Domain name service

1. Domain Name Service(DNS)
• A person can be identified by several names: Full
name, nick name etc. Similarly identifiers for hosts are:
• Users prefer hostnames and routers or any machine
refers IP address so a service is needed that converts
hostnames / domains to values which is done by DNS.
Hostnames IP Address
1. Variable length(facebook,
google)
1. Fixed length(4 parts)
2. mnemonic- Easy to
remember
2. Difficult to remember the
integers.
3. Carry no information to
help route packets towards
them.
3. Routing information
embedded within them.

2. Definitions
• A name in a distributed system is a string of bits or characters that is refer to an
entity, ex Hosts, printers, disks, files, Processes, users, mailboxes, news groups,
Web pages etc
• Three types of names: –
Address: the name of an access point to an entity
Identifier: An identifier refers to at most one entity . An identifier always refers to the
same entity
Human-friendly name: • E.g. unix file name, DNS names
• Domain name- Label that defines a realm of administrative autonomy.ex facebook
domain comprises of host belonging to facebook.
• Names are always organized in a name space – A name space is an organization
mechanism for a group of names
Zone: Contiguous section of name space. Ex. Complete tree, a single node or subtree.
Domain Name Resolvers: Also called DNS resolvers, are the computers which are
used by ISPs to respond to a user request to resolve a domain name. "Resolving a
domain name" refers to the translation of a domain name into an IP Address.

3. entity
domain
root
com
edu
facebook

4. Overall work of DNS
WEB BROWSER
User enters
“www.facebook.com”Web browser
calls the service
DNS
IP Address
TCP
connection
to the IP
Address

5. Services of DNS
1. Conversion of host name to IP address.
2. Host aliasing: “alias- alternate names” .
- Many names may map finally to same address.
- Ex. www.facebook.com: www.facebok.com, www.facbook.com
- DNS prevents the user from getting trapped by false website names created by
frauds.
- Lets consider , there is a single machine running two services: Web server, e-
mail server.
For users,
Web Server, server name: www.abc.com, E-mail server name: www.xyz.com
When the user types www.xyz.com, it will connect to www.server.com.
Ex. Facebook.com (alias name)- for users, star.c10r.facebook.com(canonical
name)- original server name
If the single machine is shifted, then the users will be connected to new machine.
- Helps run multiple services from same server.(in case there is a small
Organization who cannot maintain their Own server, they create a alias for
their server but user gets connected to the server from where the organization
got its service)

6. 3. Mail server aliasing : help specify mail server of a
given domain
Ex. Mail address is abc@cbit.ac.in (as request)
DNS finds out who is the host for e-mail server
DNS sends the host name as xyz.cbit.ac.in and then
the mail is sent.
4. Load distribution: Help distributed load across
replicated servers(every server is not working
singly but group of servers to prevent failures).
DNS rotates the server IP addresses among the
replicated servers and give back the result the IP
address of the server which can take the load.

7. Implementation of DNS
1. Centralized architecture
• Host asks for IP address to a central authority and
gets back the reply.
• Problem:
1.Single point of failure
2.Has to cope with high traffic volume.
3.Location: Where will be the DNS server placed?
If Its far from user then more time will be
required to get the reply.
4.Huge database maintenance
5.Overall its not scalable.

8. 2. Hierarchical & distributed
Implementation
Local DNS server- provides DNS service to hosts within
an organization.
Root DNS Server
com
There are 13 root servers
Controlled by US
com org
edu in uk Top level domain
amazon
facebook
co
ac
gov
cbit
Authoritative DNS servers

9. Ways to resolve domain names
client
Local
DNS
Root
.com TLD
server
Facebook
authoritative
server
1
2 3
4
5
6
7
8
IP address
Of facebook.com?
1. Iterative way

10. • Iterative way:-
1. Client requests for IP address of facebook.com to its local DNS.
2. Local DNS sends the request to the root .
3. Root returns the IP address of the DNS server which handles the .com
domain
4. Same request to .com TLD server.
5. .com returns the IP address of facebook authoritative server address.
6. Same request to facebook.com server
7,8. The actual IP address is returned to client.

11. 2. Recursive solution
client facebook
.com
Root
Local
DNS
1
8
2
7
3
6
4
5
Disadvantage: Root server is bottleneck, it it breaks then other servers
cannot be contacted.