SENG2220 Web Development II

1. SENG2220Web Development II Mohammed A. Saleh http://ifm.ac.tz/staff/msaleh/teach/ 29th October 2009

2. Domain Name Service (DNS) • TCP/IP facility that lets you use names rather than numbers to refer to host computers. • Without DNS, you’d buy books from 207.171.182.16 instead of from www.amazon.comand search the Web at 216.239.51.100 instead of at www.google.com. • If you want to review the complete official specifications for DNS, look up RFC 1034 and 1035 at www.ietf.org/rfc/rfc1034.txt and www.ietf/rfc/rfc1035.txt.

3. Understanding Domain Names • Provides a standardized system for providing names to identify TCP/IP hosts and a way to look up the IP address of a host given the host’s DNS name • For example, if you use DNS to look up the name www.ebay.com, you get the IP address of Ebay’s Web host: 66.135.192.87. • Thus, DNS allows you to access Ebay’s Web site using the DNS name www.ebay.com rather than the site’s IP address

4. Concepts of DNS • To provide a unique DNS name for every host computer on the Internet, DNS uses a time-tested technique: divide and conquer • Uses a hierarchical naming system • It organizes names into domains • Each domain includes all the names that appear directly beneath it in the DNS hierarchy • Figure 1.0: DNS Domain Tree

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6. Cont … • At the very top of the tree is the root domain • Beneath the root domain are four top-level domains, named edu, com, org, and gov (exist more top level domains) • LoweWriter is a personal domain, you have to combine it with the name of its parent domain in order to identify it, hence LoweWriter.com • The parts of the domain name are separated from each other with periods • Beneath the LoweWriter node are four host nodes • The host name with the domain name are combined to get the complete DNS name, server1.LoweWriter.com

7. Nitty-gritty about DNS Names • DNS names are not case-sensitive • The name of each DNS node can be up to 63 characters long (not including the dot) and can include letters, numbers, and hyphens • A subdomain is a domain that’s beneath an existing domain • DNS is a hierarchical naming system that’s similar to the hierarchical folder system used by Windows. • one crucial difference exists between DNS and the Windows naming convention • DNS names start at the bottom of the tree and work your way up to the root ; doug.LoweWriter.com

8. Cont … • Windows paths start at the root and work their way down Windows\System32\dns, dns is the lowest node. • The DNS tree can be up to 127 levels deep. Most DNS names have just three levels (not counting the root)

9. Fully Qualified Domain Names • If a domain name ends with a trailing dot, that trailing dot represents the root domain, and the domain name is said to be a fully qualified domain name (FQDN) • FQDN is also called an absolute name • DNS names that don’t end with a trailing dot are called relative names • relative and FQDN are interchangeable, because the software that interprets them always interprets relative names in the context of the root domain • you can type www.wiley.com — without the trailing dot — rather than www.wiley.com. to go to Wiley’s home page in a Web browser

10. Top Level Domain • Appears immediately beneath the root domain • come in two categories: generic domains and geographic domains Generic Domains • Popular top-level domains that you see most often on the Internet • Table 2.0: Summarizes the original seven generic top-level domains

11. Cont … (source: www.isc.org) • Due to an almost unmanageable size in the late 1990s, the Internet authorities approved seven new top-level domains in an effort to take some of the heat off of the com domain

12. Cont … • They are not very popular.

13. Cont … Geographic Domains • They correspond to international country designations • About 150 geographic top-level domains exist • Some them are tz (Tanzania), us (United states), za (South Africa), jp (Japan) and many more The Hosts Files • Long ago, network administrators could keep track of it all in a simple text file, called hosts files • Simply listed the name and IP address of every host on the network • The key was to keep the hosts files up to date whenever a new host is added

14. Cont … • With the growth of the Internet, it became daunting handling the host files and a better solution was needed. • DNS was invented to solve this problem • Why is it important to understand the host files • Hosts files are still used for small networks, can coexist with DNS • It is the precursor to DNS • The exact location of the hosts file depends on the client operating system

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16. DNS Servers and Zones • A DNS server is a computer that runs DNS server software, helps to maintain the DNS database, and responds to DNS name resolution requests. • Most popular DNS servers are Bind (UNIX-based) and the Windows DNS service (Windows) • DNS database — that is, the list of all the domains, sub-domains, and host mappings — is a massively distributed database • No single DNS server contains the entire DNS database • The database is delegated to different servers throughout the Internet • What happens if someone requests the IP address of coyote.acme.com from LoweWrite.com DNS server?

17. Zones • Simplify the management of the DNS database • The entire DNS namespace is divided into zones • Responsibility for each zone is delegated to a particular DNS server • Zones correspond directly to domains • Example, the LoweWriter.com domain will have LoweWriter.com zone responsible for the whole domain • The sub-domains that make up a domain can be parceled out to separate zones

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19. Cont … • Figure 3.0 show how zones can be separated • A domain named LoweWriter.com has been divided into two zones • One zone, us.LoweWriter.com • The other zone, LoweWriter.com, is responsible for the entire LoweWriter.com domain except the us.LoweWriter.com sub-domain • Why would you do that? The main reason is to delegate authority for the zone to separate servers

20. Types of zones • Primary zone • Master copy of a zone • Data is stored in the local database of the DNS server • Only one DNS server can host a particular primary zone • Secondary zone • Read-only copy of a zone • It obtains its copy of the zone from the zone’s primary server by using a process called zone transfer • must periodically check primary servers for updates

21. Servers • Primary servers • The DNS server hosts a primary zone. • Every zone must have one primary server • Secondary servers • The DNS server obtains the data for a secondary zone from a primary server TIP: A secondary server should be on a different subnet than the zone’s primary server.

22. Cont … • Root servers • The core of DNS • Authoritative for the entire Internet • The main function of the root servers is to provide the address of the DNS servers that are responsible for each of the top-level domains • A total of13 root servers are located throughout the world • DNS servers learn how to reach the root servers

23. Caching • An intelligent way of handling the same user requests • A user visits www.wiley.com today, he’ll probably do it again tomorrow. As a result, name servers keep a cache of query results • Cached data can quickly become obsolete • DNS data is given a relatively short expiration time • The expiration value for DNS data is called the TTL, which stands for time to live

24. DNS Queries • When a DNS client needs to resolve a DNS name to an IP address, it uses a library routine called a resolver to handle the query. • DNS client can make two basic types of queries: recursive and iterative ✦Recursive queries: the server must reply with either the IP address of the requested host name or an error message indicating that the host name doesn’t exist. ✦Iterative queries: it returns the IP address of the requested host name if it knows the address. If it doesn’t know the address, it returns a referral; the address of a DNS server that should know

25. Questions