Raj Jain The Ohio State University24-1The Domain Name SystemThe
Domain Name System(DNS)(DNS)Raj JainThe Ohio State
UniversityColumbus, OH
[email protected]://www.cis.ohio-state.edu/~jain/Raj
Jain The Ohio State University24-2K Naming hierarchyK Server
hierarchyK Name resolutionK Other information in name
serversOverviewRaj Jain The Ohio State University24-3Why Names?Why
Names?K Computers use addressesK Humans cannot remember IP
addresses Need namesExample, Liberia for 164.107.51.28K Simplest
Solution: Each computer has a unique nameand has a built in table
of name to address translationK Problem: Not scalableK Solution:
DNS (Adopted in 1983)K Hierarchical Names:
Liberia.cis.ohio-state.eduRaj Jain The Ohio State
University24-4Name HierarchyName Hierarchyrestoncom edu gov au
usdec ohio-state nsf co vacis netlab eecnriUnnamed root...Raj Jain
The Ohio State University24-5Name HierarchyName HierarchyK Unique
domain suffix is assigned by InternetAuthorityK The domain
administrator has complete control overthe domainK No limit on
number of subdomains or number oflevelsK
computer.site.division.company.comK
computer.site.subdivision.division.company.comK Domains within an
organization do not have to beuniform in number of subdomains or
levelsRaj Jain The Ohio State University24-6Name Hierarchy
(Cont)Name Hierarchy (Cont)K Name space is not related to physical
interconnection,e.g., math.ohio-state and cis.ohio-state could be
on thesame floor or in different citiesK Geographical hierarchy is
also allowed, e.g.,cnri.reston.va.usK A name could be a subdomain
or an individual objectRaj Jain The Ohio State University24-7Top
Level DomainsTop Level DomainsDomain Name Assignmentcom
Commercialedu Educationalgov Governmentmil Militarynet Networkorg
Other organizationsarpa Advanced Research Project Agencycountry
code au, uk, caRaj Jain The Ohio State University24-8Server
HierarchyServer Hierarchypeanut almond
walnutcandysoapfoobarcompeanut almond walnutcandysoapfoobarcomRaj
Jain The Ohio State University24-9Server Hierarchy (Cont)Server
Hierarchy (Cont)K Servers are organized in a hierarchyK Each server
has an authority over a part of the naminghierarchyK The server
does not need to keep all names.K It needs to know other servers
who are responsible forother subdomainsK Contiguous space A single
node in the naming treecannot be splitK A given level of hierarchy
can be partitioned intomultiple serversRaj Jain The Ohio State
University24-10Server Hierarchy (Cont)Server Hierarchy (Cont)K
Authority has the name to address translation tableK Responsible
Either has the name to addresstranslation table or knows the server
who hasK A single server can serve multiple domains,
e.g.,purdue.edu and laf.in.usK Root server knows about servers for
top-leveldomains, e.g., comK Each server knows the root serverRaj
Jain The Ohio State University24-11Server Hierarchy: ExampleServer
Hierarchy: ExampleServerfor comServerfor eduServerfor govServerfor
auServerfor usServer for dec.comServerfor osu.eduServer
fornsf.govserver forco.auServer forva.usRootServerRaj Jain The Ohio
State University24-12Server Hierarchy: BetterServer Hierarchy:
BetterK Fewer serversServer for dec.comServerfor osu.eduServer
fornsf.govserver forco.auServer forva.usRootServerRaj Jain The Ohio
State University24-13Name ResolutionName
ResolutionUserNameResolverNameServerNameServerCache
CacheCacheQueryResponseQueryResponseQueryResponseData-baseData-baseNameServerCacheData-baseQueryResponseRaj
Jain The Ohio State University24-14Name Resolution (Cont)Name
Resolution (Cont)UserNameResolverNameServerNameServerCache
CacheCacheQueryReferralQueryResponseQueryResponseData-baseData-baseNameServerCacheData-baseQueryResponseRaj
Jain The Ohio State University24-15Name Resolution (Cont)Name
