Domain names

1
Domain names
2
Resolving host names

• Domain Name Service
• DNS maps host names (e.g. www.port.ac.uk) ...
• ... to their address (e.g. 148.197.175.1)
• Others e.g. Microsoft WINS or Suns NIS
• Historically
• Initially (1970s) HOSTS.TXT downloaded by FTP
  from a computer at Stanford University
• Name-to-address mapping of all the (few hundred)
  hosts on the then ARPAnet

3
Domain Name System (DNS)

• DNS has been used since about 1984
• Designed by Paul Mockapetris, USC
• It is a distributed database
• Name servers each keep details about some segment
  of the Internet
• Clients called resolvers query the database by
  means of calls to name servers

4
Pros and cons of distribution

• Resilience (pro)
• not reliant on one machine or one file
• Performance (pro)
• many name servers can be running simultaneously
  and the load can be balanced among them
• Consistency (con)
• need to have a mechanism to keep servers
  consistent

5
Structure of namespace

• Structure of the namespace (and hence the
  database) is hierarchical
• There are 13 root name servers (A-M)
• These comprise the root of an inverted tree of
  domains
• 10 are in the USA, 1 in the UK, 1 in Sweden and 1
  in Japan
• See http//www.wia.org/pub/rootserv.html

6
(No Transcript)
7
Top Level Domains

• Since 1980s, three-letter generic top-level
  domains (gTLDs) are .com, .net, .org
• .biz .info .name and .pro added 2001-2002
• .arpa contains Internet infrastructure databases
• .aero .asia .cat .coop .edu .gov .int .jobs .mil 
  .mobi .museum .tel .travel are "sponsored" TLDs
• Two-letter country code top-level domains
  (ccTLDs) - ISO 3166-1 codes
• .gb versus .uk

8
Rules

• Within each domain
• The rules for allocating names etc. are up to the
  domain administrator, e.g.
• InterNIC (http//www.internic.net/) administers
  the .COM, .NET and .ORG domains
• Nominet.uk (http//www.nic.uk/) is the registry
  for most .UK domain names, CO.UK, ORG.UK, etc.
• The domain administrator can delegate parts of
  the domain to ease management

9
Zones

• The billions of resource records in the DNS
  directory are split into millions of files called
  zones
• Zones are kept on authoritative servers which
  answer DNS queries
• Caching servers simply query the authoritative
  servers and cache any replies
• Name servers can be both authoritative and
  caching for different zones

10
Delegation
11
DNS servers

• DNS servers
• Most are authoritative for just one or a few
  zones
• Larger servers may be authoritative for many
  thousands of zones
• Can be many authoritative servers for a zone
• One would be the primary master name server
• loads from a zone file
• The rest are slave servers
• load from the primary server

12
Resolvers

• DNS clients that access name servers on behalf of
  user applications, e.g. web browsers
• Resolvers
• Query a name server
• Interpret the response
• Return the IP address to the user application
• Resolvers must know at least one DNS
• e.g. as set up under Windows

13
Querying a domain name

• Assume that your web browser wants to look up a
  page on sunsite.ic.ac.uk
• The resolver will contact the name server and
  send it a recursive query to resolve
  sunsite.ic.ac.uk
• If name server has sunsite.ic.ac.uk in its cache,
  it will return the address
• If it doesn't, it will send a series of iterative
  queries to allow it to resolve the name

14
Iterative queries

• It will ask a root name server for the address of
  name servers for the .uk
• It will ask one of those name servers for address
  of the servers for .ac.uk
• It will ask one of those name servers for address
  of the servers for .ic.ac.uk
• It will ask one of those name servers for the
  address of sunsite.ic.ac.uk
• If it already knows the address of a server it
  can short-circuit the process

15
(No Transcript)
16
Time to live (TTL)

• Whenever a name is resolved, the authoritative
  name server annotates it with a "time to live"
  (TTL)
• A caching server will only cache the entry (if it
  can) for that length of time
• After that it will redo a full resolution
• This means that changes to addresses eventually
  get noticed

17
DNS failure

• The resolver will wait 5 seconds for a response
  from a name server
• If there is no reply within that time, it will
  try another name server
• Once it has tried and failed on all the servers
  it knows, it tries again with longer timeouts
• Limit to the number of retries (normally 3)
• Normally, your resolver will give up after 75-80
  seconds with a message like "host name lookup
  failure"

18
Inverse queries

• Is the resolution of names from addresses
• Uses the .in-addr.arpa domain
• E.g. to find name of 148.197.175.1
• Lookup on 1.175.197.148.in-addr.arpa
• Notes
• Inverse query not guaranteed
• No attempt made to pass query on

19
Domain names

• You need to do two things
• You need to choose a name for your domain
• You need to find a parent domain willing to adopt
  you
• Obviously you need to choose a name that is not
  already taken in the parent domain

20
Choosing a good name

• Keep it short - people will certainly have to
  remember it and type it
• Relate it to
• the name of your company
• your department
• your role - anything to make it memorable
• Make it distinct from other names
• Don't try to spoof another legitimate name
• see http//www.yaaho.com/

21
Registering a domain name

• Most name registration authorities provide a web
  interface and search
• For example
• http//bulkurl.com/
• http//www.domainnamebuyersguide.com/
• http//www.register.md/register_home.jsp
• E.g. to register me.port.ac.uk you need to
  persuade the port.ac.uk domain administrator to
  adopt you

22
What if a name is already taken?

• Choose a different name
• Choose a different parent domain
• Persuade the owners of the name to give/sell you
  it
• Initiate the Uniform Domain-Name
  Dispute-Resolution Policy (see
  http//www.icann.org/udrp/udrp.htm) (or
  http//www.nic.uk/ref/drs.html)
• Give up!

23
Your own DNS?

• To administer your own zone of the DNS, you need
  to
• Set up a computer as a name server
• Get the administrator of your parent domain to
  delegate authority for your sub-domain to you
• DNS software e.g. BIND for Unix
• http//www.isc.org/products/BIND/
• Win2K Server also has a DNS

24
Commercial services 1

• Internet Service Providers (ISPs)
• Both Narrow or Broadband providers
• Most ISPs include web space and a domain name
• DNS entry usually derived from ISP name, e.g.
• www.nodename.freeserve.co.uk
• Intended for private use (generally)

25
Commercial services 2

• Web hosting companies
• E.g. OneOne at http//oneandone.co.uk
• Typical package would include
• Domain name (in a tld, e.g. .com, .net, etc.)
• Significant web space
• Email accounts all separately manageable
• Tools for web page design
• Database support e.g. MySQL or SQL Server
• Technical support including backup and restore

26
Commercial services 3

• Dedicated and managed server hosts
• Your server is located in a managed environment
  at a server farm
• They provide 24/7 support but you have management
  authority
• Server farms usually near major Internet nodes,
  e.g. Canary Wharf (or Fareham)
• E.g. http//www.rackspace.com/index.php

27
Summary

• We have looked at
• Domain Name System DNS
• Choosing a domain name
• Registering a domain name
• Setting up a DNS
• Commercial services
• ISP basic provision
• Web hosting companies
• Managed server farms