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  1. 1. Domain names WUCM1 1
  2. 2. Resolving host names • Domain Name Service – DNS maps host names (e.g. ... – ... to their address (e.g. – Others – e.g. Microsoft WINS or Sun’s 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 WUCM1 2
  3. 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 WUCM1 3
  4. 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 WUCM1 4
  5. 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 WUCM1 5
  6. 6. WUCM1 6
  7. 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 WUCM1 7
  8. 8. Rules • Within each domain: – The rules for allocating names etc. are up to the domain administrator, e.g.: • InterNIC ( administers the .COM, .NET and .ORG domains • ( 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 WUCM1 8
  9. 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 WUCM1 9
  10. 10. Delegation “” = d e le g a tio n n e t zo n e net so u rce fo rg e .n e t zo n e p h p .n e t s o u rc e fo rg e php zo n e e tc . s g fx b ria n s g fx .s o u rc e fo rg e .n e t m a x im u m zo n e m o o s ta r b ria n .s o u rc e fo rg e .n e t lin u x zo n e m a x im u m lin u x .s o u rc e fo rg e .n e t zo n e m o o s ta r.s o u rc e fo rg e .n e t zo n e WUCM1 10
  11. 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 WUCM1 11
  12. 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 WUCM1 12
  13. 13. Querying a domain name • Assume that your web browser wants to look up a page on – The resolver will contact the name server and send it a recursive query to resolve – If name server has 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 WUCM1 13
  14. 14. Iterative queries 1. It will ask a root name server for the address of name servers for the .uk 2. It will ask one of those name servers for address of the servers for 3. It will ask one of those name servers for address of the servers for 4. It will ask one of those name servers for the address of 5. If it already knows the address of a server it can short-circuit the process WUCM1 14
  15. 15. q u e ry fo r a d d re ss o f su n site .ic.a c.u k “” “” re fe rra l to u k n a m e s e rv e r n a m e s e rve r q u e ry fo r a d d re s s o f nam e su n site .ic.a c.u k uk s e rv e r re fe rra l to a c.u k n a m e s e rv e r n a m e se rve r uk tv fr q u e ry fo r a d d re ss o f su n site .ic.a c.u k a c .u k re fe rra l to ic.a c.u k n a m e s e rv e r n a m e se rve r ac gov q u e ry fo r a d d re s s o f su n site .ic.a c.u k ic .a c .u k re s o lv e r q u e ry a d d re s s o f n a m e s e rv e r su n site .ic.a c.u k ls e ic qm w answ er R e so lu tio n p ro c e ss re s o lv e r (a fte r A lb n itz , (2 0 0 1 )) WUCM1 15
  16. 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 WUCM1 16
  17. 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" WUCM1 17
  18. 18. Inverse queries • Is the resolution of names from addresses • Uses the domain – E.g. to find name of – Lookup on • Notes – Inverse query not guaranteed – No attempt made to pass query on WUCM1 18
  19. 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 WUCM1 19
  20. 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 WUCM1 20
  21. 21. Registering a domain name • Most name registration authorities provide a web interface and search • For example: – – – • E.g. to register you need to persuade the domain administrator to adopt you WUCM1 21
  22. 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 (or • Give up! WUCM1 22
  23. 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 – • Win2K Server also has a DNS WUCM1 23
  24. 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. • – Intended for private use (generally) WUCM1 24
  25. 25. Commercial services 2 • Web hosting companies – E.g. One&One at • 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 WUCM1 25
  26. 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. WUCM1 26
  27. 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 WUCM1 27