NAME SERVICES
ARCHANA ANNAMANENI
Name Service



The name service is used by client processes to obtain attributes such as the
addresses of resources or objects when given their names
Name services are often used to hold the addresses and other details of users,
computers, network domains, services and remote objects
Name services like Directory service, Discovery service, look up services when
given some of their attributes
Need For Names



Names facilitate communication and resource sharing.
Name is needed to request a computer system to act upon a specific resource
chosen out of many.
Processes cannot share particular resources managed by a computer system unless
they can name them consistently
Descriptive attributes are also another means of identification
Names, addresses and other attributes




A name is said to be resolved when it is translated into data about the named
resource or object, in order to invoke an action upon it.
The association between a name and an object is called a binding
Names are bound to attributes of the named objects, rather than the
implementation of the objects themselves
An attribute is the value of a property associated with an object.
General Name Service Requirements
Name services were originally quite simple. But the interconnection of networks and the
increased scale of distributed systems have produced a much larger name-mapping
problem.
Examples:
Grapevine was one of the earliest extensible, multi domain name services.
Global Name Service is a descendent of Grapevine with more goals

To handle an essentially arbitrary number of names and to serve an arbitrary
number of administrative organizations.
 A long lifetime
 High Availability
 Fault isolation
 Tolerance of mistrust
DNS: Heavily relies upon replication and caching of naming data.
Domain Name System (DNS)
The DNS is a name service design whose principal naming database is used across the
Internet (DNS is a Distributed Database).
The mechanism of DNS is it maps between high-level machine names and IP addresses.
DNS defines:
 A hierarchical namespace for hosts and IP addresses
 A host table implemented as a distributed database
 A “resolver” –library routines query this database
 Improved routing for email
 A mechanism for finding services on a network
 A protocol for exchanging naming information
Name Space
Name space is the collection of all valid names recognized by a particular service.
Flat Namespace:
Each name is consisted of a sequence of characters without any further structure
Advantage: Names are convenient and short
Disadvantage: Cannot generalize to large set of machines for both technical and
administrative reasons
Hierarchical Namespace



The most important advantage is each part of a name is resolved relative to a
separate context. So the same name may be used with different meanings in
different contexts
Hierarchic name spaces are potentially infinite. So they enable a system to grow
indefinitely
Different contexts can be managed by different people
DNS name space has a hierarchic structure.
Domain Names
A domain name consists of a sequence of sub names separated by a delimiter character,
the period.
The DNS calls each section label.
For example the domain name
csc.villanova.edu
Contains three labels: csc, villanova, and edu
Any suffix of a label in a domain name is also called a domain.
Here csc.villanova.edu (the domain name for computer science department at villanova)
is lowest level domain
The second level domain name is villanova.edu (the domain for Villanova university)
Top-level domain is edu (the domain for educational institutions)
Domain names are written with the local label first and the top domain last.
DNS names are not case sensitive.
The Internet DNS name space is partitioned, organizationally and according to geography
The top-level organizational domains in use today across the Internet are
com - Commercial organizations
edu – Universities and other educational institutions
gov – US governmental agencies
mil - US military organizations
net – Major network support centers
org - Organizations not mentioned above
int - International organizations
Every country has its own domains
us – United States
uk –United kingdom
fr – France
The DNS name space is a tree of “domains”. Each domain represents a distinct chunk of
the namespace and is loosely managed by a single administrative entity. The root of the
tree is called “.” or dot, and beneath it are the top level domains
DNS servers do not recognize relative names: all names are referred to the global root.
In practical implementations, client software keeps a list of domain names that are
appended automatically to any single-component name before resolution.
For example the name webmail, presented in the domain villanova.edu, probably refers
to webmail.villanova.edu
Client software will append the default domain villanova.edu and attempts to resolve the
name.
If this fails then further default domain names may be appended
Finally the (absolute) name webmail.villanova.edu is presented to the root for resolution
Names with more than one component are normally presented to DNS as absolute names.
Aliases




One domain name is defined to stand for another.
Provides transparency
Generally used to specify the names of machines that run a web server or an FTP server
e.g. www.dcs.qmw.ac.uk is an alias for copper.dcs.qmw.ac.uk

Advantage is if the web server is moved to another computer, all that need to be
done is to update the alias in the DNS database
Name resolution

Resolution is an iterative process where a name is repeatedly presented to naming
contexts.
Name servers and navigation




The process of locating naming data from among more than one name server
in order to resolve a name is called navigation
The client name resolution software carries out navigation on behalf of the
client. It communicates with name servers as necessary to resolve a name.
The software is provided as library code and linked into clients, e.g BIND
implementation for DNS
Another alternative, used with X500, is to provide name resolution in a
separate process that is shared by all of the client processes on that computer.
Types of navigation
Iterative navigation




