are also some restrictions on the length. Labels must be 63 characters or less. For example, the following strings identify hosts in the Internet: A.ISI.EDU XX.LCS.MIT.EDU SRI-NIC.ARPA
The order of transmission of the header and data described in this
document is resolved to the octet level. Whenever a diagram shows a
Mockapetris [Page 8]
RFC 1035 Domain Implementation and Specification November 1987 group of octets, the order of transmission of those octets is the normal order in which they are read in English. For example, in the following diagram, the octets are transmitted in the order they are numbered. 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 3 | 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 5 | 6 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Whenever an octet represents a numeric quantity, the left most bit in the diagram is the high order or most significant bit. That is, the bit labeled 0 is the most significant bit. For example, the following diagram represents the value 170 (decimal). 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |1 0 1 0 1 0 1 0| +-+-+-+-+-+-+-+-+ Similarly, whenever a multi-octet field represents a numeric quantity the left most bit of the whole field is the most significant bit. When a multi-octet quantity is transmitted the most significant octet is transmitted first.
For all parts of the DNS that are part of the official protocol, all
comparisons between character strings (e.g., labels, domain names, etc.)
are done in a case-insensitive manner. At present, this rule is in
force throughout the domain system without exception. However, future
additions beyond current usage may need to use the full binary octet
capabilities in names, so attempts to store domain names in 7-bit ASCII
or use of special bytes to terminate labels, etc., should be avoided.
When data enters the domain system, its original case should be
preserved whenever possible. In certain circumstances this cannot be
done. For example, if two RRs are stored in a database, one at x.y and
one at X.Y, they are actually stored at the same place in the database,
and hence only one casing would be preserved. The basic rule is that
case can be discarded only when data is used to define structure in a
database, and two names are identical when compared in a case
insensitive manner.
Mockapetris [Page 9]
RFC 1035 Domain Implementation and Specification November 1987 Loss of case sensitive data must be minimized. Thus while data for x.y and X.Y may both be stored under a single location x.y or X.Y, data for a.x and B.X would never be stored under A.x, A.X, b.x, or b.X. In general, this preserves the case of the first label of a domain name, but forces standardization of interior node labels. Systems administrators who enter data into the domain database should take care to represent the data they supply to the domain system in a case-consistent manner if their system is case-sensitive. The data distribution system in the domain system will ensure that consistent representations are preserved.
Various objects and parameters in the DNS have size limits. They are listed below. Some could be easily changed, others are more fundamental. labels 63 octets or less names 255 octets or less TTL positive values of a signed 32 bit number. UDP messages 512 octets or less
Domain names in messages are expressed in terms of a sequence of labels.
Each label is represented as a one octet length field followed by that
number of octets. Since every domain name ends with the null label of
the root, a domain name is terminated by a length byte of zero. The
high order two bits of every length octet must be zero, and the
remaining six bits of the length field limit the label to 63 octets or
less.
To simplify implementations, the total length of a domain name (i.e.,
label octets and label length octets) is restricted to 255 octets or
less.
Although labels can contain any 8 bit values in octets that make up a
label, it is strongly recommended that labels follow the preferred
syntax described elsewhere in this memo, which is compatible with
existing host naming conventions. Name servers and resolvers must
compare labels in a case-insensitive manner (i.e., A=a), assuming ASCII
with zero parity. Non-alphabetic codes must match exactly.
Mockapetris [Page 10]
RFC 1035 Domain Implementation and Specification November 1987
All RRs have the same top level format shown below:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| |
/ /
/ NAME /
| |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| TYPE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| CLASS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| TTL |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| RDLENGTH |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
/ RDATA /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
NAME an owner name, i.e., the name of the node to which this
resource record pertains.
TYPE two octets containing one of the RR TYPE codes.
CLASS two octets containing one of the RR CLASS codes.
TTL a 32 bit signed integer that specifies the time interval
that the resource record may be cached before the source
of the information should again be consulted. Zero
values are interpreted to mean that the RR can only be
used for the transaction in progress, and should not be
cached. For example, SOA records are always distributed
with a zero TTL to prohibit caching. Zero values can
also be used for extremely volatile data.
RDLENGTH an unsigned 16 bit integer that specifies the length in
octets of the RDATA field.
