Domain Name System DNS

1
Domain Name System(DNS?)

• By
• Jarett Boxberger

2
First, Some History

• Before DNS
• One computer was used as a host that knew all
  addresses.
• Table had to be updated by Stanford
• Others hosts then downloaded updated tables
• System worked well for finding computers, problem
  revolved around the updates

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DNS to the Rescue

• 1984, Paul Mockapetris designed DNS
• No one organization responsible for updating the
  tables
• Known as a distributed database, no one server
  stores all the information
• DNS allows for almost unlimited growth

4
How does DNS Work?

• DNS makes remembering Web Addresses much easier
• Which is easier to remember
• www.fhsu.edu
• 198.22.249.2

5
Domain Name Space

• Each node represents a domain, and everything
  below that node is in its domain
• Domains are broken down into

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Domain Name Space continued

• There are also 2 letter codes that represent
  countries, these are defined as country or
  geographic domains
• They are defined in ISO 3166

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Nodes

• Each has a label, which is 0-63 octets in length
• The domain name of a node is the list of the
  labels on the path from the node to the root of
  the tree
• Labels are read from left to right, or from the
  most specific, farthest from the root, to the
  least specific, closest to the root.

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Nodes Tree
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Domain Name Servers

• Servers are organized in a hierarchy
• Each server has an authority over a part of the
  naming hierarchy
• Each server does not keep all names
• Needs to know other servers who are responsible
  for the other subdomains

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Servers cont.

• The authority server, has the name to address
  translation table, and either is able to answer a
  query, or knows the server that does
• A server can serve more than one domain
• The root server knows about the servers for the
  top-level domains
• Each server knows the root server

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Server Types

• Root Severs
• Ultimate authority
• 1 primary, backed by 12 secondary
• Primary Severs
• Has authority over a zone.
• Secondary Servers
• Back up Primary Servers
• Caching Servers
• Not authoritative for any domain
• Answers queries by asking other authoritative
  servers, then storing that information in cache
• Used to relieve burden of authoritative servers
• Slave Servers
• Can function as primary, or secondary servers.
• Have no contact with servers outside the
  organization.

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Server Hierarchy
13
Updating Tables

• When new hosts are added to a zone, the
  administrator adds the name and IP address to
  file, called a record.
• Primary name server is then notified, and it
  updates its tables.
• Secondary hosts query the primary on a regular
  basis, usually every 3 hours.

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DNS Records

• A records Given a domain name, return its IP
  address
• PTR records Given an IP address, return the
  corresponding domain name
• HINFO and TXT records Given a domain name,
  return descriptive information

15
Queries

• Two types
• Recursive Give me an answer
• Done by your computer to a resolver
• Iterative Give me an answer or a referral to the
  next server
• Done by the resolver to the name servers

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How Queries get answered

• Programs interact with the DNS servers through
  resolvers, which answers the query, or queries
  other hosts
• Message formats are specified in RCF 1035
• Queries and responses can be done either by TCP
  or UDP

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The Process
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DNS Message Format

• Identification is set by the client and returned
  by the server
• Flags provide additional information about the
  query/response
• Number of the total number of entries in the
  corresponding sections
• Questions section contains the query
• Answer, Authority, Additional Information
  sections consist of a set of resource records,
  which is the answer to the query. Each record
  describes one name.

Sent by your computer to the resolver.
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Query Reply RR Fields

• Domain name is the query name from the query.
• Type type of information
• Class is 1 for Internet domain
• Time to live is the time the information can be
  cached, expressed in seconds.
• Resource data length is the number of bytes of
  resource data

Sent by the server to your computer.
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Query Types
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Abbreviations

• Servers respond to a full name only
• What happens when we give a partial address?
• Resolvers try and fill in the rest
• Try various suffixes
• Example You entered www.fhsu
• The resolver would try putting different suffixes
  on, like .com .edu .gov ect.
• If that doesnt work an error is displayed

22
Types of DNS Entries

• DNS is not just used for name to address
  resolution
• Also used for finding mail servers, and pop
  servers

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Summary

• DNS makes remembering web addresses much easier
  by mapping names to addresses
• Names are hierarchical
• No one computer contains all information
• Tables are exchanged between servers to keep the
  information up-to-date