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query
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... 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 ...
... 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 ...
... 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
...
... 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:
...
... |
+---------+ +----------+ | +--------+
| | user queries | |queries | | |
| User |-------------->| |---------|->|Foreign |
...
... +---------+ +----------+ | +--------+
| | user queries | |queries | | |
| User |-------------->| |---------|->|Foreign |
| Program | | Resolver | | | Name |
...
... 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 ...
... host and
its operating system. User queries will typically be operating system
calls, and the resolver and its cache ...
... 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 ...
... 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
...
... 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 ...
... 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
...
... hence the resolution of a user query may involve several network
accesses and an arbitrary amount of time. The queries to foreign name
servers and the corresponding responses have a standard format described
in this memo, and may be datagrams ...
... | Master |-------------->| Server | | |Resolver|
| files | | | |<--------|--| |
| |/ | | queries | +--------+
+---------+ +----------+ |
...
... 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.
...
... | Master |-------------->| Server | | |Resolver|
| files | | | |<--------|--| |
| |/ | | queries | +--------+
+---------+ +----------+ |
A |maintenance | +--------+
...
... A |maintenance | +--------+
| +------------|->| |
| queries | |Foreign |
| | | Name |
+------------------|--| Server |
...
... name server to acquire a copy of a zone or
to check that an existing copy has not changed. The messages sent for
these maintenance activities follow the same form as queries and
responses, but the message sequences are somewhat different.
...
... |
+---------+ +----------+ | +--------+
| | user queries | |queries | | |
| User |-------------->| |---------|->|Foreign |
...
... +---------+ +----------+ | +--------+
| | user queries | |queries | | |
| User |-------------->| |---------|->|Foreign |
| Program | | Resolver | | | Name |
...
... | Master |-------------->| Server | | |Resolver|
| files | | | |<--------|--| |
| |/ | | queries | +--------+
+---------+ +----------+ |
A |maintenance | +--------+
...
... A |maintenance | +--------+
| +------------|->| |
| queries | |Foreign |
| | | Name |
+------------------|--| Server |
...
... | |----------------+ | |
+---------+ recursive | | |
queries | | |
V | |
+---------+ recursive +----------+ | +--------+
...
... V | |
+---------+ recursive +----------+ | +--------+
| | queries | |queries | | |
| Stub |-------------->| Recursive|---------|->|Foreign |
...
... +---------+ recursive +----------+ | +--------+
| | queries | |queries | | |
| Stub |-------------->| Recursive|---------|->|Foreign |
| Resolver| | Server | | | Name |
...
...
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
...
...
QCLASS fields appear in the question section of a query. QCLASS values
are a superset of CLASS values; every CLASS ...
... 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 ...
... 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 ...
... 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 ...
... address to host name mapping, and to
facilitate queries to locate all gateways on a particular network in the
...
... 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
...
... 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
...
...
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 ...
...
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 ...
... 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 ...
... 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.
...
... 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 ...
... 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 ...
... question to a name server. These fields are a query type (QTYPE), a
query class (QCLASS), and a query domain name ...
... (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
...
... name server; the additional records section contains RRs
which relate to the query, but are not strictly answers for the
question.
...
... 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
...
... identifier is copied
the corresponding reply and can be used by the requester
to match up replies to outstanding queries. ...
... A four bit field that specifies kind of query in this
message. This value is set by the originator of a query
...
... 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: ...
... AA bit
corresponds to the name which matches the query name, or
the first owner name in the answer section. ...
... Recursion Desired - this bit may be set in a query and
is copied into the response. If RD is set, it directs
...
... RD is set, it directs
the name server to pursue the query recursively.
Recursive query support is optional. ...
... Recursion Available - this be is set or cleared in a
response, and denotes whether recursive query support is
available in the name server. ...
... Reserved for future use. Must be zero in all queries
and responses. ...
... Server failure - The name server was
unable to process this query due to a
problem with the name server. ...
... name
server, this code signifies that the
domain name referenced in the query does
not exist. ...
