2. Objectives
• Discuss the Need of DNS
• List the Services Provided by DNS
• Demonstrate the working of DNS
• Analyze the DNS Records
• Apply the format of DNS Messages to
prepare DNS message
3. Objectives
• Discuss the Need of DNS
• List the Services Provided by DNS
• Demonstrate the working of DNS
• Analyze the DNS Records
• Apply the format of DNS Messages to
prepare DNS message
4. Need of DNS
• What is your voter’s id number?
• What is your driving license number?
• What is your voter’s id number?
• What is your university register number?
• What is your name?
5. Need of DNS…
• One identifier for a host is its hostname.
• Hostnames are mnemonic and are therefore
appreciated by humans.
• such as:
– cnn.com,
– www.yahoo.com,
– gaia.cs.umass.edu and
– surf.eurecom.fr
6. Need of DNS…
• Hostnames provide little information about the
location within the Internet of the host.
• A hostname such as surf.eurecom.fr, which
ends with the country code .fr, tells us that the
host is in France, but doesn't say much more.
• Furthermore, because hostnames can consist of
variable-length alpha-numeric characters, they
would be difficult to process by routers.
• For these reasons, hosts are also identified by
so-called IP addresses.
7. Need of DNS…
• An IP address consists of four bytes and has a rigid
hierarchical structure.
• An IP address looks like 121.7.106.83, where each
period separates one of the bytes expressed in
decimal notation from 0 to 127.
• An IP address is hierarchical because as we scan
the address from left to right, we obtain more and
more specific information about where the host is
located in the Internet. (like a postal address)
• An IP address is included in the header of each IP
datagram, and Internet routers use this IP address
to route datagram towards its destination
8. Objectives
• Discuss the Need of DNS
• List the Services Provided by DNS
• Demonstrate the working of DNS
• Analyze the DNS Records
• Apply the format of DNS Messages to
prepare DNS message
9. Services Provided by DNS
• Among the two identities people prefer the
more mnemonic hostname identifier, while
routers prefer fixed-length, hierarchically-
structured IP addresses.
• Hence we need a directory service that
translates hostnames to IP addresses.
• This is the main task of the Internet's Domain
Name System (DNS).
10. Services Provided by DNS…
The DNS is
(i) a distributed database implemented in a
hierarchy of name servers and
(ii) an application-layer protocol that allows
hosts and name servers to communicate in
order to provide the translation service.
11. Services Provided by DNS…
• DNS is commonly employed by other
application-layer protocols -- including
HTTP, SMTP and FTP - to translate user
supplied host names to IP addresses.
• In order for the user's machine to be able to
send an HTTP request message to the Web
server www.someschool.edu, the user's
machine must obtain the IP address of
www.someschool.edu.
12. Services Provided by DNS…
• The same user machine runs the client-side of
the DNS application.
• The browser extracts the
hostname, www.someschool.edu, from the URL
and passes the hostname to the client-side of the
DNS application.
• As part of a DNS query message, the DNS client
sends a query containing the hostname to a DNS
server.
• The DNS client eventually receives a reply, which
includes the IP address for the hostname.
13. Services Provided by DNS…
• The browser then opens a TCP connection to the
HTTP server process located at that IP address.
• All IP datagrams sent to from the client to server
as part of this connection will include this IP
address in the destination address field of the
datagrams.
• The IP datagram(s) that encapsulate the HTTP
request message use this IP address
14. Services Provided by DNS…
• DNS provides a few other important services in
addition to translating hostnames to IP
addresses:
– Host aliasing:
– Mail server aliasing:
– Load Distribution:
15. Host aliasing
• A host with a complicated hostname can have
one or more alias names.
• For ex: relay1.west-coast.enterprise.com
could have, say, two aliases such as
enterprise.com and www.enterprise.com.
• In this case, the hostname relay1.west-
coast.enterprise.com is said to be canonical
hostname.
• DNS can be invoked by an application to obtain
the canonical hostname for a supplied alias
hostname as well as the IP address of the host.
16. Mail server aliasing
• The email address must be mnemonic like
bob@hotmail.com.
• The canonical hostname might be something
like relay1.west-coast.hotmail.com
• DNS can be invoked by a mail application to
obtain the canonical hostname for a supplied
alias hostname as well as the IP address of the
host.
17. Load Distribution
• DNS is also being used to perform load
distribution among replicated servers, such as
replicated Web servers.
• Ex: www.google.com directed to
www.google.co.in
• A set of IP addresses is associated with one
canonical hostname.
18. Objectives
• Discuss the Need of DNS
• List the Services Provided by DNS
• Demonstrate the working of DNS
• Analyze the DNS Records
• Apply the format of DNS Messages to
prepare DNS message
19. Working of DNS
• Centralized design
• Problems of centralized design
– A single point of failure.
– Traffic volumes.
– Distant centralized database.
– Maintenance
20. Working of DNS…
• DNS uses a large number of name
servers, organized in a hierarchical fashion and
distributed around the world.
• three types of name servers:
– local name servers,
– root name servers,
– and authoritative name servers.
21. Local name servers
• The local name server is typically "close" to
the client.
• In the case of an institutional ISP, it may be on
the same LAN as the client host.
• For a residential ISP, the name server is
typically separated from the client host by no
more than a few routers.
23. Authoritative name servers:
Client
Local NS
Root NS
IP ADDRESS
Authoritative
NS
a name server is
authoritative for a host if it
always has a
DNS record that translates
the host's hostname to
that host's IP address.
25. For a given
hostname, the
root name server
may only know
the IP address of
an intermediate
name server that
in turn knows the
IP address of an
authoritative
name server for
the hostname.
27. DNS caching.
• When a name server receives a DNS mapping
for some hostname, it caches the mapping in
local memory (disk or RAM) while passing the
message along the name server chain.
28. DNS Records
• The name servers that together implement
the DNS distributed database, store Resource
Records (RR) for the hostname to IP address
mappings.
• A resource record is a four-tuple that contains
the following fields:
(Name, Value, Type, TTL)
29. DNS Records
• TTL is the time to live of the resource record;
it determines the time at which a resource
should be removed from a cache.
30. DNS Records
• The meaning of Name and Value depend on Type:
Type Name Value
A Host Name IP address for the hostname.
NS domain Value is the hostname of a server that knows
how to
obtain the IP addresses for hosts in the domain.
CNAME alias hostname Name canonical hostname for the alias hostname Name
MX alias hostname Name. hostname of a mail server
31. DNS Records...
• Example
Type Name Value
A relay1.bar.foo.com 145.37.93.126
NS foo.com dns.foo.com
CNAME foo.com relay1.bar.foo.com
MX foo.com mail.bar.foo.com,
