Name Services and DNS

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Name Services and DNS

• Distributed Computer Systems

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Contents

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

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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,...

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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)

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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.

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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
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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

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Composed naming domains

• Names often consist of several components which
  must be looked up in separate parts of the
  database

P.saeidi_at_soc.staffs.ac.uk
Name Type Value
User login name
Computer network address
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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,...
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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

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SNS data and operations

• Clients perceive SNS to store attributes of the
  form ltType, Valuegt 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

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Attributes stored by SNSSome example in an
abstract form

Type
Value
Users Computers Services
ltlogin name, mail server, ...
ltnetwork addresses, architecture, OS, ownergt
ltService address, version no.gt
Group
ltmailing lists, group1, group2,...gt
ltnamegt
Alias
Directory
ltname component 1gt, ltname component 2gt,...
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Name service operations in the SNS

• Name resolution (Lookup)
• Binding (Bind)
• Unbinding(Unbind)

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Bind operation- Creates a binding

Bind (accessId Permission, nameText, attr
Attributes)
Success, Not Allowed, AlreadyBound, NoDirectory
Failure Types
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Unbind operation- Deletes a binding

Unbind (accessId Permission, nameText)
Success, NotFound, Not Allowed,
DirectoryNotEmpty
Failure Types
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Lookup operation

• Looks up a name and returns attributes if name is
  bound

Lookup (nameText, typeInt, attr Attributes)
Success, NotFound
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Name resolution

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

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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 ...
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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.

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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
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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.

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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

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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...

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Example of DNS name space hierarchy

• The host name soc.staffs.ac.uk is 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 ....

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The DNS example...
root

org com edu gov ....
uk
................
co
ac
staffs
shef
bs47c ....
soc
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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.

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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)

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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
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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
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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) ...