Name Resolution and DNS

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Name Resolution and DNS
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Domain names and IP addresses

• People prefer to use easy-to-remember names
  instead of IP addresses
• Domain names are alphanumeric names for IP
  addresses e.g., syslab.csd.uwo.ca,
  www.google.com, ietf.org
• The domain name system (DNS) is an Internet-wide
  distributed database that translates between
  domain names and IP addresses
• How important is DNS?
• Imagine what happens when the local DNS server
  is down.

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Before there was DNS .

• . there was the HOSTS.TXT file
• Before DNS (until 1985), the name-to-IP address
  was done by downloading a single file (hosts.txt)
  from a central server with FTP.
• Names in hosts.txt are not structured.
• The hosts.txt file still works on most operating
  systems. It can be used to define local names.

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DNS Domain Name System

• Distributed database implemented in hierarchy of
  many name servers
• Application-layer protocol host, routers, name
  servers to communicate to resolve names
  (address/name translation)
• note core Internet function, implemented as
  application-layer protocol
• complexity at networks edge

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Hierarchy of Name Servers

• The resolution of the hierarchical name space is
  done by a hierarchy of name servers
• Each server is responsible (authoritative) for a
  contiguous portion of the DNS namespace, called a
  zone.
• Zone is a part of the subtree
• DNS server answers queries about hosts in its zone

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Hierarchical Names

• Internet hosts and other resources need globally
  unique names
• Difficult to keep unstructured names unique
• would require a single list of all names in use
• Hierarchical names are much easier to make unique

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Why Not Centralize DNS?

• Single point of failure
• Traffic volume
• Distant centralized database
• Maintenance
• doesnt scale!

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Design principle of DNS

• The naming system on which DNS is based is a
  hierarchical and logical tree structure called
  the domain namespace.
• An organization obtains authority for parts of
  the name space, and can add additional layers of
  the hierarchy
• Names of hosts can be assigned without regard of
  location on a link layer network, IP network or
  autonomous system
• Lets discuss the different levels

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Domain Name Hierarchy
Root domain

.
Top-Level-Domains
. . . . . .
edu
com
gov
mil
net
org
ro
fr
at
jp
Second Level Domains
ici
rnc
ase
pub
utt
vsat
ac
co
gv
or . . .
eunet
uni-linz
tuwien
. . . . . . .
univie
roearn ns std
cs
lmn
dsp
cc
mat
exp
itc
. . . . . .
ulise paul
phytia alpha chris
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DNS Name hierarchy

• DNS hierarchy can be represented by a tree
• Root and top-level domains are administered by an
  Internet central name registration authority
  (ICANN)
• Below top-level domain, administration of name
  space is delegated to organizations
• Each organization can delegate further

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DNS Root Name Servers

• Contacted by local name server that can not
  resolve name
• Root name server
• Contacts authoritative name server if name
  mapping not known
• Gets mapping
• Returns mapping to local name server
• Does not have name

a Verisign, Dulles, VA c Cogent, Herndon, VA
(also Los Angeles) d U Maryland College Park,
MD g US DoD Vienna, VA h ARL Aberdeen, MD j
Verisign, ( 11 locations)
k RIPE London (also Amsterdam, Frankfurt)
i Autonomica, Stockholm (plus 3 other locations)
m WIDE Tokyo
e NASA Mt View, CA f Internet Software C. Palo
Alto, CA (and 17 other locations)
13 root name servers worldwide
b USC-ISI Marina del Rey, CA l ICANN Los
Angeles, CA
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Top-level domains

• Types of top-level domains
• Organizational 3-character code indicates the
  function of the organization
• Used primarily within the US
• Examples gov, mil, edu, org, com, net
• Geographical 2-character country or region code
• Examples us, va, jp, de
• There are more than 200 top-level domains.

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Organizational top-level domains (TLD)
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TLD and Authoritative Servers

• Top-level domain (TLD) servers responsible for
  com, org, net, edu, etc, and all top-level
  country domains uk, fr, ca, jp.
• Authoritative DNS servers organizations DNS
  servers, providing authoritative hostname to IP
  mappings for organizations servers (e.g., Web
  and mail).
• Can be maintained by organization or service
  provider

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Local Name Server

• Does not strictly belong to hierarchy
• Each ISP (residential ISP, company, university)
  has one.
• Also called default name server
• When a host makes a DNS query, query is sent to
  its local DNS server
• Acts as a proxy, forwards query into hierarchy.

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Distributed, Hierarchical Database

• Client wants IP for www.amazon.com 1st
  approximation
• Client queries a root server to find com DNS
  server
• Client queries com DNS server to get amazon.com
  DNS server
• Client queries amazon.com DNS server to get IP
  address for www.amazon.com

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Example
root DNS server
2

• Host at cis.poly.edu wants IP address for
  gaia.cs.umass.edu.
• First, check locally.
• Then, check root. Root notes .edu suffix and
  suggests TLD servers for .edu.
• Check TLD server. This provides the
  authoritative server.
• Check authoritative server, and get the IP
  address!
• This uses both iterative and recursive queries.
  (Usually the way its done.)

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TLD DNS server
4
5
6
7
1
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authoritative DNS server dns.cs.umass.edu
requesting host cis.poly.edu
gaia.cs.umass.edu
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Recursive queries

• recursive query
• puts burden of name resolution on contacted name
  server
• heavy load?
• iterated query
• contacted server replies with name of server to
  contact
• I dont know this name, but ask this server

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DNS Caching and Updating Records

• Once (any) name server learns mapping, it caches
  mapping
• Cache entries timeout (disappear) after some time
• TLD servers typically cached in local name
  servers
• Thus root name servers not often visited
• Update/notify mechanisms under design by IETF
• RFC 2136
• http//www.ietf.org/html.charters/dnsind-charter.h
  tml

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Summary

• We have examined how DNS works