1. Domain Name System (DNS)

2. DNS The “Domain Name System”
What Internet users use to reference anything by name on the Internet
It translates easily memorized domain names to the numerical IP addresses needed for the purpose of locating computer services and devices worldwide

3. DNS
A lookup mechanism for translating names to addresses and vice versa
Users generally prefer names to numbers
Computers prefer numbers to names
DNS provides the mapping between the two
I have “x”, give me “y”
Copyright © 2001, Nominum, Inc.

4. DNS Components There are 3 components: Name Space:
Specifications for a structured name space and data associated with the names
Resolvers:
Client programs that extract information from Name Servers.
Name Servers:
Server programs which hold information about the structure and the names.

5. Overview Introduction to the DNS DNS Components The name space
The servers
The resolvers
Copyright © 2001, Nominum, Inc.

6. The Name Space The name space is the structure of the DNS database
An inverted tree with the root node at the top
Each node has a label
The root node has a null label, written as “”
Copyright © 2001, Nominum, Inc.

8. Copyright © 2001, Nominum, Inc.
Labels
Each node in the tree must have a label
A-Z, 0-9, and “-” only with a-z and A-Z treated as the same
Sibling nodes must have unique labels
The null label is reserved for the root node
Copyright © 2001, Nominum, Inc.

9. Domain Name Each node in the DNS tree represents a domain name.
Some examples of DNS names are
DNS domains
computers and services (host)

10. Copyright © 2001, Nominum, Inc.
Domain Names
A domain name is the sequence of labels from a node to the root, separated by dots (“.”s), read left to right
The name space has a maximum depth of 127 levels
Domain names are limited to 255 characters in length
A node’s domain name identifies its position in the name space
Copyright © 2001, Nominum, Inc.

11. DNS domain A DNS domain is a branch under the node.
DNS domains can contain both hosts (computers or services) and other domains (referred to as subdomains ).

12. Subdomains
One domain is a subdomain of another if its apex node is a descendant of the other’s apex node
More simply, one domain is a subdomain of another if its domain name ends in the other’s domain name
So sales.nominum.com is a subdomain of
nominum.com
com
nominum.com is a subdomain of com
Copyright © 2001, Nominum, Inc.

13. The parts of a domain name
mail.ksu.edu
The particular host is called main
The organization that controls it is called ksu
This host is at an educational organization
A domain is a group of related hosts.

14. How are these hosts related?
bpastudio.csudh.edu
sws.csudh.edu
chemistry.stanford.edu
They all belong to educational institutions.
They are members of the edu top-level domain (TLD).

15. How are these hosts related?
sms.ksu.edu
chemistry.ksu.edu
They all belong to the same organization, ksu.
They are members of the ksu.edu second-level domain.

16. A unique host mail.ksu.edu
This is the domain name of a unique host. It is a member of the ksu.edu domain, but it is the only one with the name mail.

17. There are a limited number of top-level domains (TLDs), including:
.edu, educational
.com, commercial
.gov, government
.org, non profit
.net, networking organizations
These are called “generic” TLDs.

18. There are also country code top-level domain names for every nation, like:
.us, United States
.sa, Saudi Arabia
.cl, Chile
.uk, United Kingdom
These are called “country code” top-level domains (ccTLDs).

19. “root” zone
TLDs & ccTLDs
2lds
3lds
4lds

20. Overview Introduction to the DNS DNS Components The name space
The servers
The resolvers
Copyright © 2001, Nominum, Inc.

21. Zones
In addition to dividing DNS namespace into domains, DNS namespace are divided into zones that store name information about one or more DNS domains.
A zone is the authoritative source for information about each DNS domain name that is included in the zone.

23. Name Servers
Name servers store information about the name space in units called “zones”
The name servers that load a complete zone are said to “have authority for” or “be authoritative for” the zone
Usually, more than one name server are authoritative for the same zone
This ensures redundancy and spreads the load
Also, a single name server may be authoritative for many zones
Server responsible for answering DNS queries
Exists at all levels of hierarchy
Authoritative name servers hold part of the DNS database
One name server can serve more then one zone
Many name servers “should” serve the same zone
Some name servers are authoritative for certain zones
Copyright © 2001, Nominum, Inc.

