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  1. Domain names Dr Jim Briggs WUCM1

  2. From the Internet's perspective WUCM1

  3. 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 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

  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 WUCM1

  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 WUCM1

  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/ WUCM1

  7. WUCM1

  8. Current root server locations WUCM1

  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 WUCM1

  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 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 WUCM1

  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 WUCM1

  12. Delegation WUCM1

  13. 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

  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 WUCM1

  15. Example configuration WUCM1

  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 WUCM1

  17. Iterative queries • It will ask a root name server for the address of name servers for the .ukdomain • 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 WUCM1

  18. WUCM1

  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 WUCM1

  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 WUCM1

  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 WUCM1

  22. From the WEBSITE Manager's perspective WUCM1

  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 WUCM1

  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/ WUCM1

  25. 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 • To register me.port.ac.uk you need to persuade the port.ac.uk domain administrator to adopt you WUCM1

  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) • Give up! WUCM1

  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 • Windows Server also has DNS software WUCM1

  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) WUCM1

  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 WUCM1

  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.uk WUCM1

  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 WUCM1