EE 122 Domain Name Server DNS Ion Stoica Nov 25 2002 based in part on on line slides by J Kurose K Rose and Raj Jain Names Addresses What is a name What is an address What is the difference between names and addresses istoica cs berkeley edu 2 Internet Centric View Addresses 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 people Examples IP address 169 229 131 109 Name arachne berkeley edu istoica cs berkeley edu 3 Name Service Name 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 tables Resolution procedure that when invoked with a name returns the corresponding value Name server specific implementation of a resolution mechanism that is available on the network and that can be queried by sending messages istoica cs berkeley edu 4 General View In general there are multiple mappings Host name arachne berkeley edu DNS resolution IP address 169 229 131 109 ARP Address Resolution Protocol Ethernet MAC address 12 34 56 78 90 12 istoica cs berkeley edu 5 Mapping Multiple 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 www yahoo com can be mapped to multiple machines istoica cs berkeley edu 6 Name Hierarchy root edu berkeley eecs com gov mil org net uk fr cmu sims sonoma istoica cs berkeley edu 7 Name Hierarchy Unique domain suffix is assigned by the Internet Authority The domain administrators have complete control over the domain No limit on the number of subdomains or number of levels Name 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 istoica cs berkeley edu 8 Top Level Domains Domain Name Assignment com Commercial edu Educational gov Government mil Military net Network org Other organizations country code au uk ca istoica cs berkeley edu 9 DNS Name Servers Why not centralize DNS Single point of failure Traffic volume Distant centralized database Maintenance Doesn t scale istoica cs berkeley edu 10 Server Hierarchy Zones A zone corresponds to an administrative authority that is responsible for that portion of the hierarchy root edu berkeley eecs divine com gov mil org net uk fr cmu sims istoica cs berkeley edu 11 Server Hierarchy Server are organized in hierarchies Each 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 hierarchy istoica cs berkeley edu 12 Server Hierarchy Authority each server has the name to address translation table for all names in the name space it controls Every server knows the root Root server knows about all top level domains istoica cs berkeley edu 13 DNS Name Servers No server has all name to IP address mappings Local name servers Each ISP company has local default name server Host DNS query first go to local name server Authoritative name servers For a host stores that host s name IP address Can perform name address translation for that host s name istoica cs berkeley edu 14 DNS Root Name Servers Contacted by local name server that can not resolve name Root name server Contacts authoritative name server if name mapping not known Gets mapping Returns mapping to local name server Dozen root name servers worldwide istoica cs berkeley edu 15 Simple DNS Example root name server Host whsitler cs cmu edu wants IP address of www berkeley edu 1 Contacts its local DNS server mango srv cs cmu edu 2 mango srv cs cmu edu contacts root name server if necessary 3 Root name server contacts authoritative name server ns1 berkeley edu if necessary 2 5 3 4 local name server authorititive name server mango srv cs cmu edu 1 6 requesting host whistler cs cmu edu istoica cs berkeley edu ns1 berkeley edu www berkeley edu 16 DNS Example root name server Root name server May not know 2 7 authoritative name server May know intermediate name server who to contact to find local name server authoritative name mango srv cs cmu edu server 1 8 requesting host 6 3 intermediate name server edu server 4 5 authoritative name server ns1 berkeley edu whistler cs cmu edu www berkeley edu istoica cs berkeley edu 17 DNS Iterated Queries Recursive query root name server Puts burden of name resolution on contacted name server Heavy load iterated query 2 3 5 Iterated query Contacted server replies with name of server to contact I don t know this name but ask this server 4 local name server mango srv cs cmu edu 1 8 requesting host intermediate name server edu server 6 7 authoritative name server ns1 berkeley edu whistler cs cmu edu istoica cs berkeley edu www berkeley edu 18 Discussion Robustness 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 name What about update notify Mechanisms under design by IETF RFC 2136 http www ietf org html charters dnsind charter html istoica cs berkeley edu 19 Summary DNS maps names onto IP address Name space and the administration are both hierarchical Replication used to increase robustness Caching used to increase the performance istoica cs berkeley edu 20
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