Naming with the Domain Name System

1
Naming with the Domain Name System

• Prof. Martins
• Department of Computer Science and Computer
  Information Systems

2
Goals

• In this chapter you will learn about
• Naming hierarchy
• Organization of name servers
• Details of client-server interaction among them
• How caching improves efficiency of the naming
  system

3
Structure of Computer Names

• The naming scheme used in the Internet is called
  the Domain Name System (DNS)
• Each computer name consists of alpha-numeric
  segments separated by periods
• For example, a computer at Cisco Systems
  Incorporated has the domain name
• anakin.cisco.com

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Structure of Computer Names

• Domain names are hierarchical
• The most significant part of the name is on the
  right
• The left-most segment of a name is the name of an
  individual computer.
• For example, Cisco gives the name of a company.

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Structure of Computer Names

• The domain name system does specify values for
  the most significant segment
• The most significant segment is called the
  top-level domain (TLD).

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Structure of Computer Names
Fig. 31.1 - Values for the most significant
segment of a domain name. DNS does not
distinguish between names in upper or lower case.
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Structure of Computer Names

• Top-level domains are controlled by the Internet
  Corporation for Assigned Names and Numbers
  (ICANN).
• Each organization that chooses to participate in
  the domain name system must apply for a name
  under one of the existing TLDs.

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Structure of Computer Names

• Once an organization has been assigned a domain,
  the suffix is reserved for the organization.
• If, for example, foobar.com has been assigned,
  another organization named Foobar could apply for
  foobar.biz or foobar.org, but not foobar.com.

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To summarize

• To obtain a domain, an organization must register
  with an approved registrar. A unique domain
  suffix is assigned to each organization.

10
Geographic Structure

• In addition to the familiar organizational
  structure, the DNS allow organizations to use
  geographic registration.
• For example, the Corporation for National
  Research Initiatives registered the domain
• Cnri.reston.va.us
• The corporation is located in the town of Reston,
  Virginia in the United States.

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Geographic Structure

• Some foreign countries have adopted a combination
  of geographic and organizational domain names.
• Example
• Cs.york.ac.uk/rts
• Where ac is an abbreviation for academic and uk
  is the official country code for the United
  Kindgom

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Domain Names Within An Organization
Fig. 31.2 A graphical representation that
illustrates one way a DNS hierarchy might be
structured in a corporation. Names for individual
computers can be added to the diagram as well.
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Domain Names that Begin With WWW

• Although a domain name denotes a computer, many
  organizations assign a domain names that reflect
  the service that the computer provides.
• For example, the FTP service.
• If Foobar corporation provides the FTP service,
  it can choose a computer to run the service, and
  assign the domain name
• ftp.foobar.com

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Domain Names that Begin With WWW

• Similarly, a computer that runs a Web Server, can
  be assigned the name
• www.foobar.com
• Although the descriptive names are easy for
  humans to remember and use, they are not
  required.
• In particular, the use of www to name computers
  than run a Web server is merely a convention.

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Domain Names that Begin With WWW

• An arbitrary computer can run a Web server the
  computers domain name does not need to contain
  www.
• Furthermore, a computer that has a domain name
  beginning with www is not required to run a Web
  server.

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To summarize
Using the first label in a domain name to denote
the service that a computer offers is merely a
convention to help humans a computer that runs
a Web server does not need to be named www, and a
computer named www does not need to run a web
server.
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The DNS Client-Server Model

• One of the main features of the Domain Naming
  System is autonomy.
• An organization control all names with a
  particular suffix.
• DNS uses client-server interaction to aid
  autonomy.
• The entire naming system operates as a large,
  distributed database

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The DNS Client-Server Model

• Most organizations that have an Internet run a
  domain name server
• Each server contains information that links the
  server to other domain name servers
• The resulting set of servers functions as a
  large, coordinated database of names.

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The DNS Client-Server Model

• Whenever an application needs to translate a name
  to an IP address, the application becomes a
  client of the naming system.
• The client places the name to be translated in a
  DNS request message, and sends the request to a
  DNS server.

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The DNS Client-Server Model

• The server extracts the name from the request,
  translates the name to an equivalent IP
  address, and returns the resulting address to
  the application in a reply message.

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The DNS Server Hierarchy

• DNS servers are arranged in a hierarchy that
  matches the naming hierarchy.
• A root server occupies the top of the hierarchy,
  and is an authority for the top-level domains
  (e.g., com)

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The DNS Server Hierarchy
Fig. 31.3 Two examples of ways the domain name
hierarchy from Figure 31.2 can be divided among
three servers. Each organization chooses how to
divide names among its servers.
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Server Architectures

• When does an organization need more than one
  server?
• The smallest organizations do not run a server
  themselves. Instead, the organization contracts
  with an Internet Service Provider that runs a
  domain name server on behalf of its customers.

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Server Architectures

• A small organization can minimize cost by placing
  all its domain information in a single server.
• Larger organizations use two or more servers.
  Two reasons
• Speed
• Less coordination effort required compared to one
  centralized server.

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Locality of Reference and Multiple Servers

• The domain name system follows the locality of
  reference principle in two ways
• A user tends to look up the names of local
  computers more often than the names of remote
  computers
• A user tends to look up the same set of domain
  names repeatedly.
• Having multiple servers within an org works well
  because a server can be placed within each group.

