ES 101. Module 3 Domain Name System (DNS)

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ES 101. Module 3Domain Name System (DNS)
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Last Lecture

• Routing and IP addressing

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This Lecture

• Domain Name System

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Domain Name System History

• DNS provides host IP address to mnemonic name
  mapping
• Originally this was accomplished by a set of
  configuration files on each client and server
• /etc/hosts
• /etc/networks
• These files at first were manually configured by
  the System Administrator using a text editor
• Highly subject to typographical errors

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DNS History (contd)

• In the early days of the Internet, the Network
  Information Center (NIC) maintained a single
  etc/hosts file, which could be downloaded for
  host configuration
• HOSTS.TXT
• ftpd by all hosts (RFC 952 and 953)
• The bandwidth consumed by this process is
  proportional to the square of the number of hosts
• Outgoing ftp load on the NIC host was very high
• Made worse by the astronomical number of hosts on
  the Internet

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DNS History (contd)

• This situation caused the development of a
  network-wide, distributed database system which
  did this host name to IP address mapping
• Originally known as Berkeley Internet Domain
  Server, or BIND
• Later renamed to Domain Name System (DNS)
• We will study the host configuration files to
  gain insight into the operation of the DNS
• Host files are still used for small private
  networks on both Unix and Microsoft networks

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Internet Hosts vs. Time
Date No. Hosts 1984 1,000 1989 100,000 1992 gt
1.0 Million 1994 3.2 Million Today ????
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Internet Names

• With millions of Hosts on the Internet, how do
  you easily specify the host that you want to
  interact with?
• IP addresses are very hard to memorize for humans
• DNS was developed to provide an easy to remember
  addressing scheme
• Originally specified in RFCs 1032, 1033, 1034,
  and 1035

Checkout the following web site for access to all
RFCs http//www.cis.ohio-state.edu/cs/Services/rf
c
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/etc/hosts File

• Every computer on a TCP/IP network has an IP
  address, canonical host name, and zero or more
  host name aliases
• The /etc/hosts file is the classical method for
  mapping IP addresses to host names
• Format of the hosts file consists of one IP
  address per line
• Note that the 127.0.0.0 address is the local
  loopback IP address

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Host File Example
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/etc/networks File

• Networks and subnets are also named
• The handling of network address to network name
  mapping is accomplished by the /etc/networks file

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Network File Example
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Introduction to DNS

• DNS is essentially a hierarchical, distributed
  local hosts database
• The DNS name provides an easily recognized
  structure
• DNS can be conceptually divided into three parts
• Domain name space
• Name servers
• Resolvers
• Each of these will be discussed in the following
  charts

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Domain Name Space

• DNS is partitioned into different Domains
• It is organized hierarchically
• Top level domain names are globally specified
• Second-level domain names are unique to each
  organization, and must be registered with via a
  registrar
• Domain names are not associated with a location
• You can search for a unique name using the
  whois utility of the registrar
• When you find no match, the name is available
• You can then register the name with a registrar
  to reserve it

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Top Level Domain Names
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Fully Qualified Domain Names

• The highest level of the hierarchical structure
  (root) is dot
• The proper syntax for a fully qualified domain
  name includes the trailing period
• Standard practice is not to include it, since it
  is understood

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Domain Name Hierarchy
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Second Level Domain Names

• These names are registered by qualified
  registrars
• www.networksolutions.com originally
• Many other registrars are now available, for
  example
• Domain Direct (http//www.domaindirect.com/)
• Each domain is given its own authority to create
  subdomains
• Example http//eecs.vuse.vanderbilt.edu
• A (sub)domain may contain an entire network, or
  only parts of a network

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Domain Physical Representation
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Domain Name Tree

• The easiest representation of the DNS name is as
  a tree
• Each domain represents a branch of the tree
• The leafs of the tree are individual host
  machines

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DNS Servers

• The DNS database is distributed among a number of
  servers
• These servers perform the DNS name to IP address
  mapping
• They are referred to as name servers
• Each domain is responsible for maintaining a
  minimum of one primary and secondary DNS servers
• This function can be subcontracted to others
• No single DNS server could hold the database for
  the entire Internet

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DNS Servers (contd)

