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Name Services and DNS

Name Services and DNS. Distributed Computer Systems. Contents. What is a name service ? SNS : A Simple Name Service DNS : Domain Name Service The X.500 Directory Service. 2. What is a name service?. Client processes use name services to keep track of entities such as:

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Name Services and DNS

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  1. Name Services and DNS Distributed Computer Systems

  2. Contents • What is a name service? • SNS: A Simple Name Service • DNS: Domain Name Service • The X.500 Directory Service 2

  3. What is a name service? • Client processes use name services to keep track of entities such as: • Address of named resources or objects. • These objects may be: • users • file and directory names • computers • network domains and services, ports,... 3

  4. A mechanism is needed to convert names to addresses • TCP/IP systems convert names to IP addresses by using: • A host file • Domain Name System (DNS) • NFS systems use Network Information Service (NIS) 4

  5. The host file • TCP/IP systems normally have a file called hosts that contains: • a simple list of IP addresses and • names that relate to them • Each address can have several names: • usually a formal name and • a number of nicknames or aliases. 5

  6. Example of a hosts file • Parts of the /etc/hosts file in bs47c # "Large" multi-user/server Alphas/scismes etc 193.60.1.13 bs47a.staffs.ac.uk bs47a 193.60.1.14 bs47b.staffs.ac.uk bs47b 193.60.1.15 bs47c.staffs.ac.uk bs47c 193.60.1.16 bs41.staffs.ac.uk bs41 193.60.1.17 bsmail.staffs.ac.uk bsmail bs33m 193.60.1.18 web.staffs.ac.uk web www bs33w 193.60.1.19 news.staffs.ac.uk news bs33n # # IP Hosts Database # # Allocation of ranges: # # 193.60.1.n - scismes, scismstations, DECstations # # 193.60.2.n - Suns, Acorns # # 193.60.3.n - Apples, FastPaths (*** Internet Access enabled ***) # # 193.60.4.n - PCs and clones (*** Internet Access enabled ***) # # 193.60.5.n - Comms. devices (bridges, servers, etc.) # Reserved for School of Computing 193.61.121.1 mail.soc.staffs.ac.uk 193.61.121.252 bsroute121.staffs.ac.uk bsroute121 193.61.121.254 crroute121.staffs.ac.uk crroute121 # [no smtp] 6

  7. Name Services • A name service stores a database of bindings between textual names and attributes for objects. • Major operations of a name service: • Name Resolution-lookup an address, etc. • Creating new bindings • Deleting bindings • Listing bound names 7

  8. Composed naming domains • Names often consist of several components which must be looked up in separate parts of the database: P.saeidi@soc.staffs.ac.uk Name Type: Value: User login name Computer: network address 8

  9. The SNS - a Simple Name Service model • Stores attributes of named objects such as users, computers and services and group names. Value Named object Email server, login info, encoded passwords, home directory Users Computers Services Network addresses, architecture, OS, owner Service address, version no. Group Mailing lists, group1, group2,... 9

  10. SNS basic design requirements • Specify the Types of named objects: • users, services, computers and group names and directories. • Other types of objects may be integrated; • The names are used only within the organization; • Efficient name lookup; • Access control: • everyone can read but Authorized write; 10

  11. SNS data and operations • Clients perceive SNS to store attributes of the form <Type, Value> against names. • Type declares the generic type of the object: • User, Service or Computer • Group, alias or directory • The format of the Value depends upon type 11

  12. Attributes stored by SNSSome example in an abstract form Type Value Users Computers Services <login name, mail server, ... <network addresses, architecture, OS, owner> <Service address, version no.> Group <mailing lists, group1, group2,...> <name> Alias Directory <name component 1>, <name component 2>,... 12

  13. Name service operations in the SNS • Name resolution (Lookup) • Binding (Bind) • Unbinding(Unbind) 13

  14. Bind operation- Creates a binding Bind (accessId: Permission, name:Text, attr: Attributes) {Success, Not Allowed, AlreadyBound, NoDirectory} Failure Types 14

  15. Unbind operation- Deletes a binding Unbind (accessId: Permission, name:Text) {Success, NotFound, Not Allowed, DirectoryNotEmpty} Failure Types 15

  16. Lookup operation • Looks up a name and returns attributes if name is bound Lookup (name:Text, type:Int, attr: Attributes) {Success, NotFound} 16

  17. Name resolution • Is an iterative process whereby a name is repeatedly presented to naming contexts. • A naming context is represented by a directory andmaps a given name onto: • a set of attributes directly, or • a further naming context 17

  18. Name servers and navigation • Multiple servers can store different partitions of the naming database. • This enables different departments to control their own sub-domains. .rootServer engineering computing Arts ... .engServer .compServer .staffServer Staff students ... students staff cmtps cmtmcw ...

  19. Navigation in SNS • The process of locating naming data from among more than one server. • Every computer runs a process called a user agent (UA). • The UA carries out a navigation on behalf of clients at its computer. • The UA caches attributes it receives from name servers.

  20. How does navigation work? • The client code sends a name to a local UA. • If the UA does not hold the cached attributes (from a previous lookup) it starts to navigate by itteratively contacting name servers until the name is resolved. Name Servers NS1 NS2 UA NS3

  21. Domain Naming System (DNS) • DNS is a distributed mapping system between host names and IP numbers • Each host on the Internet with a registered name has one name associated with its IP number.

  22. DNS functions • Two main services are provided by dedicated name servers: • Lookup services • name resolution (mapping host names onto IP addresses) • Provision of a database • that defines these mappings

  23. The DNS hierarchy • DNS is implemented by a hierarchical system of name servers • The root level is maintained by InterNIC • InterNIC assigns domain names in all root level domains including: • EDU • COM • MIL • NET • ORG...

  24. Example of DNS name space hierarchy • The host namesoc.staffs.ac.ukis part of domain staffs.ac.uk (staffordshire University) • staffs.ac.uk is part of academic community in uk: • the section of DNS name space that is allocated to staffordshire university • ac is the domain for academic communities in • uk ....

  25. The DNS example... root uk org com edu gov .... ................ co ac staffs shef bs47c .... soc

  26. DNS Name Resolution • The nearest DNS name server maintains ONLY a small portion of the total DNS name space. • DNS uses a name resolution protocol to resolve names by • Using a special program called resolver that searches for a proper DNS name server.

  27. The X.500 Directory Service • A directory service that can be used the same way as in a conventional name services. • A service for accessing real-world entities satisfying also imprecise queries such as those in yellow pages. • Organizes data in a tree structure called the Directory Information Base (DIB)

  28. The X.500 service architecture • Servers are Directory Service Agents (DSAs) and the clients are termed Directory User Agents (DUAs). DSA DSA DUA DUA DSA DSA DSA DUA DSA

  29. The X.500 service architecture • The architecture is similar to DNS: • Each DUA client interacts with a single DSA process which may access other DSAs DSA DSA DUA DUA DSA DSA DSA DUA DSA

  30. The DIB • Each entry consists of a name and a set of attributes • The full name corresponds to a path in DIT (Directory information Tree) The X500 Service (Root) ...France(country) Great Britain (country) Germany(country) ... ....BT PLc (org) Staffordshire university (org) ... ..SOC(org unit) Engineering(org unit) ... ...Staff(org unit) Students(org unit) ...

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