Mockapetris [Page 11]
RFC 1035 Domain Implementation and Specification November 1987 RDATA a variable length string of octets that describes the resource. The format of this information varies according to the TYPE and CLASS of the resource record.
TYPE fields are used in resource records. Note that these types are a subset of QTYPEs. TYPE value and meaning
NS 2 an authoritative name server MD 3 a mail destination (Obsolete - use MX) MF 4 a mail forwarder (Obsolete - use MX) CNAME 5 the canonical name for an alias SOA 6 marks the start of a zone of authority MB 7 a mailbox domain name (EXPERIMENTAL) MG 8 a mail group member (EXPERIMENTAL) MR 9 a mail rename domain name (EXPERIMENTAL) NULL 10 a null RR (EXPERIMENTAL) WKS 11 a well known service description PTR 12 a domain name pointer HINFO 13 host information MINFO 14 mailbox or mail list information MX 15 mail exchange TXT 16 text strings
QTYPE fields appear in the question part of a query. QTYPES are a
superset of TYPEs, hence all TYPEs are valid QTYPEs. In addition, the
following QTYPEs are defined:
Mockapetris [Page 12]
RFC 1035 Domain Implementation and Specification November 1987 AXFR 252 A request for a transfer of an entire zone MAILB 253 A request for mailbox-related records (MB, MG or MR) MAILA 254 A request for mail agent RRs (Obsolete - see MX) * 255 A request for all records
CLASS fields appear in resource records. The following CLASS mnemonics
and values are defined:
IN 1 the Internet
CS 2 the CSNET class (Obsolete - used only for examples in
some obsolete RFCs)
CH 3 the CHAOS class
HS 4 Hesiod [Dyer 87]
QCLASS fields appear in the question section of a query. QCLASS values are a superset of CLASS values; every CLASS is a valid QCLASS. In addition to CLASS values, the following QCLASSes are defined: * 255 any class
The following RR definitions are expected to occur, at least
potentially, in all classes. In particular, NS, SOA, CNAME, and PTR
will be used in all classes, and have the same format in all classes.
Because their RDATA format is known, all domain names in the RDATA
section of these RRs may be compressed.
<domain-name> is a domain name represented as a series of labels, and
terminated by a label with zero length. <character-string> is a single
length octet followed by that number of characters. <character-string>
is treated as binary information, and can be up to 256 characters in
length (including the length octet).
Mockapetris [Page 13]
RFC 1035 Domain Implementation and Specification November 1987
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ CNAME /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
CNAME A <domain-name> which specifies the canonical or primary
name for the owner. The owner name is an alias.
CNAME RRs cause no additional section processing, but name servers may
choose to restart the query at the canonical name in certain cases. See
the description of name server logic in [RFC-1034] for details.
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ CPU /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ OS /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
CPU A <character-string> which specifies the CPU type.
OS A <character-string> which specifies the operating
system type.
Standard values for CPU and OS can be found in [RFC-1010].
HINFO records are used to acquire general information about a host. The
main use is for protocols such as FTP that can use special procedures
when talking between machines or operating systems of the same type.
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ MADNAME /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
MADNAME A <domain-name> which specifies a host which has the
specified mailbox.
Mockapetris [Page 14]
RFC 1035 Domain Implementation and Specification November 1987 MB records cause additional section processing which looks up an A type RRs corresponding to MADNAME.
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ MADNAME /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
MADNAME A <domain-name> which specifies a host which has a mail
agent for the domain which should be able to deliver
mail for the domain.
MD records cause additional section processing which looks up an A type
record corresponding to MADNAME.
MD is obsolete. See the definition of MX and [RFC-974] for details of
the new scheme. The recommended policy for dealing with MD RRs found in
a master file is to reject them, or to convert them to MX RRs with a
preference of 0.
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ MADNAME /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
MADNAME A <domain-name> which specifies a host which has a mail
agent for the domain which will accept mail for
forwarding to the domain.
MF records cause additional section processing which looks up an A type
record corresponding to MADNAME.
MF is obsolete. See the definition of MX and [RFC-974] for details ofw
the new scheme. The recommended policy for dealing with MD RRs found in
a master file is to reject them, or to convert them to MX RRs with a
preference of 10.
