EE 122: Lecture 20 (Domain Name Server - DNS)Names & AddressesInternet Centric ViewGeneral ViewName ServiceSlide 6MappingName HierarchySlide 9Top Level DomainsDNS Name ServersServer Hierarchy: ZonesServer HierarchySlide 14Slide 15DNS: Root Name ServersSimple DNS ExampleDNS ExampleDNS: Iterated QueriesDNS RecordsDNS protocol, messagesSlide 22DiscussionSummaryEE 122: Lecture 20(Domain Name Server - DNS)Ion StoicaNov 15, 2001(* based on the some on-line slides of J. Kurose & K. Rose and of Raj Jain)[email protected] 2Names & AddressesWhat is a name?What is an address?What is an identifier?What is the difference between name, addresses, and [email protected] 3Internet Centric ViewAddresse: -Says how to reach an object it has location semantics associated to it-It’s in a format easy to process by computers Name: -Does not have any location semantics associated to it-It’s in a format easier to understand/read/remember by peopleExamples:-IP address: 128.32.27.22-Name: [email protected] 4General ViewDifferentiating between names and addresses introduces several consistency problems:-What is a multicast IP address?-What is the IP address of a mobile host?-What is cnri.reston.va.us?Solution: interpret an address as just another name (that may happen to have location semantics)[email protected] 5Name ServiceName space: define the set of possible names-Hierarchical (e.g., Unix and Windows file names)-Flat Bindings: the mapping between names and values (e.g., bindings can be implemented by using tables)Resolution: procedure that, when invoked with a name, returns the corresponding valueName server: specific implementation of a resolution mechanism that is available on the network and that can be queried by sending [email protected] 6General ViewIn general there are multiple mappingsHost name: amber.berkeley.edu IP address: 128.32.27.22 Ethernet MAC address: 12.34.56.78.90.12 DNS resolutionARP (Address Resolution Protocol)[email protected] 7MappingMultiple names can map onto the same name at the underlying level-Example: www.berkeley.edu and amber.berkeley.edu can map to the same machine (I.e., the same IP address)One name can map onto multiple names at the underlying level-Example: www.yahoo.com can be mapped to multiple [email protected] 8Name Hierarchyrooteducomgov milorgnet uk [email protected] 9Name HierarchyUnique domain suffix is assigned by the Internet AuthorityThe domain administrators have complete control over the domainNo limit on the number of subdomains or number of levelsName space is not related with the physical interconnection Geographical hierarchy is allowed (e.g., cnri.reston.va.us)A name could be a domain or an individual [email protected] 10Top Level DomainsDomain Name Assignmentcom Commercialedu Educational gov Government mil Military net Network Org Other organizationsArpa Advances research project agencycountry code au, uk, [email protected] 11DNS Name ServersWhy not centralize DNS? -Single point of failure-Traffic volume-Distant centralized database-MaintenanceDoesn’t [email protected] 12Server Hierarchy: ZonesA zone corresponds to an administrative authority that is responsible for that portion of the hierarchyrooteducomgov milorgnet uk [email protected] 13Server HierarchyServer are organized in hierarchiesEach server has authority over a portion of the hierarchy-A single node in the name hierarchy cannot be split-A server maintains only a subset of all names-It needs to know other servers that are responsible for the other portions of the [email protected] 14Server HierarchyAuthority: each server has the name to address translation table for all names in the name space it controlsEvery server knows the rootRoot server knows about all top-level [email protected] 15DNS Name ServersNo server has all name-to-IP address mappingsLocal name servers:-Each ISP (company) has local (default) name server-Host DNS query first go to local name serverAuthoritative name servers:-For a host: stores that host’s IP address, name-Can perform name/address translation for that host’s [email protected] 16DNS: Root Name ServersContacted by local name server that can not resolve nameRoot name server:-Contacts authoritative name server if name mapping not known-Gets mapping-Returns mapping to local name server~ Dozen root name servers [email protected] 17Simple DNS ExampleHost whsitler.cs.cmu.edu wants IP address of www.berkeley.edu1. Contacts its local DNS server, mango.srv.cs.cmu.edu2. mango.srv.cs.cmu.edu contacts root name server, if necessary3. Root name server contacts authoritative name server, ns1.berkeley.edu, if necessary requesting hostwhistler.cs.cmu.eduwww.berkeley.eduroot name serverauthorititive name serverns1.berkeley.edulocal name [email protected] 18DNS ExampleRoot name server:May not know authoritative name serverMay know intermediate name server: who to contact to find authoritative name server?requesting hostwhistler.cs.cmu.eduwww.berkeley.eduroot name serverlocal name servermango.srv.cs.cmu.edu123456authoritative name serverns1.berkeley.eduintermediate name server(edu server)[email protected] 19DNS: Iterated QueriesRecursive query:Puts burden of name resolution on contacted name serverHeavy load?Iterated query:Contacted server replies with name of server to contact“I don’t know this name, but ask this server”requesting hostwhistler.cs.cmu.eduwww.berkeley.eduroot name serverlocal name servermango.srv.cs.cmu.edu123467authoritative name serverns1.berkeley.eduintermediate name server(edu server)58iterated [email protected] 20DNS RecordsDNS: distributed database storing resource records (RR)Type=NS-name is domain (e.g. foo.com)-value is IP address of authoritative name server for this domainRR format: (name, value, type,ttl)Type=A-name is hostname-value is IP addressType=CNAME-name is an alias name for some “cannonical” (the real) name-value is cannonical nameType=MX-value is hostname of mailserver associated with [email protected] 21DNS protocol, messagesDNS protocol : query and reply messages, both with
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