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Locality of Reference and Multiple Servers

• Because the DNS obeys the locality principle,
  the local server can handle most requests.
• Multiple DNS servers
• Are easier to administer
• Help balance the load ( and thereby reduce the
  problems of contention a centralized server may
  cause)

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Links Among Servers

• Servers in the domain name are linked together,
  making it possible for a client to find the
  correct server by following links.
• Each server is configured to know the locations
  of servers of subparts of the hierarchy.

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To summarize
All domain name servers are linked together to
form a unified system. Each server knows how to
reach a root server and how to reach servers
that are authorities for names further down the
hierarchy.
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Resolving A Name

• The translation of a domain name into an
  equivalent IP address is called name resolution.
• The name is said to be resolved to an address.
• Software to perform the translation is known as
  name resolver software (or simply resolver).

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Resolving A Name

• Many operating systems provide name resolver
  software as a library routine that an
  application can call.
• Example UNIX systems, an application can call a
  library routine gethostbyname to resolve a name

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Resolving a Name

• How does a resolver software work?

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Resolving a Name

• When an incoming request specifies a name for
  which a server is an authority, the server
  answers directly.
• That is, the server looks up the name it is local
  database, and sends a reply to the resolver.

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Resolving a Name

• When a request arrives for a name outside the
  set for which the server is an authority, further
  client-server interaction results.
• The server temporarily becomes a client of
  another name server.
• When the other server returns an answer, the
  original server sends a copy of the answer back
  to the resolver from which the request arrived.

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Resolving a Name

• How does a DNS server know which other DNS
  server is the authority for a given name?
• It does not
• Each server knows the address of a root server.
• Knowing the location of the root server is
  sufficient.

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Resolving a Name

• The name can be resolved from the root server.
• The root server may not be an authority for the
  name, but the response from the root server gives
  the location of a server for the right-most
  segment.

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Example

• Suppose servers for Foobar Corporation are
  organized as in Figure 31.3b, and a resolver at
  a remote site (e.g. at a University) sends a
  request to its local server, L, for the name
• Venus.walnut.candy.foobar.com
• Server L is not an authority for the name, so it
  proceeds to act as a client of other servers.
• In the first step, L sends a request to the root
  server.

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Example

• The root server is not an authority for the name,
  but the response from the root server gives the
  location of a server for foobar.com
• When it receives the response from the root
  server, server L contacts the server for
  foobar.com
• Although it is not an authority for names in the
  walnut subdivision, the main server at Foobar
  knows the location of the server for walnut.

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Example

• Thus, it returns a response to inform L.
• Finally, L contacts the server that is the
  authority for names of the form
• Computer.walnut.candy.foobar.com
• The server returns an authoritative answer to L,
  either the IP address for the name or an
  indication that no such name exists.

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To summarize
The resolver software in a host always requests
recursive resolution in which a name is resolved
to an equivalent address. When it becomes a
client of another server, a server can request
iterative resolution to step through the server
hierarchy one level at a time.
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Optimization of DNS Performance

• The domain name system as described is hopelessly
  inefficient.
• Without optimizations, traffic at root server
  would be intolerable.
• Optimizations used in the DNS
• Replication and
• Caching

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Optimization of DNS Performance

• Each root server is replicated
• Many copies of the server exist around the world
• When a new site joins the internet, the site
  configures its local DNS server with a list of
  root servers
• The sites server uses whichever root server is
  most responsive at a given time.

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Optimization of DNS Performance

• The geographically closest server usually
  responds best.
• Thus, a site in Europe will tend to use a root
  server in Europe
• A site in California will choose to use a root
  server on the west cost of the US.

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Optimization of DNS Performance

• Caching is more important than replication
• Caching works well because name resolution
  shows a strong tendency toward temporal locality
  of reference.
• On a given day, a user is likely to look up the
  same name repeatedly
• When an application looks up a name for the
  first time, the local DNS server caches the
  binding.

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Optimization of DNS Performance

• The server can then answer subsequent requests by
  returning the binding from its cache instead of
  contacting the authoritative server again.

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Types of DNS Entries

• Each entry in a DNS database consists of three
  items
• A domain name
• A record type how the value is to be interpreted
• A value

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Types of DNS Entries

• A DNS support different types of binding
• A binding address binding a domain name and an
  equivalent address
• MX binding Mail exchanger map the computer
  name found in an e-mail to an IP address

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Aliases Using the CNAME Type

• Check out page 475

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An Important Consequence of Multiple Types

• Check out page 475

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Abbreviations and the DNS

• Check out page 476

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Summary

• The domain name system provides automated
  mapping between computer names and equivalent IP
  addresses.
• Each name is a character string that consists of
  a sequence of alpha-numeric segments separated
  by periods.
• Names are allocated hierarchically
• Segments in the name correspond to levels in the
  hierarchy.

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Summary

• There is no standard for the number of segments
  in a name because each organization is free to
  choose how to assign levels of the hierarchy.
• Two groups within a given organization may use
  two different levels of hierarchy.
• A set of online servers provides answers to
  resolution requests.

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Summary

• Servers are linked together to form a unified
  system
• An application program that calls a resolver
  becomes a client of the domain name system
• The client sends a request to its local server.
• The local server either answers the request
  directly, or contacts other servers to find the
  answer.

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Summary

• DNS servers use two performance optimization
  techniques
• Replication
• Caching
• Root servers are replicated to reduce the load on
  a given server
• Because name resolution follows the principle of
  locality, all DNS servers place a copy of
  resolved bindings in their cache for later use.