24. Name Servers and Zones Name Servers Zones nominum.com isc.org
serves data for both nominum.com and isc.org zones
Name Servers
Zones
nominum.com
serves data for nominum.com zone only
isc.org
serves data for isc.org zone only
Copyright © 2001, Nominum, Inc.

25. Types of Name Servers Two main types of servers Other types exist…
Authoritative – maintains the data
Master – where the data is edited
Slave – where data is replicated to
Caching – stores data obtained from an authoritative server
The most common name server implementation (BIND) combines these two into a single process
Other types exist…
No special hardware necessary
Copyright © 2001, Nominum, Inc.

26. Name Server Architecture
You can think of a name server as part:
database server, answering queries about the parts of the name space it knows about (i.e., is authoritative for),
cache, temporarily storing data it learns from other name servers, and
agent, helping resolvers and other name servers find data that other name servers know about
Also, the caching resolver can insert data into the cache, and can "query" the cache and the database server.
Copyright © 2001, Nominum, Inc.

27. Name Server Architecture
Zone
data
file
From
disk
Authoritative Data
(primary master and
slave zones)
Agent
(looks up queries
on behalf of resolvers)
Cache Data
(responses from
other name servers)
Name Server Process
Master
server
Zone transfer
Copyright © 2001, Nominum, Inc.

28. on behalf of resolvers)
Authoritative Data
Authoritative Data
(primary master and
slave zones)
Agent
(looks up queries
on behalf of resolvers)
Cache Data
(responses from
other name servers)
Name Server Process
Response
Resolver
Query
Copyright © 2001, Nominum, Inc.

29. Using Other Name Servers
Authoritative Data
(primary master and
slave zones)
Agent
(looks up queries
on behalf of resolvers)
Cache Data
(responses from
other name servers)
Name Server Process
Response
Response
Arbitrary
name
server
Resolver
Query
Query
Copyright © 2001, Nominum, Inc.

30. on behalf of resolvers)
Cached Data
Authoritative Data
(primary master and
slave zones)
Agent
(looks up queries
on behalf of resolvers)
Cache Data
(responses from
other name servers)
Name Server Process
Response
Resolver
Query
Copyright © 2001, Nominum, Inc.

31. Overview Introduction to the DNS DNS Components The name space
The servers
The resolvers
Copyright © 2001, Nominum, Inc.

32. Name Resolution
Name resolution is the process by which resolvers and name servers cooperate to find data in the name space
To find information anywhere in the name space, a name server only needs the names and IP addresses of the name servers for the root zone (the “root name servers”)
The root name servers know about the top-level zones and can tell name servers whom to contact for all TLDs
Copyright © 2001, Nominum, Inc.

33. Name Resolution
A name server receiving a query from a resolver looks for the answer in its authoritative data and its cache
If the answer isn’t in the cache and the server isn’t authoritative for the answer, the answer must be looked up
Types of resolution: Recursive Resolution or Iterative Resolution
Copyright © 2001, Nominum, Inc.

34. The Resolution Process
Let’s look at the resolution process step-by-step:
annie.west.sprockets.com
ping
Copyright © 2001, Nominum, Inc.

35. The Resolution Process
The workstation annie asks its configured name server, dakota, for address
dakota.west.sprockets.com
What’s the IP address of
annie.west.sprockets.com
ping
Copyright © 2001, Nominum, Inc.

36. The Resolution Process
The name server dakota asks a root name server, m, for address
m.root-servers.net
dakota.west.sprockets.com
What’s the IP address of
annie.west.sprockets.com
ping
Copyright © 2001, Nominum, Inc.

37. The Resolution Process
The root server m refers dakota to the com name servers
This type of response is called a “referral”
m.root-servers.net
Here’s a list of the com name servers. Ask one of them.
dakota.west.sprockets.com
annie.west.sprockets.com
ping
Copyright © 2001, Nominum, Inc.

