Summer 2009Introduction to “Domain Name Service”(DNS)1. Host name to IP number mapping was originally doneby downloading a static file2. The UNIX version of this file is /etc/hosts (the filewe ftp’d in was called hosts.txtCNT 4603Summer 2009Introduction to the Domain NameService (DNS)3. The central file was maintained by the Stanford ResearchInstitute Network Information Center (SRI-NIC)CNT 4603Summer 2009Introduction to the Domain NameService (DNS)4. As the Internet grew this scheme became unworkable+ The size of the file became too large+ The load on SRI-NIC site became too heavy+ The file was always inconsistent with reality+ Hostname collisions became frequent (anyone couldname their machine “whitehouse.gov” if they wantedto)CNT 4603Summer 2009Overview of DNSIn 1984 Paulk Mockapetris of USC designed thearchitecture of DNS. It’s based on the idea of “resourcerecords”.CNT 4603Summer 2009Overview of DNSThe InterNIC was set up to manage DNS; however,this responsibility was given over to ICANN (InternetCorporation for Assigned Names and Numbers), and nowthe “InterNIC” is just a website. One of the mostimportant activities of ICANN is to accredit registrars.CNT 4603Summer 2009Overview of DNSToday, there are hundreds of registrars, and prices fordomain names have dropped under $10.CNT 4603Summer 2009Features+ Local control: each segment is updated locally+ Global access: each segment is available (almost)immediately to the rest of the world upon updateCNT 4603Summer 2009Features+ Robustness: achieved through replication+ Adequate performance: is achieved through cachingCNT 4603Summer 2009Software+ Servers: called name servers, contain information aboutsome segment of the network and make it available toclients (“BIND” = “Berkeley Internet Name Daemon”,includes “named”, libraries, “nslookup”, “dig”, “host”)CNT 4603Summer 2009Software+ Client: resolvers, a set of library routines that resolvenames by accessing a server (originally a separate library,like libresolv.a, now usually part of libc.a)CNT 4603Summer 2009Software+ Domain name server software is also available for non-UNIX platforms, such as Windows 2008 and MacintoshOS X.CNT 4603Summer 2009Domain structure+ Similar to the structure of a hierarchical file system+ The root’s name is the null label “ “ but is written asa single dot “.”CNT 4603Summer 2009Domain structure+ Each node represents a ’domain’+ Every domain is namedCNT 4603Summer 2009Domain structure+ The full domain name is the sequence of labels fromthe domain to the root, separated by periodsCNT 4603Summer 2009Domain structure+ Unlike a file system pathname the name is read fromleaf to root (right to left rather than left to right)xi.cs.fsu.eduCNT 4603Summer 2009Domain management+ Each domain may be managed by a differentorganization+ The organization may divide itself into subdomainsCNT 4603Summer 2009Domain management+ Then delegate responsbility for maintaining them+ ICANN (currently) manages the “provisioning” of top-level domainsCNT 4603Summer 2009Domain managementWhat is ICANN?The Internet Corporation for Assigned Names andNumbers (ICANN) is responsible for managing andcoordinating the Domain Name System (DNS) to ensurethat every address is unique and that all users ofthe Internet can find all valid addresses. It doesthis by overseeing the distribution of unique IPaddresses and domain names. It also ensures thateach domain name maps to the correct IP address.ICANN is also responsible for accrediting thedomain name registrars. "Accredit" means to identifyand set minimum standards for the performance ofregistration functions, to recognize persons orCNT 4603Summer 2009entities meeting those standards, and to enterinto an accreditation agreement that sets forththe rules and procedures applicable to theprovision of Registrar Services.CNT 4603Summer 2009Host names+ Each host on a network has a domain+ The domain points to information about the hostCNT 4603Summer 2009Host names+ This may include:ó An IP address (A records)ó Mail routing information (MX records)CNT 4603Summer 2009Host namesó Aliases which point to the real (“canonical”) host name(CNAME records)CNT 4603Summer 2009The domain name space+ There may be any number of branches at a node+ Some implementations limit the tree’s depthCNT 4603Summer 2009The domain name space+ Each name may contain up to 63 characters+ The suggested length is 12 or less charactersCNT 4603Summer 2009The domain name space+ A domain name that is written relative to the root iscalled a ’fully-qualified domain name’ - FQDN+ Names without trailing dots (“leading dots”) aresometimes interpreted as relative to some domain otherthan rootCNT 4603Summer 2009The domain name space+ Sibling nodes must have unique names+ The name of a domain is the domain name of the nodeat the top of the domain (example purdue.edu)CNT 4603Summer 2009The domain name space+ Again, similar to a file system+ A node is in multiple domainsCNT 4603Summer 2009The domain name space+ So, a domain is just a subtree of the domain namespace (“subdomain”)+ Must not use “ ”, although other similiar namingschemes (prominently, such as NIS) have allowedthis.CNT 4603Summer 2009Hosts+ Where are the hosts?+ A domain name is just an index into the DNS databaseCNT 4603Summer 2009Hosts+ The ’hosts’ are domain names that point to individualmachine information+ The hosts are related ’logically’ usually by geographyor organizationCNT 4603Summer 2009Hosts+ They are NOT necessarily related by network or IPaddress or hardware type+ You could have 10 different hosts on 10 differentnetworks in ten different countries all in the same domain(hp.com)CNT 4603Summer 2009Hosts+ Nodes at the leaves of the tree usually representindividual hosts+ Interior nodes may point to both host information andto subdomain information For example, “hp.com” isboth the name of a domain and the name of a machinethat routes mailCNT 4603Summer 2009The domain name space+ Termsó top-level domain (TLD): a child of root (edu)ó first-level domain: a child of root (edu)ó second-level domain: a child of 1st level domain(fsu.edu)CNT 4603Summer 2009The domain name spaceCNT 4603Summer 2009The domain name space+ Naming rules - the original 7 top-level domains were:ó com - commerical organizationsó edu - educational organizationsó gov - governmental bodiesó mil - military organizationsó net - networking organizationsó org