DNS supports the iterative navigation model.
Here to resolve a name, a client presents the name to the local name server, which
attempts to resolve it.
If the local name server has the name, it returns the result immediately. If it does not, it
will suggest another server that will be able to help.
Resolution proceeds at the new server, with further navigation as necessary until the
name is located or is discovered to be unbound.
Multicast navigation: Client multicasts the name to be resolved, to the group of name
servers. Only the server that holds the named attribute responds to the request
Non-recursive and recursive server-controlled navigation
Under non-recursive server controlled navigation, the client may choose any name server.
This server communicates by multicast or iteratively with its peers, as though it were a client.
Under recursive server-controlled navigation, the client contacts a single server. If this server
does not know the name, the server contacts its peer , which in turn attempts to resolve it. This
procedure continues recursively until the name is resolved.
Caching


Client name resolution software and servers maintain a cache of the results of previous
name resolutions
When a client requests a name lookup, the name resolution software consults its cache. If
it holds a recent result from a previous lookup for the name, it returns it to the client;
otherwise it sets about finding it from a server. That server, in turn, may return data
cached from other servers.
Advantages
 Caching is a key to a name service’s performance and assists in maintaining the
availability of both the name service and other services despite name server crashes.
 It enhances the response times by saving communication with name servers
 Caching can be used to eliminate high-level name servers- the root server
 Caching is successful because naming data are changed relatively rarely.
DNS queries
Host name resolution: Applications use the DNS to resolve host names into IP addresses.
Mail host location: Electronic mail software uses the DNS to resolve domain names into the IP
addresses of mail hosts –computers that will accept mail for those domains.
Reverse resolution: A domain name is returned, given an IP address.
Host information: DNS stores the machine architecture type and operating system against the
domain names of hosts.
Well-known services: A list of the services run by a computer (telnet, FTP) and the protocol used
to obtain them can be returned.
The DNS DATABASE





The DNS database is distributed across a logical network of servers. Each server holds
part of the naming database –primarily data for the local domain.
Most queries concern computers in the local domain and are satisfied by servers within
that domain.
Each server records the domain names and addresses of other name servers, so that
queries pertaining to objects outside the domain can be satisfied
A server may hold authoritative data for zero or more zones.
System administrators enter the data for a zone into a master file




Two types of servers are considered to provide authoritative data for the zone.
A primary or master server reads zone data directly from a local master file.
Secondary servers download zone data from a primary server. These servers
communicate periodically with the primary server to check whether their stored version
matches with the primary server.
The frequency of the secondary’s check is set by administrator, as a zone parameter, and
its value is typically once or twice a day.
The above figure shows the arrangement of some of the DNS database.
A domain’s DNS database is a set of text files maintained by the system administration on the
domain’s master name server. These text files are often called zone files.
Zone files contain “resource records” (RR)
Resource records
Each zone of the DNS hierarchy has a set of resource records associated with it.
The basic format of a resource record is
[name] [ttl] [class] type data
The name field identifies the entity (usually a host or domain) that the record describes
The ttl (time to live) field specifies the length of time in seconds that the data item can be cached
and still be considered valid
The class specifies the network type. The default value for the class is IN.
Resource records are divided into four groups:




Zone records – identify domains and their name servers
Basic records – map names to addresses and route mail
Security records – add authentication and signature to zone files
Optional records – provide extra information about hosts or domains
The content of the data field depend on the record type.
The data for a zone starts with a SOA-type record. It contains the zone parameters that specify,
for example the version number and how often secondary servers should refresh their copies.
Each zone has exactly one SOA record. The zone continues until another SOA is encountered.
SOA type records are followed by records of type NS specifying the name servers for the domain
and a list of records of type MX giving preferences and domain names of mail hosts.
A (address) records provide the mapping from hostnames to IP addresses.
An example for SOA record
cs.colorado.edu IN SOA ns.colorado.edu admin.cs.colorado.edu(
1999121501 ;serial
21600
;Refresh 6hours
1800
;Retry 30 minutes
1209600
;Expire, 2weeks
432000 )
;Minimum, 5days
An example for NS record
The format is
Zone [ttl] IN NS hostname
cs.colorado.edu IN NS ns.cs.colorado.edu
cs.colorado.edu IN NS anchor.cs.colorado.edu
cs.colorado.edu IN NS ns.cs.utah.edu
A record (address record)
The format is
hostname [ttl] IN A ipaddr
anchor
IN A 128.138.243.100
The BIND software
BIND, the Berkeley Internet Name Domain system, is an open source software package from ISC
that implements the DNS protocol and provides name service on UNIX systems(now on
Windows NT)
The BIND system has three components:
 A daemon called named that answers queries
 Library routines that resolve host queries by contacting the servers of the DNS distributed
database
 Command-line interfaces to DNS: nslookup, dig, and host
named: the BIND name server
In DNS parlance, a daemon like named (or the machine on which it runs) is called a “name
server”, and the client code that contacts it is called a “resolver”.
named answers queries about hostnames and IP addresses. If named doesn’t know the answer
to a query, it asks other servers and caches their responses.
DIRECTORY AND DISCOVERY SERVICES



