Domain name system

NEED OF DNS
 To identify an entity, the Internet uses the IP address,
which uniquely identifies the connection of a host to the
Internet. However, people prefer to use names instead of
addresses. Therefore, we need a system that can map a
name to an address or an address to a name.

HISTORY OF DNS
 When the Internet was small, mapping was done using a
host file. The host file had only two columns: one for the
name and one for the address. Every host could store the
host file on its disk and update it periodically from a
master host file. When a program or user wanted to map
a name to an address, the host consulted the host file and
found the mapping.
 Today, however, it is impossible to have one single host
file relate every address to a name and vice versa. The
host file would be too large to store in every host.

 In addition, it would be impossible to update all the host
files in the world every time there is a change.
 One solution would be to store the entire host file in a
single computer and allow access to this centralized
information to every computer that needs a mapping. But
this would create huge amount of traffic on the Internet.

INTRODUCTION
 Another solution, the one used today, is to divide this
huge information into smaller parts and store each part
on a different computer. In this method, the host that
needs mapping can contact the closest computer holding
the needed information. This method is used by the
Domain Name System (DNS).

Design principle of DNS
 The naming system on which DNS is based is a hierarchical and logical tree
structure called the domain namespace.
 An organization obtains authority for parts of the name space, and can add
additional layers of the hierarchy
 In practice, allocation of the domain names generally follows the allocation of IP
address, e.g.,
 All hosts with network prefix 128.143/16 have domain name suffix
virginia.edu
 All hosts on network 128.143.136/24 are in the Computer Science
Department of the University of Virginia

NAME SPACE
 A name space that maps each address to a unique name
can be organized in two ways:
a. FLAT NAME SPACE
In this, a name is assigned to an address. A name in
this space is a sequence without structure.
Disadvantage: Cannot be used in large systems like
Internet because it must be centrally controlled to
avoid ambiguity and duplication.

b. HIERARCHIAL NAME SPACE
• Each name is made of several parts.
• The first part can define the nature of organization, the
second part can define the name, the third part can define
departments and so on.
• The authority to assign and control the name spaces can be
decentralized.

DOMAIN NAME SPACE
 To have a hierarchical name space, Domain Name Space
was designed. In this design, the names are defined in an
inverted-tree structure with the root at the top. The tree
can have only 128 levels: level 0 (root) to level 127. Each
level of the tree defines a hierarchical level.

The DNS Name Space
A portion of the Internet domain name space.

 LABEL: Each node in a tree has a label, which is a string
with a maximum of 63 characters. The root label is a null
string. DNS requires that children of a node have different
labels, which guarantees the uniqueness of the domain
names.
 DOMAIN NAME: Each node in the tree has a domain
name. A full domain name is a sequence of labels
separated by dots(.). The domain names are always read
from the node upto the root.

Domain name system
 Each node in the DNS tree represents
a DNS name
 Each branch below a node is a DNS
domain.
 DNS domain can contain hosts or
other domains (subdomains)
 Example:
DNS domains are
., edu, virginia.edu, cs.virginia.edu
virginia.edu
cs.virginia.eduwww.virginia.edu
neon.cs.virginia.edu
edu
.

 FULLY QUALIFIED DOMAIN NAME (FQDN):
 Every node in the DNS domain tree can be identified by a unique
Fully Qualified Domain Name (FQDN). The FQDN gives the
position in the DNS tree.
 In this, the label is terminated by a null string.
 It is a domain name that contains the full name of the host.
 It contains all labels, from the most specific to the most general,
that uniquely define the host.
 PARTIALLY QUALIFIED DOMAIN NAME (PQDN)
 In this, the label is not terminated by a null string.
 A PQDN starts with a node but does not reach the root.
 It is used when the name to be resolved belongs to the same site
as the client.

Top-level domains
 Three types of top-level domains:
 Organizational: 3-character code indicates the function of the
organization
 Used primarily within the US
 Examples: gov, mil, edu, org, com, net
 Geographical: 2-character country or region code
 Examples: us, va, jp, de
 Reverse domains: A special domain (in-addr.arpa) used for IP
address-to-name mapping
There are more than 200 top-level domains.

Organizational top-level domains
com Commercial organizations
edu Educational institutions
gov Government institutions
int International organizations
mil U.S. military institutions
net Networking organizations
org Non-profit organizations

Hierarchy of name servers
 The resolution of the
hierarchical name space is
done by a hierarchy of name
servers
 Each server is responsible
(authoritative) for a
contiguous portion of the DNS
namespace, called a zone.
 Zone is a part of the subtree
 DNS server answers queries
about hosts in its zone
root server
com servergov serveredu serverorg server
uci.edu
server
.virginia.edu
server
cs.virginia.edu
server
DISTRIBUTION OF NAME SPACE

Authority and Delegation
 Authority for the root domain is with the Internet
Corporation for Assigned Numbers and Names (ICANN)
 ICANN delegates to accredited registrars (for gTLDs) and
countries for country code top level domains (ccTLDs)
 Authority can be delegated further
 Chain of delegation can be obtained by reading domain
name from right to left.
 Unit of delegation is a “zone”.