Mockapetris [Page 15]
RFC 1035 Domain Implementation and Specification November 1987
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ MGMNAME /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
MGMNAME A <domain-name> which specifies a mailbox which is a
member of the mail group specified by the domain name.
MG records cause no additional section processing.
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ RMAILBX /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ EMAILBX /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
RMAILBX A <domain-name> which specifies a mailbox which is
responsible for the mailing list or mailbox. If this
domain name names the root, the owner of the MINFO RR is
responsible for itself. Note that many existing mailing
lists use a mailbox X-request for the RMAILBX field of
mailing list X, e.g., Msgroup-request for Msgroup. This
field provides a more general mechanism.
EMAILBX A <domain-name> which specifies a mailbox which is to
receive error messages related to the mailing list or
mailbox specified by the owner of the MINFO RR (similar
to the ERRORS-TO: field which has been proposed). If
this domain name names the root, errors should be
returned to the sender of the message.
MINFO records cause no additional section processing. Although these
records can be associated with a simple mailbox, they are usually used
with a mailing list.
Mockapetris [Page 16]
RFC 1035 Domain Implementation and Specification November 1987
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ NEWNAME /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
NEWNAME A <domain-name> which specifies a mailbox which is the
proper rename of the specified mailbox.
MR records cause no additional section processing. The main use for MR
is as a forwarding entry for a user who has moved to a different
mailbox.
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| PREFERENCE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ EXCHANGE /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
PREFERENCE A 16 bit integer which specifies the preference given to
this RR among others at the same owner. Lower values
are preferred.
EXCHANGE A <domain-name> which specifies a host willing to act as
a mail exchange for the owner name.
MX records cause type A additional section processing for the host
specified by EXCHANGE. The use of MX RRs is explained in detail in
[RFC-974].
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ <anything> /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Anything at all may be in the RDATA field so long as it is 65535 octets
or less.
Mockapetris [Page 17]
RFC 1035 Domain Implementation and Specification November 1987 NULL records cause no additional section processing. NULL RRs are not allowed in master files. NULLs are used as placeholders in some experimental extensions of the DNS.
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ NSDNAME /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
NSDNAME A <domain-name> which specifies a host which should be
authoritative for the specified class and domain.
NS records cause both the usual additional section processing to locate
a type A record, and, when used in a referral, a special search of the
zone in which they reside for glue information.
The NS RR states that the named host should be expected to have a zone
starting at owner name of the specified class. Note that the class may
not indicate the protocol family which should be used to communicate
with the host, although it is typically a strong hint. For example,
hosts which are name servers for either Internet (IN) or Hesiod (HS)
class information are normally queried using IN class protocols.
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ PTRDNAME /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
PTRDNAME A <domain-name> which points to some location in the
domain name space.
PTR records cause no additional section processing. These RRs are used
in special domains to point to some other location in the domain space.
These records are simple data, and don't imply any special processing
similar to that performed by CNAME, which identifies aliases. See the
description of the IN-ADDR.ARPA domain for an example.
Mockapetris [Page 18]
RFC 1035 Domain Implementation and Specification November 1987
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ MNAME /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ RNAME /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| SERIAL |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| REFRESH |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| RETRY |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| EXPIRE |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| MINIMUM |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
MNAME The <domain-name> of the name server that was the
original or primary source of data for this zone.
RNAME A <domain-name> which specifies the mailbox of the
person responsible for this zone.
SERIAL The unsigned 32 bit version number of the original copy
of the zone. Zone transfers preserve this value. This
value wraps and should be compared using sequence space
arithmetic.
REFRESH A 32 bit time interval before the zone should be
refreshed.
RETRY A 32 bit time interval that should elapse before a
failed refresh should be retried.
EXPIRE A 32 bit time value that specifies the upper limit on
the time interval that can elapse before the zone is no
longer authoritative.
Mockapetris [Page 19]
RFC 1035 Domain Implementation and Specification November 1987 MINIMUM The unsigned 32 bit minimum TTL field that should be exported with any RR from this zone. SOA records cause no additional section processing. All times are in units of seconds. Most of these fields are pertinent only for name server maintenance operations. However, MINIMUM is used in all query operations that retrieve RRs from a zone. Whenever a RR is sent in a response to a query, the TTL field is set to the maximum of the TTL field from the RR and the MINIMUM field in the appropriate SOA. Thus MINIMUM is a lower bound on the TTL field for all RRs in a zone. Note that this use of MINIMUM should occur when the RRs are copied into the response and not when the zone is loaded from a master file or via a zone transfer. The reason for this provison is to allow future dynamic update facilities to change the SOA RR with known semantics.