... Not Implemented - The name server does
not support the requested kind of query. ...
...
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:
...
... a two octet code which specifies the type of the query.
The values for this field include all codes valid for a
...
... a two octet code that specifies the class of the query.
For example, the QCLASS field is IN for the Internet ...
... used for any DNS activity, datagrams are preferred for queries due to
their lower overhead and better performance ...
... UDP is not acceptable for zone transfers, but is the recommended method
for standard queries in the Internet. Queries sent using UDP ...
... for standard queries in the Internet. Queries sent using UDP may be
lost, and hence a retransmission ...
... UDP may be
lost, and hence a retransmission strategy is required. Queries or their
responses may be reordered by the network, or by processing in name
...
... client should try other servers and server addresses
before repeating a query to a specific address of a server. ...
... refresh operation) to be made on a single connection.
Since the server would be unable to answer queries anyway, a
unilateral close or reset may be used instead of a graceful
close. ...
... UDP requests while it waits for TCP
data for refreshing or query activities. Similarly, a name server
should not attempt to provide recursive service ...
... data structure. The
main purpose of this structure is to find the nearest ancestor
zone, if any, for arriving standard queries. ...
... . In designing the tree
framework the designer should recognize that query processing will need
to traverse the tree using case-insensitive ...
... database via a simple pointer
replacement. It is very important that when a zone is
refreshed, queries should not use old and new data
simultaneously. ...
... Errors in zone definitions that cause overlapping zones, etc.,
may cause erroneous responses to queries, but problem
determination is simplified, and the contents of one "bad"
zone can't corrupt another. ...
... cache data use absolute times. Absolute numbers are taken with respect
to some known origin and converted to relative values when placed in the
response to a query. When an absolute TTL is negative after conversion
to relative, then the data is expired and should be ignored.
...
... Standard query processing ...
...
When processing queries with QCLASS=*, or some other QCLASS which
matches multiple classes, the response should never be authoritative
...
... zone within the its expiration parameter. In this case, the name server
should answer queries as if it were not supposed to possess the zone.
...
... Inverse queries (Optional) ...
...
Inverse queries are an optional part of the DNS. Name servers are not
required to support any form of inverse queries ...
... queries are an optional part of the DNS. Name servers are not
required to support any form of inverse queries. If a name server
receives an inverse query ...
... queries. If a name server
receives an inverse query that it does not support, it returns an error
response with the "Not Implemented" error set in the header. While
...
... error
response with the "Not Implemented" error set in the header. While
inverse query support is optional, all name servers must be at least
able to return the error response.
...
... The contents of inverse queries and responses ...
... Inverse
queries reverse the mappings performed by standard query operations;
while a standard query ...
... Inverse
queries reverse the mappings performed by standard query operations;
while a standard query maps a domain name ...
... queries reverse the mappings performed by standard query operations;
while a standard query maps a domain name to a resource, an inverse
query maps a resource to a ...
... while a standard query maps a domain name to a resource, an inverse
query maps a resource to a domain name. For example, a standard query
...
... to a resource, an inverse
query maps a resource to a domain name. For example, a standard query
might bind a domain name to a host ...
...
Inverse queries take the form of a single RR in the answer section of
the message, with an empty question section. The owner name of the
...
... take the form of a single RR in the answer section of
the message, with an empty question section. The owner name of the
query RR and its TTL are not significant. The response carries
...
... TTL are not significant. The response carries
questions in the question section which identify all names possessing
the query RR WHICH THE NAME SERVER KNOWS. Since no name server ...
... about all of the domain name space, the response can never be assumed to
be complete. Thus inverse queries are primarily useful for database
management and debugging activities. Inverse ...
... are primarily useful for database
management and debugging activities. Inverse queries are NOT an
acceptable method of mapping host ...
... Where possible, name servers should provide case-insensitive comparisons
for inverse queries. Thus an inverse query asking for an MX RR of
...
... case-insensitive comparisons
for inverse queries. Thus an inverse query asking for an MX RR of
"Venera.isi.edu" should get the same response as a query ...