38. The Resolution Process
The name server dakota asks a com name server, f, for address
What’s the IP address of
m.root-servers.net
dakota.west.sprockets.com
f.gtld-servers.net
annie.west.sprockets.com
ping
Copyright © 2001, Nominum, Inc.

39. The Resolution Process
The com name server f refers dakota to the nominum.com name servers
Here’s a list of the nominum.com name servers. Ask one of them.
m.root-servers.net
dakota.west.sprockets.com
f.gtld-servers.net
annie.west.sprockets.com
ping
Copyright © 2001, Nominum, Inc.

40. The Resolution Process
The name server dakota asks an nominum.com name server, ns1.sanjose, for address
What’s the IP address of
m.root-servers.net
dakota.west.sprockets.com
ns1.sanjose.nominum.net
f.gtld-servers.net
annie.west.sprockets.com
ping
Copyright © 2001, Nominum, Inc.

41. The Resolution Process
The nominum.com name server ns1.sanjose responds with address
m.root-servers.net
dakota.west.sprockets.com
Here’s the IP address for
ns1.sanjose.nominum.net
f.gtld-servers.net
annie.west.sprockets.com
ping
Copyright © 2001, Nominum, Inc.

42. The Resolution Process
The name server dakota responds to annie with address
Here’s the IP address for
m.root-servers.net
dakota.west.sprockets.com
ns1.sanjose.nominum.net
f.gtld-servers.net
annie.west.sprockets.com
ping
Copyright © 2001, Nominum, Inc.

43. Resolution Process (Caching)
After the previous query, the name server dakota now knows:
The names and IP addresses of the com name servers
The names and IP addresses of the nominum.com name servers
The IP address of
Let’s look at the resolution process again
annie.west.sprockets.com
ping ftp.nominum.com.
Copyright © 2001, Nominum, Inc.

44. Resolution Process (Caching)
The workstation annie asks its configured name server, dakota, for ftp.nominum.com’s address
m.root-servers.net
dakota.west.sprockets.com
What’s the IP address of ftp.nominum.com?
ns1.sanjose.nominum.net
f.gtld-servers.net
annie.west.sprockets.com
ping ftp.nominum.com.
Copyright © 2001, Nominum, Inc.

45. Resolution Process (Caching)
dakota has cached an NS record indicating ns1.sanjose is an nominum.com name server, so it asks it for ftp.nominum.com’s address
What’s the IP address of ftp.nominum.com?
m.root-servers.net
dakota.west.sprockets.com
ns1.sanjose.nominum.net
f.gtld-servers.net
annie.west.sprockets.com
ping ftp.nominum.com.
Copyright © 2001, Nominum, Inc.

46. Resolution Process (Caching)
The nominum.com name server ns1.sanjose responds with ftp.nominum.com’s address
m.root-servers.net
dakota.west.sprockets.com
Here’s the IP address for ftp.nominum.com
ns1.sanjose.nominum.net
f.gtld-servers.net
annie.west.sprockets.com
ping ftp.nominum.com.
Copyright © 2001, Nominum, Inc.

47. Resolution Process (Caching)
The name server dakota responds to annie with ftp.nominum.com’s address
Here’s the IP address for ftp.nominum.com
m.root-servers.net
dakota.west.sprockets.com
ns1.sanjose.nominum.net
f.gtld-servers.net
annie.west.sprockets.com
ping ftp.nominum.com.
Copyright © 2001, Nominum, Inc.

48. Recursive Resolution root server 3 edu server 2 com server 4 8 7
recursive request
“What is the IP address of
udel server 1
google
server 5 9
recursive response
“The IP address of is ” 6 10
client

49. Iterative Resolution 5 7 3 iterative response (referral)
a.root server 5
a3.nstld.com 3
a.gtld- server 7
iterative response (referral)
“I don't know. Try a.gtld-servers.net.” 6
iterative request
“What is the IP address of
udel server 1
ns1.google.com 9
iterative response (referral)
“I don't know. Try a.root-servers.net.” 4
iterative response (referral)
“I don't know. Try ns1.google.com.” 8
iterative response (referral)
“I don't know. Try a3.nstld.com.” 2
iterative response
“The IP address of is ” 10
client