A service that stores collection of bindings between names and attributes and that
looks up entries that match attribute-based specifications is called a directory
service.
Examples Microsoft’s Active Directory Services, X.500 and LDAP
Directory services are sometimes called yellow pages services and also called as
attribute-based name services
Example:
For example, a user might ask ‘which computers in this building are Macintoshes
running the Mac 8.6 OS?’
‘Where can I print a high-resolution color image?’
Discovery services





A discovery service is a directory service that registers the services provided in a
spontaneous networking environment
In spontaneous networks, devices are liable to connect without warning and
without administrative preparation.
The requirement is for the set of clients and services to change dynamically but to
be integrated without user intervention.
To meet these needs, a discovery service provides an interface for automatically
registering and de-registering services, as well as an interface for clients to look
up the services that they require from those that are currently available.
Recent developments in discovery services include the Jini discovery service
Jini








Jini is a system that is designed to be used for spontaneous networking.
It is entirely java-based – it assumes that JVMs run in all of the computers,
allowing them to communicate with one another by means of RMI and to
download code as necessary.
Jini provides facilities for service discovery, for transactions, for shared data
spaces called JavaSpace
The lookup service allows Jini services to register the services they offer, and Jini clients
to request services that match their requirements.
A Jini service, such as printing service, may be registered with one or more lookup
services.
A Jini service provides, and the lookup services store, an object that provides the service,
as well as the attributes of the service.
Jini clients query lookup services to find Jini services that match their requirements; if a
match is found, they download an object that provides the service from the lookup
service.
The matching of service offers to clients’ requests can be based on attributes or on Java
typing, for example allowing a client to request a color printer for which it has the
corresponding Java interface.
Bootstrapping
Entities that wish to start participating in a distributed a system of JiniTM technology-enabled
services and/or devices, known as a djinn, must first obtain references to one or more Jini lookup
services. The protocols that govern the acquisition of these references are known as the discovery
protocols. Once these references have been obtained, a number of steps must be taken for entities
to start communicating usefully with services in a djinn; these steps are described by the join
protocol.
The Discovery Protocols
There are three closely related discovery protocols: one is used to discover one or more lookup
services on a local area network (LAN), another is used to announce the presence of a lookup
service on a local network, and the last is used to establish communications with a specific
lookup service over a wide-area network (WAN).
Protocol Roles
The multicast discovery protocols work together over time. When an entity is initially started, it
uses the multicast request protocol to actively seek out nearby lookup services. After a limited
period of time performing active discovery in this way, it ceases using the multicast request
protocol and switches over to listening for multicast lookup announcements via the multicast
announcement protocol.
Protocol Participants
Several components take part in the multicast request protocol. Of these, two run on an entity
that is performing multicast requests, and two run on the entity that listens for such requests and
responds.
On the requesting side live the following components:


A multicast request client performs multicasts to discover nearby lookup services.
A multicast response server listens for responses from those lookup services.
These components are paired; they do not occur separately. Any number of pairs of such components
may coexist in a single JVM at any given time. The lookup service houses the other two participants:



A multicast request server listens for incoming multicast requests.
A multicast response client responds to callers, passing each a proxy that allows it to
communicate with its lookup service.
Figure DJ.2.1: Multicast Request Protocol Participants


Jini makes use of leases, when Jini services register with the lookup service they are provided with a
lease that guarantees their registration entry for a limited period of time.
If a service does not communicate with the lookup service to renew its lease before the lease has
expired, then it is assumed to have failed, and the lookup service can delete the entry.
An Example for Service discovery in Jini



There may be several instances of the lookup service reachable by multicast communication from a
given Jini client or service.
Every such service instance is configured with one or more group names such as ‘admin’,’finance’
and ‘sales’
When clients and services request a lookup service, they may specify any groups that they are
particularly interested in, and only lookup services bound to those same group names will respond.
References: Distributed Systems Concepts and Design by Coulouris, Dollimore, and Kindberg
UNIX System Administration Hand Book by Nemeth, Snyder, Seebass,and Hein
http://www.sun.com/jini/specs/jini1.1html/discovery-spec.html#39235