... query asking for an MX RR of
"Venera.isi.edu" should get the same response as a query for
"VENERA.ISI.EDU"; an inverse query for HINFO RR ...
... "Venera.isi.edu" should get the same response as a query for
"VENERA.ISI.EDU"; an inverse query for HINFO RR "IBM-PC UNIX ...
... PC UNIX" should
produce the same result as an inverse query for "IBM-pc unix". However,
this cannot be guaranteed because name servers may possess RRs that
...
...
When a name server processes an inverse query, it either returns:
...
...
When the response to an inverse query contains one or more QNAMEs, the
owner name and TTL of the RR ...
... TTL of the RR in the answer section which defines the
inverse query is modified to exactly match an RR found at the first
QNAME.
...
...
RRs returned in the inverse queries cannot be cached using the same
mechanism as is used for the replies to standard queries. One reason
...
... RRs returned in the inverse queries cannot be cached using the same
mechanism as is used for the replies to standard queries. One reason
for this is that a name might have multiple RRs of the same type, and
...
... for this is that a name might have multiple RRs of the same type, and
only one would appear. For example, an inverse query for a single
address of a multiply homed host might ...
... Inverse query and response example ...
... The overall structure
of an inverse query for retrieving the domain name that corresponds to
Internet address 10.1.0.52 is shown below:
...
...
This query asks for a question whose answer is the Internet style
address 10.1.0.52. Since the owner name is not known, any ...
...
Note that the QTYPE in a response to an inverse query is the same as the
TYPE field in the answer section of the inverse query. Responses to
...
... Note that the QTYPE in a response to an inverse query is the same as the
TYPE field in the answer section of the inverse query. Responses to
inverse queries may contain multiple questions when the inverse is not
...
... TYPE field in the answer section of the inverse query. Responses to
inverse queries may contain multiple questions when the inverse is not
unique. If the question section in the response is not empty, then the
RR in the answer section is modified to correspond to be an exact copy
...
... Inverse query processing ...
...
Name servers that support inverse queries can support these operations
through exhaustive searches of their databases, but this becomes
...
... Completion queries and responses ...
... Transforming a user request into a query ...
... cache in the usual manner, but still used by the current
request, even after intervals of many seconds due to system
load, query retransmission timeouts, etc. ...
... resolver from accessing the name servers it needs. While
local limits on the number of times a resolver will retransmit
a particular query to a particular name server address are
...
... Sending the queries ...
... As described in [RFC-1034], the basic task of the resolver is to
formulate a query which will answer the client's request and direct that
query to name servers which can provide the information. The resolver
...
... formulate a query which will answer the client's request and direct that
query to name servers which can provide the information. The resolver
will usually only have very strong hints about which servers to ask, in
...
... the form of NS RRs, and may have to revise the query, in response to
CNAMEs, or revise the set of name servers the resolver is asking, in
response to ...
... and some internally generated. Each request is represented by some
state information, and the desired behavior is that the resolver
transmit queries to name servers in a way that maximizes the probability
that the request is answered, minimizes the time that the request takes,
and avoids excessive transmissions. The key algorithm ...
... information of the request to select the next name server address to
query, and also computes a timeout which will cause the next action
should a response not arrive. The next action will usually be a
transmission to some other server, but may be a temporary error to the
...
...
The resolver always starts with a list of server names to query (SLIST).
This list will be all NS RRs ...
... Some name servers send their responses from different
addresses than the one used to receive the query. That is, a
resolver cannot rely that a response will come from the same
address ...
... resolver cannot rely that a response will come from the same
address which it sent the corresponding query to. This name
server bug is typically encountered in UNIX systems.
...
... The results of an inverse query should not be cached.
...
... The results of standard queries where the QNAME contains "*"
labels if the data might be used to construct wildcards. The
...
... RRs for ISI.EDU. The MX RRs at ISI.EDU name
VENERA.ISI.EDU and VAXA.ISI.EDU, and type A queries can find the host
addresses.
...
...
Several outcomes are possible for this query:
...