CS5008: Internet Computing Lecture 12: Domains and DNS A. O Riordan, 2009, latest revision 2016 Some slides based on Fitzgerald and Dennis textbook
Domains Purpose of Domains: Domains are administrative entities. The purpose and expected use of domains is to divide the name management required of a central administration and assign it to sub-administrations. There are no geographical, topological, or technological constraints on a domain. RFC 920 Easier-to-remember domain names (such as www.icann.org) rather than the all-numeric IP addresses (such as 192.0.34.65) A domain name consists of one or more parts, technically called labels, separated by dots, e.g. www.ucc.ie Used by application programs such as for Web and email The Internet Corporation for Assigned Names and Numbers (ICANN) has overall responsibility for managing the domain names 2
Name space Tree-structured global name space IANA root zone database http://www.iana.org/domains/root/db/ traditionally only names that consist of letters, digits and the hyphen (-) permitted but now Unicode supported case-insensitive Top level domains (TLD) e.g. ie (for Ireland) as in www.ucc.ie last label of a fully qualified domain name Second level domains e.g. ucc as in ucc.ie (for UCC) and third-level domain names that are typically open for reservation by endusers 3
gtlds TLDs with three or more characters are generic TLDs, or gtlds Original ones dating back to ARPANET (1985) com - commercial organizations, unrestricted edu - educational establishments, primarily U.S. gov - government entities within the United States int - international organizations established by treaty mil - the U.S. DoD net - originally for network infrastructures, now unrestricted org - originally for organizations now unrestricted arpa - reserved exclusively to support ARPANET/Internet infrastructure New ones added from 2001-2004. aero - the air transport industry asia companies, organisations in the Asia- Pacific region biz - business use cat - Catalan language/culture coop - cooperatives info - informational sites, but unrestricted jobs - employment-related sites mobi -sites catering to mobile devices museum - museums name - families and individuals pro - certain professions tel - services involving the telephone network travel - travel agents, airlines, etc. 4
cctlds TLDs with two letters have been established, since 1985, for over 250 countries and territories and are referred to as country-code TLDs (cctlds) Two letter codes (usually ISO 3166 code, but not for all) e.g. United States (us), Germany (de), Japan (jp), UK (uk), Ireland (ie), EU (eu) A number of the world's smallest countries have licensed their TLDs for worldwide commercial use Tuvalu (tv), e.g. u.tv, FS Micronesia (fm) e.g. last.fm, Andorra (ad) for some advertising sites Changes occur e.g. cs (Serbia and Montenegro) split into rs (Serbia) and me (Montenegro) after Montenegrin independence. Foreign registration permitted for some domains 5
Opened up Since 2011, ICANN's board voted to end most restrictions on the generic top-level domain names (gtld); hundreds of new ones including since 2013 sets of non-latin characters (such as Cyrillic, Arabic, Chinese, etc.) Internationalized cctlds (since 2010), e.g.. 中国 (for China),.рф (for Russia) Some gtlds have unrestricted use, i.e. any territory, any business e.g. com, net, org Others are restricted, requiring proof of eligibility within the guidelines set for each e.g. biz, name, mil, pro 6
Mapping Domains to IP Addresses In ARPANET the NIC (Network Information Centre) maintained a flat table (called hosts.txt) of all name-to-address bindings emailed out to network administrators every few days but as Internet grew there was a need for a better approach Domain Name System (DNS) was introduced in 1983 shortly after TCP/IP was deployed RFC 882 and RFC 883 superseded by RFC 1034 and RFC 1035 Application layer protocol; popular (and de facto) implementation is BIND 7
DNS Used to determine IP address for a given domain Provided through a group of name servers databases containing directories of domain names and their corresponding IP addresses large organizations maintain their own name servers; smaller organizations rely on name servers provided by their ISPs Other uses of DNS: mail transfer agents, email blacklists, software updates 8
How it works DNS uses a hierarchical distributed tree-shape namespace Tree sub-divides into zones beginning at the root zone Designated authoritative name servers for each domain The most common types of records stored in the DNS database are for DNS zone authority (SOA), IP addresses (A and AAAA), SMTP mail exchangers (MX), name servers (NS) and aliases (CNAME) 9
Inside DNS: Resource records (RRs) Resource records (RRs): Name Domain name Type e.g. A record is used to translate from a domain name to an IPv4 address Class IN for Internet TTL Time to live RDATA data of type-specific relevance To provide resilience in the event of computer failure, multiple (two) DNS servers are usually provided for coverage of each domain Due to caching, changes to DNS records do not always take effect immediately TTL is set by the administrator of the DNS server handing out the response 10
How DNS resolves names Clients maintains a address table containing URLs used and corresponding IP addresses If desired URL in client s address table use the corresponding IP address If desired URL not in client s address table: ask DNS server a name server would start its search for an IP address by contacting one of the root name servers. The root servers know the IP address for all of the name servers that handle the top-level domains work way down the tree there are currently #13 root name servers run by VeriSign, ICANN and others under the auspices of DNS Root Server System Advisory Committee, an ICANN committee 11
University of Toronto DNS Request DNS Response DNS Server Asks for a web page on Indiana University s server Client computer LAN DNS Request How DNS Works Internet DNS Response Root DNS Server for.edu domain DNS Request Indiana University DNS Server LAN Copyright 2005 John Wiley & Sons, Inc DNS Response 12