Domainnamesystem

Domain Name
System
Mrs.G.Chandraprabha,M.Sc.,M.Phil.,
Assistant Professor,
Department of Information Technology,
V.V.Vanniaperumal College for Women,
Virudhunagar.

• What is DNS?
• Internet Directory Service
• A client-server application that maps host names into their
corresponding IP addresses
• Mapping host names into their corresponding IP
addresses is called name resolution or name translation
or name mapping or Address Resolution

DNS Name Space
 For the Internet, the top of the naming hierarchy is
managed by an organiza- tion called ICANN (Internet
Corporation for Assigned Names and Numbers).
ICANN was created for this purpose in 1998, as part of the
maturing of the Inter- net to a worldwide, economic
concern.
 Conceptually, the Internet is divided into over 250 top-
level domains, where each domain covers many hosts.
Each domain is partitioned into subdomains, and these
are further partitioned, and so on.
 The leaves of the tree represent domains that have no
subdomains (but do contain machines, of course).
A leaf domain may contain a single host, or it may
represent a company and contain thousands of hosts.

A portion of the Internet domain name space.
inTop level domains
Portion of the Internet domain name space.

DNS Name Space
 The top-level domains come in two flavors: generic and countries.
The generic domains are given below,
 Domain Intended use Start date Restricted?
com Commercial 1985 No
edu Educational institutions 1985 Yes
gov Government 1985 Yes
int International organizations 1988 Yes
mil Military 1985 Yes
net Network providers 1985 No
org Non-profit organizations 1985 No
aero Air transport 2001 Yes
biz Businesses 2001 No
coop Cooperatives 2001 Yes
info Informational 2002 No
museum Museums 2002 Yes
name People 2002 No
pro Professionals 2002 Yes
cat Catalan 2005 Yes
jobs Employment 2005 Yes
mobi Mobile devices 2005 Yes
tel Contact details 2005 Yes
travel Travel industry 2005 Yes
xxx Sex industry 2010 No

DNS Name Space
 The country domains include one entry for every country, as
defined in ISO 3166.
 Internationalized country domain names that use non-Latin
alphabets were introduced in 2010.
These domains let people name hosts in Arabic, Cyrillic, Chinese,
or other languages.
Getting a second-level domain, such as name-of-company.com, is
easy. The top-level domains are run by registrars appointed by
ICANN.
Getting a name merely requires going to a corresponding registrar
(for com in this case) to check if the desired name is available and
not somebody else’s trademark.
If there are no problems, the requester pays the registrar a small
annual fee and gets the name.

DNS Name Space
 Each domain is named by the path upward from it to the
(unnamed) root.
The components are separated by periods (pronounced ‘‘dot’’)
Domain names can be either absolute or relative.
An absolute domain name always ends with a period (e.g.,
eng.cisco.com.), whereas a relative one does not.
Relative names have to be interpreted in some context to uniquely
determine their true meaning.
In both cases, a named domain refers to a specific node in the tree
and all the nodes under it.
Domain names are case-insensitive, so edu, Edu, and EDU mean
the same thing.
 Component names can be up to 63 characters long, and full path
names must not exceed 255 characters.

Domain Resource Records
Every domain, whether it is a single host or a top-level domain, can
have a setof resource records associated with it.
 These records are the DNS database.
 For a single host, the most common resource record is just its IP
address, but many other kinds of resource records also exist.
 When a resolver gives a domain name to DNS, what it gets back
are the resource records associated with that name.
Thus, the primary function of DNS is to map domain names onto
resource records.A resource record is a five-tuple.
Although they are encoded in binary for efficiency,in most
expositions resource records are presented as ASCII text, oneline
per resource record.
The format we will use is as follows:
Domain name Time to live Class Type Value

The Domain name tells the domain to which this record applies.
Normally, many records exist for each domain and each copy of the
database holds information about multiple domains. This field is
thus the primary search key used to satisfy queries. The order of the
records in the database is not significant.
The Time to live field gives an indication of how stable the record
is. Information that is highly stable is assigned a large value, such
as 86400 (the numberof seconds in 1 day). Information that is
highly volatile is assigned a small value, such as 60 (1 minute)
The third field of every resource record is the Class. For Internet
information, it is always IN. For non-Internet information, other
codes can be used, but in practice these are rarely seen.
The Type field tells what kind of record this is. There are many
kinds of DNS records.

