Chapter 24 Chapter 24 Chapter 24 The Domain Name System The Domain Name System The Domain Name System (DNS) (DNS) (DNS) Raj Jain The Ohio State University Columbus, OH 43210 Jain@CIS.Ohio-State.Edu http://www.cis.ohio-state.edu/~jain/ Raj Jain The Ohio State University 24-1
Overview K Naming hierarchy K Server hierarchy K Name resolution K Other information in name servers Raj Jain The Ohio State University 24-2
Why Names? Why Names? Why Names? K Computers use addresses K Humans cannot remember IP addresses ⇒ Need names Example, Liberia for 164.107.51.28 K Simplest Solution: Each computer has a unique name and has a built in table of name to address translation K Problem: Not scalable K Solution: DNS (Adopted in 1983) K Hierarchical Names: Liberia.cis.ohio-state.edu Raj Jain The Ohio State University 24-3
Name Hierarchy Name Hierarchy Name Hierarchy Unnamed root ... com edu gov au us dec ohio-state nsf co va cis netlab ee reston cnri Raj Jain The Ohio State University 24-4
Name Hierarchy Name Hierarchy Name Hierarchy K Unique domain suffix is assigned by Internet Authority K The domain administrator has complete control over the domain K No limit on number of subdomains or number of levels K computer.site.division.company.com K computer.site.subdivision.division.company.com K Domains within an organization do not have to be uniform in number of subdomains or levels Raj Jain The Ohio State University 24-5
Name Hierarchy (Cont) Name 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 the same floor or in different cities K Geographical hierarchy is also allowed, e.g., cnri.reston.va.us K A name could be a subdomain or an individual object Raj Jain The Ohio State University 24-6
Top Level Domains Top Level Domains Top Level Domains Domain Name Assignment com Commercial edu Educational gov Government mil Military net Network org Other organizations arpa Advanced Research Project Agency country code au, uk, ca Raj Jain The Ohio State University 24-7
Server Hierarchy Server Hierarchy Server Hierarchy com com foobar foobar soap soap candy candy peanut almond walnut peanut almond walnut Raj Jain The Ohio State University 24-8
Server Hierarchy (Cont) Server Hierarchy (Cont) Server Hierarchy (Cont) K Servers are organized in a hierarchy K Each server has an authority over a part of the naming hierarchy K The server does not need to keep all names. K It needs to know other servers who are responsible for other subdomains K Contiguous space ⇒ A single node in the naming tree cannot be split K A given level of hierarchy can be partitioned into multiple servers Raj Jain The Ohio State University 24-9
Server Hierarchy (Cont) Server Hierarchy (Cont) Server Hierarchy (Cont) K Authority ⇒ has the name to address translation table K Responsible ⇒ Either has the name to address translation table or knows the server who has K A single server can serve multiple domains, e.g., purdue.edu and laf.in.us K Root server knows about servers for top-level domains, e.g., com K Each server knows the root server Raj Jain The Ohio State University 24-10
Server Hierarchy: Example Server Hierarchy: Example Server Hierarchy: Example Root Server Server Server Server Server Server for com for edu for gov for au for us Server for Server Server for server for Server for dec.com for osu.edu nsf.gov co.au va.us Raj Jain The Ohio State University 24-11
Server Hierarchy: Better Server Hierarchy: Better Server Hierarchy: Better Root Server Server for Server Server for server for Server for dec.com for osu.edu nsf.gov co.au va.us K Fewer servers Raj Jain The Ohio State University 24-12
Name Resolution Name Resolution Name Resolution Cache Cache Query Name Name Server Server Response Data- Data- Response base base Query Query Query Name Name User Resolver Server Response Response Data- base Cache Cache Raj Jain The Ohio State University 24-13
Name Resolution (Cont) Name Resolution (Cont) Name Resolution (Cont) Cache Cache Name Name Server Server Data- Query Data- base base Referral Response Query Query Query Name Name User Resolver Server Response Response Data- base Cache Cache Raj Jain The Ohio State University 24-14
