1. Domain names Dr Jim Briggs WUCM11

2. FROM THE INTERNET'S PERSPECTIVE WUCM12

3. Resolving host names Domain Name Service – DNS maps host names (e.g. www.port.ac.uk)...www.port.ac.uk –... to their address (e.g. 148.197.175.1) – 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 WUCM13

4. 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 WUCM14

5. 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 WUCM15

6. Structure of namespace Structure of the namespace (and hence the database) is hierarchical There are logically 13 root name servers (A-M) – most replicated: actually 274 These form the root of an inverted tree of domains See http://www.root-servers.org/http://www.root-servers.org/ WUCM16

7. 7

8. Current root server locations WUCM18

9. 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 WUCM19

10. 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 domainshttp://www.internic.net/ Nominet.uk (http://www.nic.uk/) is the registry for most.UK domain names, CO.UK, ORG.UK, etc.http://www.nic.uk/ – The domain administrator can delegate parts of the domain to ease management WUCM110

11. 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 WUCM111

12. Delegation WUCM112

13. 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 WUCM113

14. 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 server – e.g. as set up under Windows WUCM114

15. Example configuration WUCM115

16. 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 WUCM116

17. Iterative queries 1.It will ask a root name server for the address of name servers for the.uk domain 2.It will ask one of those name servers for address of the servers for.ac.uk 3.It will ask one of those name servers for address of the servers for.ic.ac.uk 4.It will ask one of those name servers for the address of sunsite.ic.ac.uk 5.If it already knows the address of a server it can short-circuit the process WUCM117

18. WUCM118

19. 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 WUCM119

20. 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" The application software (e.g. browser) will decide how to report this to the user WUCM120

21. 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 WUCM121

22. FROM THE WEBSITE MANAGER'S PERSPECTIVE WUCM122

23. 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 WUCM123

24. 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/http://www.yaaho.com/ WUCM124

25. Registering a domain name Most name registration authorities provide a web interface and search For example: – http://bulkurl.com/ http://bulkurl.com/ – http://www.domainnamebuyersguide.com/ http://www.domainnamebuyersguide.com/ – http://www.register.md/register_home.jsp http://www.register.md/register_home.jsp To register me.port.ac.uk you need to persuade the port.ac.uk domain administrator to adopt you WUCM125

26. 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)http://www.icann.org/udrp/udrp.htmhttp://www.nic.uk/ref/drs.html Give up! WUCM126

27. 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/software/bind http://www.isc.org/software/bind – Windows Server also has DNS software WUCM127

28. Commercial services 1 Internet Service Providers (ISPs) – 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) WUCM128

29. Commercial services 2 Web hosting companies Typical package would include: – Domain name (in a tld, e.g..com,.net, etc.) – Web space – Email accounts – Tools for web page design – Database support – e.g. MySQL or SQL Server – Dynamic page support – e.g. PHP or CGI – Technical support – including backup and restore WUCM129

30. Commercial services 3 Dedicated and managed server hosts – Server located in a managed environment at a server farm – Supplier provides 24/7 support but you have management authority – Server farms often near major Internet nodes – E.g. http://www.rackspace.co.ukhttp://www.rackspace.co.uk WUCM130

31. 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 WUCM131