the domain name system (dns)
TRANSCRIPT
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The Domain Name SystemThe Domain Name System(DNS)(DNS)
Raj JainThe Ohio State University
Columbus, OH [email protected]
http://www.cis.ohio-state.edu/~jain/
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❑ Naming hierarchy
❑ Server hierarchy
❑ Name resolution
❑ Other information in name servers
Overview
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Why Names?Why Names?
❑ Computers use addresses
❑ Humans cannot remember IP addresses⇒ Need namesExample, Liberia for 164.107.51.28
❑ Simplest Solution: Each computer has a unique nameand has a built in table of name to address translation
❑ Problem: Not scalable
❑ Solution: DNS (Adopted in 1983)
❑ Hierarchical Names: Liberia.cis.ohio-state.edu
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Name HierarchyName Hierarchy
reston
com edu gov au us
dec ohio-state nsf co va
cis netlab ee
cnri
Unnamed root
...
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Name HierarchyName Hierarchy
❑ Unique domain suffix is assigned by InternetAuthority
❑ The domain administrator has complete control overthe domain
❑ No limit on number of subdomains or number oflevels
❑ computer.site.division.company.com
❑ computer.site.subdivision.division.company.com
❑ Domains within an organization do not have to beuniform in number of subdomains or levels
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Name Hierarchy (Cont)Name Hierarchy (Cont)
❑ 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 cities
❑ Geographical hierarchy is also allowed, e.g.,cnri.reston.va.us
❑ A name could be a subdomain or an individual object
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Top Level DomainsTop Level Domains
Domain Name Assignmentcom Commercialedu Educationalgov Governmentmil Militarynet Networkorg Other organizationsarpa Advanced Research Project Agencycountry code au, uk, ca
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Server HierarchyServer Hierarchy
peanut almond walnut
candy
soap
foobar
com
peanut almond walnut
candy
soap
foobar
com
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Server Hierarchy (Cont)Server Hierarchy (Cont)
❑ Servers are organized in a hierarchy
❑ Each server has an authority over a part of the naminghierarchy
❑ The server does not need to keep all names.
❑ It needs to know other servers who are responsible forother subdomains
❑ Contiguous space ⇒ A single node in the naming treecannot be split
❑ A given level of hierarchy can be partitioned intomultiple servers
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Server Hierarchy (Cont)Server Hierarchy (Cont)
❑ Authority ⇒ has the name to address translation table
❑ Responsible ⇒ Either has the name to addresstranslation table or knows the server who has
❑ A single server can serve multiple domains, e.g.,purdue.edu and laf.in.us
❑ Root server knows about servers for top-leveldomains, e.g., com
❑ Each server knows the root server
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Server Hierarchy: ExampleServer Hierarchy: Example
Serverfor com
Serverfor edu
Serverfor gov
Serverfor au
Serverfor us
Server for dec.com
Serverfor osu.edu
Server fornsf.gov
server forco.au
Server forva.us
RootServer
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Server Hierarchy: BetterServer Hierarchy: Better
❑ Fewer servers
Server for dec.com
Serverfor osu.edu
Server fornsf.gov
server forco.au
Server forva.us
RootServer
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Name ResolutionName Resolution
UserName
ResolverNameServer
NameServer
Cache Cache
Cache
Que
ry
Res
pons
e
Query
Response
Query
Response
Data-base
Data-base
NameServer
Cache
Data-base
Query
Response
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Name Resolution (Cont)Name Resolution (Cont)
UserName
ResolverNameServer
NameServer
Cache Cache
Cache
Que
ry
Ref
erra
l
Query
Response
Query
Response
Data-base
Data-base
NameServer
Cache
Data-base
Query
Response
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Name Resolution (Cont)Name Resolution (Cont)
❑ Each computer has a name resolver routine, e.g.,gethostbyname in UNIX
❑ Each resolver knows the name of a local DNS server
❑ Resolver sends a DNS request to the server
❑ DNS server either gives the answer, forwards therequest to another server, or gives a referral
❑ Referral = Next server to whom request should be sent
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Name Resolution (Cont)Name Resolution (Cont)
❑ Resolvers use UDP (single name)or TCP (whole group of names)
❑ Knowing the address of the root server is sufficient
❑ Recursive Query:Give me an answer (Don't give me a referral)
❑ Iterative Query:Give me an answer or a referral to the next server
❑ Resolvers use recursive query.
❑ Servers use iterative query.
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DNS OptimizationDNS Optimization
❑ Spatial Locality: Local computers referenced moreoften than remote
❑ Temporal Locality: Same set of domains referencedrepeatedly ⇒ Caching
❑ Each entry has a time to live (TTL)
❑ Replication: Multiple servers. Multiple roots.Ask the geographically closest server.
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AbbreviationsAbbreviations
❑ Servers respond to a full name only
❑ However, humans may specify only a partial name
❑ Resolvers may fill in the rest of the suffix, e.g.,Liberia.cis = Liberia.cis.ohio-state.edu
❑ Each resolver has a list of suffixes to try
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DNS Message FormatDNS Message Format
Number of Authority Number of AdditionalQuestion Section
...Answer Section
...Authority Section
...Additional Information Section
...
Number of Questions Number of AnswersIdentification Parameter
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Format (Cont)Format (Cont)❑ Format of the query section entries:
❑ Format of other section entries:
Query Type Query Class
Query Domain Name...
Time to live Resource Data Length
Resource Domain Name
Type Class
Resource Data
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DNS Message FormatDNS Message Format
❑ Length = 0 ⇒ End of names. Length < 64Two msbs (most significant bits) = 11 ⇒ Pointer
❑ Resource data contains serial (version) number of thezone, refresh interval, retry interval, expiry interval,mailbox of the responsible person, etc.
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DNS Message Format (Cont)DNS Message Format (Cont)
Bit Meaning0 Operation: 0=Query, 1=Response1-4 Query type: 0=Standard, 1=Inverse, 2,3
obsolete5 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 exist
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Inverse MappingInverse Mapping
❑ Given an address, what is the name?
❑ nnn.nnn.nnn.nnn.in-addr.arpa
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Types of DNS EntriesTypes of DNS Entries
❑ DNS is used not just for name to address resolution
❑ But also for finding mail server, pop server,responsible person, etc for a computer
❑ DNS database has multiple types
❑ Record type A ⇒ Address of X
❑ Record type MX ⇒ Mail exchanger of X
❑ CNAME entry = Alias name (like a file link), "seename"
❑ www.foobar.com = hobbes.foobar.com
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Resource 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 of
naming hierarchy implemented)TXT Arbitrary Text
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SummarySummary
❑ DNS: Maps names to addresses
❑ Names are hierarchical. Administration is alsohierarchical.
❑ No standard for number of levels
❑ Replication and caching is used for performanceoptimization.