DNS Domains and Zones
 Each zone is anchored at a
specific domain node, but
zones are not domains.
 A DNS domain is a branch of
the namespace
 A zone is a portion of the DNS
namespace generally stored in
a file (It could consists of
multiple nodes)
 A server can divide part of its
zone and delegate it to other
servers
. (root)
.virginia.edu
.edu
.uci.edu
cs.virginia.edumath.virginia.edu
Domain
Zone
and
domain
Zone

Primary and Secondary Name Servers
 For each zone, there must be a primary name server and a secondary
name server
 The primary server (master server) maintains a zone file which has
information about the zone. Updates are made to the primary server
 The secondary server copies data stored at the primary server.
Adding a host:
 When a new host is added (“gold.cs.virginia.edu”) to a zone, the
administrator adds the IP information on the host (IP address and name) to
a configuration file on the primary server

RESOURCE RECORDS
 The database records of
the distributed data base
are called resource records
(RR)
 Resource records are
stored in configuration files
(zone files) at name
servers.
 Left Resource records for a
zone:
db.mylab.com
$TTL 86400
mylab.com. IN SOA PC4.mylab.com.
hostmaster.mylab.com. (
1 ; serial
28800 ; refresh
7200 ; retry
604800 ; expire
86400 ; ttl
)
;
mylab.com. IN NS PC4.mylab.com.
;
localhost A 127.0.0.1
PC4.mylab.com. A 10.0.1.41

Resource Records
The principal DNS resource records types.

Resource Records
db.mylab.com
$TTL 86400
mylab.com. IN SOA PC4.mylab.com. hostmaster.mylab.com. (
1 ; serial
28800 ; refresh
7200 ; retry
604800 ; expire
86400 ; ttl
)
;
mylab.com. IN NS PC4.mylab.com.
;
localhost A 127.0.0.1
Max. age of cached data
in seconds
* Start of authority (SOA) record.
Means: “This name server is
authoritative for the zone
Mylab.com”
* PC4.mylab.com is the
name server
* hostmaster@mylab.com is the
email address of the person
in charge
Name server (NS) record.
One entry for each authoritative
name server
Address (A) records.
One entry for each hostaddress

Resolver and name server
1. An application program on a host
accesses the domain system through
a DNS client, called the resolver
2. Resolver contacts DNS server, called
name server
3. DNS server returns IP address to
resolver which passes the IP address
to application
 Reverse lookups are also possible,
i.e., find the hostname given an IP
address
HTTP Resolver
Hostname (neon.tcpip-lab.edu)
IP address (128.143.71.21)
Name
server
Hostname
(neon.tcpip-lab.edu)
IPaddress(128.143.71.21)

Domain name resolution
1. User program issues a request for
the IP address of a hostname
2. Local resolver formulates a DNS
query to the name server of the host
3. Name server checks if it is
authorized to answer the query.
a) If yes, it responds.
b) Otherwise, it will query other
name servers, starting at the
root tree
4. When the name server has the
answer it sends it to the resolver.
HTTP Resolver
Hostname (neon.tcpip-lab.edu)
IP address (128.143.71.21)
Name
server
Hostname
(neon.tcpip-lab.edu)
IPaddress(128.143.71.21)

Recursive and Iterative Queries
 There are two types of queries:
 Recursive queries
 Iterative (non-recursive) queries
 The type of query is determined by a bit in the DNS query
 Recursive query: When the name server of a host cannot
resolve a query, the server issues a query to resolve the query
 Iterative queries: When the name server of a host cannot
resolve a query, it sends a referral to another server to the
resolver

Recursive queries
 In a recursive query, the resolver
expects the response from the name
server
 If the server cannot supply the
answer, it will send the query to the
“closest known” authoritative name
server (here: In the worst case, the
closest known server is the root
server)
 The root sever sends a referral to the
“edu” server. Querying this server
yields a referral to the server of
“virginia.edu”
 … and so on
root server
edu server
virginia.edu
server
cs.virginia.edu
server
Resolver
Name
server
query
response
Referral to edu name server
1st query: neon.cs.virginia.edu
2nd query: neon.cs.virginia.edu
Referral to virginia.edu name
server
3rd
query:
Referral to
cs.virginia.edu
name server
4th query:
IP address of

Iterative queries
 In an iterative query, the name server
sends a closest known authoritative
name server a referral to the root
server.
 This involves more work for the
resolver
root server
edu server
virginia.edu
server
cs.virginia.edu
server
Resolver
Name
server
query
referraltorootserver
Referral to
edu
nam
e
server
1
st query: neon.cs.virginia.edu
2
nd query: neon.cs.virginia.edu
Referral to virginia.edu name
server
3
rd query: neon.cs.virginia.edu
Referral to cs.virginia.edu
name server
4th
query:
IP address of

Caching
 To reduce DNS traffic, name servers caches information on
domain name/IP address mappings
 When an entry for a query is in the cache, the server does not
contact other servers
 Note: If an entry is sent from a cache, the reply from the
server is marked as “unauthoritative”

Domain name system

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