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / TXT-DATA / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ where: TXT-DATA One or more <character-string>s. TXT RRs are used to hold descriptive text. The semantics of the text depends on the domain where it is found.
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
ADDRESS A 32 bit Internet address.
Hosts that have multiple Internet addresses will have multiple A
records.
Mockapetris [Page 20]
RFC 1035 Domain Implementation and Specification November 1987
an A line in a master file is an Internet address expressed as four decimal numbers separated by dots without any imbedded spaces (e.g., "10.2.0.52" or "192.0.5.6").
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| PROTOCOL | |
+--+--+--+--+--+--+--+--+ |
| |
/ <BIT MAP> /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
ADDRESS An 32 bit Internet address
PROTOCOL An 8 bit IP protocol number
<BIT MAP> A variable length bit map. The bit map must be a
multiple of 8 bits long.
The WKS record is used to describe the well known services supported by
a particular protocol on a particular internet address. The PROTOCOL
field specifies an IP protocol number, and the bit map has one bit per
port of the specified protocol. The first bit corresponds to port 0,
the second to port 1, etc. If the bit map does not include a bit for a
protocol of interest, that bit is assumed zero. The appropriate values
and mnemonics for ports and protocols are specified in [RFC-1010].
For example, if PROTOCOL=TCP (6), the 26th bit corresponds to TCP port
port 25; if zero, SMTP service is not supported on the specified
address.
The purpose of WKS RRs is to provide availability information for
servers for TCP and UDP. If a server supports both TCP and UDP, or has
multiple Internet addresses, then multiple WKS RRs are used.
WKS RRs cause no additional section processing.
In master files, both ports and protocols are expressed using mnemonics
or decimal numbers.
Mockapetris [Page 21]
RFC 1035 Domain Implementation and Specification November 1987
The Internet uses a special domain to support gateway location and
Internet address to host mapping. Other classes may employ a similar
strategy in other domains. The intent of this domain is to provide a
guaranteed method to perform host address to host name mapping, and to
facilitate queries to locate all gateways on a particular network in the
Internet.
Note that both of these services are similar to functions that could be
performed by inverse queries; the difference is that this part of the
domain name space is structured according to address, and hence can
guarantee that the appropriate data can be located without an exhaustive
search of the domain space.
The domain begins at IN-ADDR.ARPA and has a substructure which follows
the Internet addressing structure.
Domain names in the IN-ADDR.ARPA domain are defined to have up to four
labels in addition to the IN-ADDR.ARPA suffix. Each label represents
one octet of an Internet address, and is expressed as a character string
for a decimal value in the range 0-255 (with leading zeros omitted
except in the case of a zero octet which is represented by a single
zero).
Host addresses are represented by domain names that have all four labels
specified. Thus data for Internet address 10.2.0.52 is located at
domain name 52.0.2.10.IN-ADDR.ARPA. The reversal, though awkward to
read, allows zones to be delegated which are exactly one network of
address space. For example, 10.IN-ADDR.ARPA can be a zone containing
data for the ARPANET, while 26.IN-ADDR.ARPA can be a separate zone for
MILNET. Address nodes are used to hold pointers to primary host names
in the normal domain space.
Network numbers correspond to some non-terminal nodes at various depths
in the IN-ADDR.ARPA domain, since Internet network numbers are either 1,
2, or 3 octets. Network nodes are used to hold pointers to the primary
host names of gateways attached to that network. Since a gateway is, by
definition, on more than one network, it will typically have two or more
network nodes which point at it. Gateways will also have host level
pointers at their fully qualified addresses.
Both the gateway pointers at network nodes and the normal host pointers
at full address nodes use the PTR RR to point back to the primary domain
names of the corresponding hosts.