An SOA record provides the name of the primary source of
information about the name server’s zone (described below), the
email address of its administrator, a unique serial number, and
various flags and timeouts.
The most important record type is the A (Address) record. It holds a
32-bit IPv4 address of an interface for some host.
The corresponding AAAA, or ‘‘quad A,’’ record holds a 128-bit IPv6
address. Every Internet host must have at least one IP address so
that other machines can communicate with it.
Some hosts have two or more network interfaces, in which case
they will have two or more type A or AAAA resource records.
 Consequently, DNS can return multiple addresses for a single
name.

A common record type is the MX record.
 It specifies the name of the host prepared to accept email for the
specified domain.
Another important record type is the NS record. It specifies a name
server for the domain or subdomain. This is a host that has a copy
of the database for a domain. It is used as part of the process to look
up names, which we will describe shortly.
CNAME records allow aliases to be created .
Like CNAME, PTR points to another name. However, unlike
CNAME, which is really just a macro definition (i.e., a mechanism
to replace one string by another), PTR is a regular DNS data type
whose interpretation depends on the context in which it is found.
RV is a newer type of record that allows a host to be identified for a
given service in a domain.

SPF is also a newer type of record. It lets a domain encode
information about what machines in the domain will send mail to
the rest of the Internet. This helps receiving machines check that
mail is valid.
 Last on the list, TXT records were originally provided to allow
domains to identify themselves in arbitrary ways. Nowadays, they
usually encode machine readable information, typically the SPF
information.

Type Meaning Value
SOA Start of authority Parameters for this zone
A IPv4 address of a host 32-Bit integer
AAAA IPv6 address of a host 128-Bit integer
MX Mail exchange Priority, domain willing to accept email
NS Name server Name of a server for this domain
CNAME Canonical name Domain name
PTR Pointer Alias for an IP address
SPF Sender policy framework Text encoding of mail sending
policy
SRV Service Host that provides it
TXT Text Descriptive ASCII text
The principal DNS resource record types.

 Finally, we have the Value field. This field can be a number, a domain name,or
an ASCII string. The semantics depend on the record type.
 For an example of the kind of information one might find in the DNS database of
a domain
 ; Authoritative data for cs.vu.nl
cs.vu.nl. 86400 IN SOA star boss (9527,7200,7200,241920,86400)
cs.vu.nl. 86400 IN MX 1 zephyr
cs.vu.nl. 86400 IN MX 2 top
cs.vu.nl. 86400 IN NS star

Name Servers
 A single name server could contain the entire DNS database and
respond to all queries about it.
 In practice, this server would be so overloaded as to be useless.
 Furthermore, if it ever went down, the entire Internet would be
crippled.
 To avoid the problems associated with having only a single source
of information, the DNS name space is divided into
nonoverlapping zones.

Name Servers
Each circled zone contains some part of the tree.
Where the zone boundaries are placed within a zone is
up to that zone’s administrator

Name Servers
Each zone is also associated with one or more name servers.
These are hosts that hold the database for the zone.
Normally, a zone will have one primary nameserver, which gets its
information from a file on its disk, and one or more secondary
name servers, which get their information from the primary name
server.
To improve reliability, some of the name servers can be located
outside the zone.
The process of looking up a name and finding an address is called
name resolution.
When a resolver has a query about a domain name, it passes the
query to a local name server.
An authoritative record is one that comes from the authoritythat
manages the record and is thus always correct.
Authoritative records are in contrast to cached records, which may
be out of date.

Domainnamesystem

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