Name Resolution (Cont) Name Resolution (Cont) Name Resolution (Cont) K Each computer has a name resolver routine, e.g., gethostbyname in UNIX K Each resolver knows the name of a local DNS server K Resolver sends a DNS request to the server K DNS server either gives the answer, forwards the request to another server, or gives a referral K Referral = Next server to whom request should be sent Raj Jain The Ohio State University 24-15
Name Resolution (Cont) Name 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 sufficient K 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 server K Resolvers use recursive query. K Servers use iterative query. Raj Jain The Ohio State University 24-16
DNS Optimization DNS Optimization DNS Optimization K Spatial Locality: Local computers referenced more often than remote K Temporal Locality: Same set of domains referenced repeatedly ⇒ Caching K Each entry has a time to live (TTL) K Replication: Multiple servers. Multiple roots. Ask the geographically closest server. Raj Jain The Ohio State University 24-17
Abbreviations Abbreviations Abbreviations K Servers respond to a full name only K However, humans may specify only a partial name K Resolvers may fill in the rest of the suffix, e.g., Liberia.cis = Liberia.cis.ohio-state.edu K Each resolver has a list of suffixes to try Raj Jain The Ohio State University 24-18
DNS Message Format DNS Message Format DNS Message Format Identification Parameter Number of Questions Number of Answers Number of Authority Number of Additional Question Section ... Answer Section ... Authority Section ... Additional Information Section ... Raj Jain The Ohio State University 24-19
Format (Cont) Format (Cont) Format (Cont) K Format of the query section entries: Query Domain Name ... Query Type Query Class K Format of other section entries: Resource Domain Name Type Class Time to live Resource Data Length Resource Data Raj Jain The Ohio State University 24-20
DNS Message Format DNS Message Format DNS Message Format K Length = 0 ⇒ End of names. Length < 64 Two msbs (most significant bits) = 11 ⇒ Pointer K Resource data contains serial (version) number of the zone, refresh interval, retry interval, expiry interval, mailbox of the responsible person, etc. Raj Jain The Ohio State University 24-21
DNS Message Format (Cont) DNS Message Format (Cont) DNS Message Format (Cont) Bit Meaning 0 Operation: 0=Query, 1=Response 1-4 Query type: 0=Standard, 1=Inverse, 2,3 obsolete 5 Set if answer authoritative 6 Set if message truncated 7 Set if recursion desired 8 Set if recursion available 9-11 Reserved 12-15 Response type: 0=No error, 1=Format error, 2=Server Failure, 3=Name does not exist Raj Jain The Ohio State University 24-22
Inverse Mapping Inverse Mapping Inverse Mapping K Given an address, what is the name? K nnn.nnn.nnn.nnn.in-addr.arpa Raj Jain The Ohio State University 24-23
Types of DNS Entries Types of DNS Entries Types of DNS Entries K DNS is used not just for name to address resolution K But also for finding mail server, pop server, responsible person, etc for a computer K DNS database has multiple types K Record type A ⇒ Address of X K Record type MX ⇒ Mail exchanger of X K CNAME entry = Alias name (like a file link), "see name" K www.foobar.com = hobbes.foobar.com Raj Jain The Ohio State University 24-24
Resource Record Types Resource Record Types Resource Record Types Type Meaning A Host Address CNAME Canonical Name (alias) HINFO CPU and O/S MINFO Mailbox Info MX Mail Exchanger NS Authoritative name server for a domain PTR Pointer to a domain name (link) RP Responsible person SOA Start of zone authority (Which part of naming hierarchy implemented) TXT Arbitrary Text Raj Jain The Ohio State University 24-25
Summary Summary Summary K DNS: Maps names to addresses K Names are hierarchical. Administration is also hierarchical. K No standard for number of levels K Replication and caching is used for performance optimization. Raj Jain The Ohio State University 24-26
Homework Homework Homework K Read Chapter 24 and RFC 1034 and 1035 K Submit answer to exercise 24.1 Raj Jain The Ohio State University 24-27
DNS: References DNS: References DNS: References K D. Comer, “Computer Networks and Internets,” Prentice Hall, NJ, 1997, Chapter 24. K D. Comer, “Internetworking with TCP/IP,” Vol 1, 3rd Ed, Prentice Hall, 1995, Chapter 22. K U. Black, “TCP/IP and Related Protocols,” 2nd Ed, McGraw-Hill, Chapter 4. Raj Jain The Ohio State University 24-28
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