If your network
devices require connectivity with devices in networks for which you do not
control name assignment, you can assign device names that uniquely identify
your devices within the entire internetwork. The global naming scheme of the
Internet, the DNS, accomplishes this task. This service is enabled by default.
The following sections summarize DNS concepts and function:
Host Names for Network
Each unique IP
address can have an associated host name. DNS uses a hierarchical scheme for
establishing host names for network nodes. This allows local control of the
segments of the network through a client-server scheme. The DNS system can
locate a network device by translating the host name of the device into its
associated IP address.
Domains Names for Groups of
IP defines a naming
scheme that allows a device to be identified by its location in the IP. This is
a hierarchical naming scheme that provides for
domains . On the
Internet, a domain is a portion of the naming hierarchy tree that refers to
general groupings of networks based on organization type or geography. Domain
names are pieced together with periods (.) as the delimiting characters. For
example, Cisco is a commercial organization that the IP identifies by a
name, so its domain name is
cisco.com . A
specific device in this domain, the File Transfer Protocol (FTP) system, for
example, is identified as
To keep track of
domain names, IP has defined the concept of a
server . Name servers are programs that have
complete information about their namespace portion of the domain tree and may
also contain pointers to other name servers that can be used to lead to
information from any other part of the domain tree. Name servers know the parts
of the domain tree for which they have complete information. A name server may
also store information about other parts of the domain tree. To map domain
names to IP addresses, you must first identify the host names, then specify a
name server, and enable the DNS service.
To speed the
process of converting names to addresses, the name server maintains a database,
cache , of
host-name-to-address mappings for use by the
telnet , and
commands, and related Telnet support operations. The cache stores the results
from previous responses. Upon receiving a client-issued DNS query, it will
check this local storage to see if the answer is available locally.
Name resolvers are
programs that extract information from name servers in response to client
requests. Resolvers must be able to access at least one name server. The
resolver either uses that name server's information to answer a query directly
or pursues the query using referrals to other names servers. A resolver will
typically be a system routine that is directly accessible to user programs.
Therefore, no protocol is necessary between the resolver and the user program.
namespace is divided into areas called zones that are points of delegation in
the DNS tree. A zone contains all domains from a certain point downward, except
those for which other zones are authoritative.
A name server is
said to be an authority for the parts of the domain tree for which it has
complete information. A zone usually has an authoritative name server, often
more than one. An authoritative name server has been configured with host table
information or has acquired host table information though a zone transfer (the
action that occurs when a secondary DNS server starts up and updates itself
from the primary server).
organization, you can have many name servers, but Internet clients can query
only those that the root name servers know. The other name servers answer
internal queries only.
A name server
handles client-issued queries to the DNS server for locally defined hosts
within a particular zone as follows:
authoritative name server responds to DNS user queries for a domain name that
is under its zone of authority by using the permanent and cached entries in its
own host table. If the query is for a domain name that is under its zone of
authority but for which it does not have any configuration information, the
authoritative name server simply replies that no such information exists..
A name server
that is not configured as the authoritative name server responds to DNS user
queries by using information that it has cached from previously received query
responses. If no device is configured as the authoritative name server for a
zone, queries to the DNS server for locally defined hosts will receive
Name servers answer
DNS queries (forward incoming DNS queries or resolve internally generated DNS
queries) according to the forwarding and lookup parameters configured for the
sequence of DNS public key (DNSKEY) RR sets and Delegation Signer (DS) RR sets
forms a chain of signed data, with each link in the chain vouching for the
next. A DNSKEY RR is used to verify the signature covering a DS RR and allows
the DS RR to be authenticated. The DS RR contains a hash of another DNSKEY RR
and this new DNSKEY RR is authenticated by matching the hash in the DS RR.