For example, the IN-ADDR.ARPA domain will contain information about the
ISI gateway between net 10 and 26, an MIT gateway from net 10 to MIT's
Mockapetris [Page 22]
RFC 1035 Domain Implementation and Specification November 1987 net 18, and hosts A.ISI.EDU and MULTICS.MIT.EDU. Assuming that ISI gateway has addresses 10.2.0.22 and 26.0.0.103, and a name MILNET- GW.ISI.EDU, and the MIT gateway has addresses 10.0.0.77 and 18.10.0.4 and a name GW.LCS.MIT.EDU, the domain database would contain: 10.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU. 10.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU. 18.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU. 26.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU. 22.0.2.10.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU. 103.0.0.26.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU. 77.0.0.10.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU. 4.0.10.18.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU. 103.0.3.26.IN-ADDR.ARPA. PTR A.ISI.EDU. 6.0.0.10.IN-ADDR.ARPA. PTR MULTICS.MIT.EDU. Thus a program which wanted to locate gateways on net 10 would originate a query of the form QTYPE=PTR, QCLASS=IN, QNAME=10.IN-ADDR.ARPA. It would receive two RRs in response: 10.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU. 10.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU. The program could then originate QTYPE=A, QCLASS=IN queries for MILNET- GW.ISI.EDU. and GW.LCS.MIT.EDU. to discover the Internet addresses of these gateways.
host address 10.0.0.6 would pursue a query of the form QTYPE=PTR,
QCLASS=IN, QNAME=6.0.0.10.IN-ADDR.ARPA, and would receive:
6.0.0.10.IN-ADDR.ARPA. PTR MULTICS.MIT.EDU.
Several cautions apply to the use of these services:
- Since the IN-ADDR.ARPA special domain and the normal domain
for a particular host or gateway will be in different zones,
the possibility exists that that the data may be inconsistent.
- Gateways will often have two names in separate domains, only
one of which can be primary.
- Systems that use the domain database to initialize their
routing tables must start with enough gateway information to
guarantee that they can access the appropriate name server.
- The gateway data only reflects the existence of a gateway in a
manner equivalent to the current HOSTS.TXT file. It doesn't
replace the dynamic availability information from GGP or EGP.
Mockapetris [Page 23]
RFC 1035 Domain Implementation and Specification November 1987
The previously defined types and classes are the ones in use as of the
date of this memo. New definitions should be expected. This section
makes some recommendations to designers considering additions to the
existing facilities. The mailing list NAMEDROPPERS@SRI-NIC.ARPA is the
forum where general discussion of design issues takes place.
In general, a new type is appropriate when new information is to be
added to the database about an existing object, or we need new data
formats for some totally new object. Designers should attempt to define
types and their RDATA formats that are generally applicable to all
classes, and which avoid duplication of information. New classes are
appropriate when the DNS is to be used for a new protocol, etc which
requires new class-specific data formats, or when a copy of the existing
name space is desired, but a separate management domain is necessary.
New types and classes need mnemonics for master files; the format of the
master files requires that the mnemonics for type and class be disjoint.
TYPE and CLASS values must be a proper subset of QTYPEs and QCLASSes
respectively.
The present system uses multiple RRs to represent multiple values of a
type rather than storing multiple values in the RDATA section of a
single RR. This is less efficient for most applications, but does keep
RRs shorter. The multiple RRs assumption is incorporated in some
experimental work on dynamic update methods.
The present system attempts to minimize the duplication of data in the
database in order to insure consistency. Thus, in order to find the
address of the host for a mail exchange, you map the mail domain name to
a host name, then the host name to addresses, rather than a direct
mapping to host address. This approach is preferred because it avoids
the opportunity for inconsistency.
In defining a new type of data, multiple RR types should not be used to
create an ordering between entries or express different formats for
equivalent bindings, instead this information should be carried in the
body of the RR and a single type used. This policy avoids problems with
caching multiple types and defining QTYPEs to match multiple types.
For example, the original form of mail exchange binding used two RR
types one to represent a "closer" exchange (MD) and one to represent a
"less close" exchange (MF). The difficulty is that the presence of one
RR type in a cache doesn't convey any information about the other
because the query which acquired the cached information might have used
a QTYPE of MF, MD, or MAILA (which matched both). The redesigned
Mockapetris [Page 24]
RFC 1035 Domain Implementation and Specification November 1987 service used a single type (MX) with a "preference" value in the RDATA section which can order different RRs. However, if any MX RRs are found in the cache, then all should be there.
