It'sinaformateasiertounderstand_read_rememberbypeople.ppt

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1、EE 122:Domain Name Server(DNS)Ion StoicaNov 25,2002(*based in part on on-line slides by J.Kurose&K.Rose and Raj Jain)Names&AddressesWhat is a name?What is an address?What is the difference between names and addresses?2istoicacs.berkeley.eduInternet Centric ViewAddresses:-Says how to reach an object

2、it has location semantics associated to it-Its in a format easy to process by computers Name:-Does not have any location semantics associated to it-Its in a format easier to understand/read/remember by peopleExamples:-IP address:169.229.131.109-Name:arachne.berkeley.edu 3istoicacs.berkeley.eduName S

3、erviceName 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.,addresses)-Bindings can be implemented by using tablesResolution:procedure that,when invoked with a name,returns the corresponding valueName server

4、:specific implementation of a resolution mechanism that is available on the network and that can be queried by sending messages4istoicacs.berkeley.eduGeneral ViewIn general there are multiple mappingsHost name:arachne.berkeley.edu IP address:169.229.131.109 Ethernet MAC address:12.34.56.78.90.12 DNS

5、 resolutionARP(Address Resolution Protocol)5istoicacs.berkeley.eduMappingMultiple names can map onto the same address-Example:www.berkeley.edu and arachne.berkeley.edu maps to the same machine(i.e.,the same IP address)One name can map onto multiple addresses-Example: can be mapped to multiple machin

6、es6istoicacs.berkeley.eduName Hierarchyrooteducomgovmilorgnetukfrberkeleycmueecssimssonoma7istoicacs.berkeley.eduName 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 le

7、velsName 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 objects 8istoicacs.berkeley.eduTop Level DomainsDomain Name AssignmentcomCommercialeduEducational govGovernment milMilitary netNetwork or

8、gOther organizationscountry codeau,uk,ca,9istoicacs.berkeley.eduDNS Name ServersWhy not centralize DNS?-Single point of failure-Traffic volume-Distant centralized database-MaintenanceDoesnt scale!10istoicacs.berkeley.eduServer Hierarchy:ZonesA zone corresponds to an administrative authority that is

9、responsible for that portion of the hierarchyrooteducomgovmilorgnetukfrberkeleycmueecssimsdivine11istoicacs.berkeley.eduServer 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

10、only a subset of all names-It needs to know other servers that are responsible for the other portions of the hierarchy12istoicacs.berkeley.eduServer HierarchyAuthority:each server has the name to address translation table for all names in the name space it controlsEvery server knows the rootRoot ser

11、ver knows about all top-level domains13istoicacs.berkeley.eduDNS 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 hosts(name,IP ad

12、dress)-Can perform name/address translation for that hosts name 14istoicacs.berkeley.eduDNS: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

13、root name servers worldwide15istoicacs.berkeley.eduSimple 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

14、.berkeley.edu,if necessary requesting hostwhistler.cs.cmu.eduwww.berkeley.eduroot name serverauthorititive name serverns1.berkeley.edulocal name servermango.srv.cs.cmu.edu12345616istoicacs.berkeley.eduDNS ExampleRoot name server:May not know authoritative name serverMay know intermediate name server

15、: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)7817istoicacs.berkeley.eduDNS:Iterated QueriesRecursive query:Pu

16、ts burden of name resolution on contacted name serverHeavy load?Iterated query:Contacted server replies with name of server to contact“I dont know this name,but ask this server”requesting hostwhistler.cs.cmu.eduwww.berkeley.eduroot name serverlocal name servermango.srv.cs.cmu.edu123467authoritative

17、name serverns1.berkeley.eduintermediate name server(edu server)58iterated query18istoicacs.berkeley.eduDiscussionRobustness-Use multiple replicas,but-what if someone mounts a denial of service attack to all root servers?Performance:-Use caching to speed-up subsequent queries to the same nameWhat abo

18、ut update/notify?-Mechanisms under design by IETF(RFC 2136;http:/www.ietf.org/html.charters/dnsind-charter.html)19istoicacs.berkeley.eduSummaryDNS:maps names onto IP addressName space and the administration are both hierarchicalReplication:used to increase robustnessCaching:used to increase the performance 20istoicacs.berkeley.edu