• DNS servers that are in different locations
  operate cooperatively to resolve the name to IP
  address mapping request
• Distributed databases have the advantage that
  they are efficient
• If the names that are accessed most frequently
  are in a local cache (DNS server), the DNS lookup
  traffic across the Internet is greatly reduced

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DNS Servers (contd)

• Distributed databases are also more reliable than
  a single machine
• One failure cannot take down the network

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DNS Forwarding

• If you do not wish to host your own DNS server,
  you can pay for a service known as DNS
  Forwarding
• Typically provided by the registrar
• What happens is that the registrar maintains the
  redundant DNS servers
• The URL of interest is locally controlled by the
  registrar
• A table entry is made into the registrars DNS
  database to go to the IP address of a server that
  is specified by you
• This is done by a control panel
• Name resolution requests can then be referred to
  any IP address on the Internet
• Even your dorm PC, assuming that it has a static
  IP address

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Static IP Addresses

• You may request a static IP address from your
  ISP
• However, this is becoming more difficult, since
  ISPs like to manage their own IP address space
• This is because IP addresses are in short supply
• When this is done, your PC requests an IP address
  from a Dynamic Host Configuration Protocol
  (DHCP) server
• Your machine is then assigned an IP address
• This address ages and is changed periodically
• However, Vanderbilt does not change this often

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Dynamic Address Determination

• How would you determine your DHCP assigned IP
  address?
• ipconfig command
• Whats my IP web site
• http//www.whatismyip.com/
• If your IP address does not change often, you can
  DNS forward to it
• If the IP changes, you simply log onto the
  registrars control panel and repoint the URL to
  the new IP address

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Assignment

• Those of you willing to spend 30/year should
  register a domain name of your choosing

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Web Servers

• Note that the DNS only points to an IP address
• A server that hosts many web sites, must still
  resolve the URL to the proper index.htm file
  for the web site of interest
• This is done by a local DNS server process that
  only points to the internal folders
• No updates from the Internet
• This information is not distributed out to the
  Internet
• An example is the Helios server here at Vanderbilt

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DNS Name Resolution

• Each client executes a DNS client software
  process known as a (DNS) name resolver
• This software communicates with the DNS Server
  established in the client by the network
  administrator
• Name servers are arranged in a tree structure
  that corresponds to the naming hierarchy
• Each lower-level server knows what the
  higher-level servers are by their IP address

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DNS Name Server Registration

• An organization registers for a second-level
  domain name
• It is required to provide and maintain a two name
  servers for that sub-domain
• These servers are known as the Authority Name
  Servers for the domain
• The name server must be linked into the DNS
  server tree

If you use an ISP, you can use their DNS servers.
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Name Server Modes

• Depending on the client request, the name server
  operates in one of two modes
• When the resolver queries a name server, the
  message contains the following information
• Name to be resolved
• Class code of the name
• Protocol group to be used, ie. IP, IPX, etc.
• For IP, the class code is IN
• Type of response desired, ie. IP address
• Action code that specifies whether the name
  server should translate the name completely

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Object Types

• DNS servers can be used for multiple functions
• Translation of a host name to an IP address
• Look up a mail server address
• This information is differentiated by Object
  Types
• Also known as resource record types

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Common DNS Resource Record Types
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DNS Query Processing

• When a domain name server receives a query from a
  client, it checks to see if the name is within
  its subdomain
• If so, it sends back the information to the
  resolver
• If the name server cannot resolve the name
  completely, it checks the action code to see
  what the client specified
• Recursive resolution (complete lookup)
• Iterative resolution (non-recursive lookup)

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Recursive Resolution Schematic
Name server asks for assistance from other
higher level name servers
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Iterative Resolution

• Name server generates an error if it cannot
  resolve the name
• Client is informed of the name server that it
  should query next

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Non-Recursive Resolution Schematic
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Name Caching

• Name resolution is performed from left to right
• Local authority up to higher levels
• Most efficient path for name resolution
• Each name server also keeps a cache of recently
  resolved names and the mapping information
• This information ages off the name server
• It is up to the clientsoftware to decide if the
  returned information is good enough

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Name Resolution Sequence

• Client queries the name server
• Name server first checks for local resolution
• Next checks its name cache to see if the name was
  recently resolved
• If found, the name server reports back to the
  resolver
• If not, the name server follows the action code
  provided
• Recursive resolution
• Non-recursive resolution

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Next Lecture

• Basic Network Troubleshooting and Utilities