All communications inside of the domain protocol are carried in a single
format called a message. The top level format of message is divided
into 5 sections (some of which are empty in certain cases) shown below:
+---------------------+
| Header |
+---------------------+
| Question | the question for the name server
+---------------------+
| Answer | RRs answering the question
+---------------------+
| Authority | RRs pointing toward an authority
+---------------------+
| Additional | RRs holding additional information
+---------------------+
The header section is always present. The header includes fields that
specify which of the remaining sections are present, and also specify
whether the message is a query or a response, a standard query or some
other opcode, etc.
The names of the sections after the header are derived from their use in
standard queries. The question section contains fields that describe a
question to a name server. These fields are a query type (QTYPE), a
query class (QCLASS), and a query domain name (QNAME). The last three
sections have the same format: a possibly empty list of concatenated
resource records (RRs). The answer section contains RRs that answer the
question; the authority section contains RRs that point toward an
authoritative name server; the additional records section contains RRs
which relate to the query, but are not strictly answers for the
question.
Mockapetris [Page 25]
RFC 1035 Domain Implementation and Specification November 1987
The header contains the following fields:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode |AA|TC|RD|RA| Z | RCODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QDCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ANCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| NSCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ARCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
ID A 16 bit identifier assigned by the program that
generates any kind of query. This identifier is copied
the corresponding reply and can be used by the requester
to match up replies to outstanding queries.
QR A one bit field that specifies whether this message is a
query (0), or a response (1).
OPCODE A four bit field that specifies kind of query in this
message. This value is set by the originator of a query
and copied into the response. The values are:
0 a standard query (QUERY)
1 an inverse query (IQUERY)
2 a server status request (STATUS)
3-15 reserved for future use
AA Authoritative Answer - this bit is valid in responses,
and specifies that the responding name server is an
authority for the domain name in question section.
Note that the contents of the answer section may have
multiple owner names because of aliases. The AA bit
Mockapetris [Page 26]
RFC 1035 Domain Implementation and Specification November 1987 corresponds to the name which matches the query name, or the first owner name in the answer section. TC TrunCation - specifies that this message was truncated due to length greater than that permitted on the transmission channel. RD Recursion Desired - this bit may be set in a query and is copied into the response. If RD is set, it directs the name server to pursue the query recursively. Recursive query support is optional. RA Recursion Available - this be is set or cleared in a response, and denotes whether recursive query support is available in the name server.
and responses.
RCODE Response code - this 4 bit field is set as part of
responses. The values have the following
interpretation:
0 No error condition
1 Format error - The name server was
unable to interpret the query.
2 Server failure - The name server was
unable to process this query due to a
problem with the name server.
3 Name Error - Meaningful only for
responses from an authoritative name
server, this code signifies that the
domain name referenced in the query does
not exist.
4 Not Implemented - The name server does
not support the requested kind of query.
5 Refused - The name server refuses to
perform the specified operation for
policy reasons. For example, a name
server may not wish to provide the
information to the particular requester,
or a name server may not wish to perform
a particular operation (e.g., zone
Mockapetris [Page 27]
RFC 1035 Domain Implementation and Specification November 1987 transfer) for particular data. 6-15 Reserved for future use. QDCOUNT an unsigned 16 bit integer specifying the number of entries in the question section. ANCOUNT an unsigned 16 bit integer specifying the number of resource records in the answer section. NSCOUNT an unsigned 16 bit integer specifying the number of name server resource records in the authority records section. ARCOUNT an unsigned 16 bit integer specifying the number of resource records in the additional records section.
The question section is used to carry the "question" in most queries,
i.e., the parameters that define what is being asked. The section
contains QDCOUNT (usually 1) entries, each of the following format:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| |
/ QNAME /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QTYPE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QCLASS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
QNAME a domain name represented as a sequence of labels, where
each label consists of a length octet followed by that
number of octets. The domain name terminates with the
zero length octet for the null label of the root. Note
that this field may be an odd number of octets; no
padding is used.
QTYPE a two octet code which specifies the type of the query.
The values for this field include all codes valid for a
TYPE field, together with some more general codes which
can match more than one type of RR.
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