Resolution (Cont)K Each computer has a name resolver routine,
e.g.,gethostbyname in UNIXK Each resolver knows the name of a local
DNS serverK Resolver sends a DNS request to the serverK DNS server
either gives the answer, forwards therequest to another server, or
gives a referralK Referral = Next server to whom request should be
sentRaj Jain The Ohio State University24-16Name Resolution
(Cont)Name Resolution (Cont)K Resolvers use UDP (single name)or TCP
(whole group of names)K Knowing the address of the root server is
sufficientK Recursive Query:Give me an answer (Don't give me a
referral)K Iterative Query:Give me an answer or a referral to the
next serverK Resolvers use recursive query.K Servers use iterative
query.Raj Jain The Ohio State University24-17DNS OptimizationDNS
OptimizationK Spatial Locality: Local computers referenced
moreoften than remoteK Temporal Locality: Same set of domains
referencedrepeatedly CachingK Each entry has a time to live (TTL)K
Replication: Multiple servers. Multiple roots.Ask the
geographically closest server.Raj Jain The Ohio State
University24-18AbbreviationsAbbreviationsK Servers respond to a
full name onlyK However, humans may specify only a partial nameK
Resolvers may fill in the rest of the suffix, e.g.,Liberia.cis =
Liberia.cis.ohio-state.eduK Each resolver has a list of suffixes to
tryRaj Jain The Ohio State University24-19DNS Message FormatDNS
Message FormatNumber of Authority Number of AdditionalQuestion
Section...Answer Section...Authority Section...Additional
Information Section...Number of Questions Number of
AnswersIdentification ParameterRaj Jain The Ohio State
University24-20Format (Cont)Format (Cont)K Format of the query
section entries:K Format of other section entries:Query Type Query
ClassQuery Domain Name...Time to live Resource Data LengthResource
Domain NameType ClassResource DataRaj Jain The Ohio State
University24-21DNS Message FormatDNS Message FormatK Length = 0 End
of names. Length < 64Two msbs (most significant bits) = 11
PointerK Resource data contains serial (version) number of thezone,
refresh interval, retry interval, expiry interval,mailbox of the
responsible person, etc.Raj Jain The Ohio State University24-22DNS
Message Format (Cont)DNS Message Format (Cont)Bit Meaning0
Operation: 0=Query, 1=Response1-4 Query type: 0=Standard,
1=Inverse, 2,3obsolete5 Set if answer authoritative6 Set if message
truncated7 Set if recursion desired8 Set if recursion available9-11
Reserved12-15 Response type: 0=No error, 1=Format error,2=Server
Failure, 3=Name does not existRaj Jain The Ohio State
University24-23Inverse MappingInverse MappingK Given an address,
what is the name?K nnn.nnn.nnn.nnn.in-addr.arpaRaj Jain The Ohio
State University24-24Types of DNS EntriesTypes of DNS EntriesK DNS
is used not just for name to address resolutionK But also for
finding mail server, pop server,responsible person, etc for a
computerK DNS database has multiple typesK Record type A Address of
XK Record type MX Mail exchanger of XK CNAME entry = Alias name
(like a file link), "seename"K www.foobar.com =
hobbes.foobar.comRaj Jain The Ohio State University24-25Resource
Record TypesResource Record TypesType MeaningA Host AddressCNAME
Canonical Name (alias)HINFO CPU and O/SMINFO Mailbox InfoMX Mail
ExchangerNS Authoritative name server for a domainPTR Pointer to a
domain name (link)RP Responsible personSOA Start of zone authority
(Which part ofnaming hierarchy implemented)TXT Arbitrary TextRaj
Jain The Ohio State University24-26SummarySummaryK DNS: Maps names
to addressesK Names are hierarchical. Administration is
alsohierarchical.K No standard for number of levelsK Replication
and caching is used for performanceoptimization.
