Distributed Systems II Lecture 6: Name Services & DNS

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Questions and Answers

What do top-level domain (TLD) servers handle in the Domain Name System (DNS)?

Responsible for university domains

Responsible for country domains only

Responsible for top-level country domains and certain generic domains (correct)

Responsible for root domain only

Recursive navigation in the DNS may tie up server threads.

True

What is the role of caching in the Domain Name System (DNS)?

Caching improves response time by storing mappings and returning them in response to queries. Cached entries may be out-of-date as they timeout after some time.

___ records store IPv6 addresses in the DNS.

AAAA Signup and view all the answers

Match the following DNS terms with their definitions:

TLD servers = Responsible for top-level country domains and certain generic domains Authoritative DNS servers = Provide authoritative hostname to IP mappings for organization's named hosts Caching = Improves response time by storing mappings Signup and view all the answers

What is the main purpose of the Domain Name System (DNS)?

Name service design used across the Internet Signup and view all the answers

In DNS name resolution, a recursive query puts the burden of name resolution on the contacted server.

True Signup and view all the answers

What is the purpose of the root DNS server?

To provide the starting point for DNS queries Signup and view all the answers

The DNS database is distributed across a logical network of __________.

servers Signup and view all the answers

Match the following top-level domains with their corresponding descriptions:

com = Commercial organizations edu = Universities and other educational institutions gov = Governmental agencies mil = Military organizations net = Major network support centres org = Organizations not mentioned above int = International organizations Signup and view all the answers

What is the purpose of names in a distributed system?

Facilitate communication and resource sharing Signup and view all the answers

What is the distinction made by Needham regarding names?

Pure names are uninterpreted bit patterns, while non-pure names contain information about the object they name Signup and view all the answers

Addresses are always reliable means of identification in a distributed system.

False Signup and view all the answers

Name spaces require a syntactic definition to separate valid names from ________ names.

invalid Signup and view all the answers

What is an alias in the context of naming services?

A name that denotes the same information as another name Signup and view all the answers

Study Notes

Name Services

In a distributed system, names are used to refer to a wide variety of resources such as computers, services, remote objects, and files, as well as users.
Naming is fundamental in distributed system design, facilitating communication and resource sharing.
A name is needed to request a computer system to act upon a specific resource chosen out of many.

Names, Addresses, and Other Attributes

Any process that requires access to a specific resource must possess a name or an identifier for it.
Examples of human-readable names are file names, URLs, and Internet domain names.
Identifiers are chosen for efficiency in lookup and storage by software.
Needham makes a distinction between pure names and non-pure names, where pure names are uninterpreted bit patterns, and non-pure names contain information about the object they name.

Addresses

An object's address is a value that identifies the location of the object rather than the object itself.
Addresses are efficient for accessing objects, but objects can be relocated, making addresses inadequate for identification.
An 'address' may be considered just another name that must be looked up, or it may contain such a name.

Name Services and the Domain Name System

A name service stores information about a collection of textual names, in the form of bindings between the names and the attributes of the entities they denote.
The collection is often subdivided into one or more naming contexts, which are individual subsets of the bindings that are managed as a unit.
The major operation that a name service supports is to resolve a name, which is to look up attributes from a given name.

Name Spaces

A name space is the collection of all valid names recognized by a particular service.
Names may have an internal structure that represents their position in a hierarchic name space.
One important advantage of a hierarchy is that it makes large name spaces more manageable.

Aliases

An alias is a name defined to denote the same information as another name.
Aliases allow more convenient names to be substituted for relatively complicated ones, and allow alternative names to be used by different people for the same entity.

Naming Domains

A naming domain is a name space for which there exists a single overall administrative authority responsible for assigning names within it.
Domains in DNS are collections of domain names; syntactically, a domain's name is the common suffix of the domain names within it.

Name Resolution

For the common case of hierarchic name spaces, name resolution is an iterative or recursive process.
A naming context either maps a given name onto a set of primitive attributes directly, or maps it onto a further naming context and a derived name to be presented to that context.

Domain Name System

The DNS is a name service design whose main naming database is used across the Internet.
The DNS is designed for use in multiple implementations, each of which may have its own name space.
The Internet DNS name space is partitioned both organizationally and according to geography.
The DNS naming data are divided into zones, which contain attribute data for names in a domain, less any subdomains administered by lower-level authorities.

DNS Name Servers

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.
Queries concerning computers in the local domain are satisfied by servers within that domain.
The DNS naming data are divided into zones, which contain the names and addresses of at least two name servers that provide authoritative data for the zone.### Domain Name System (DNS)
A distributed, hierarchical database that translates domain names into IP addresses
DNS is a naming system that allows humans to use easy-to-remember domain names instead of difficult-to-remember IP addresses

DNS Architecture

Root DNS servers
Top-Level Domain (TLD) servers
Authoritative DNS servers
DNS clients (resolvers)

Top-Level Domain (TLD) Servers

Responsible for .com, .org, .net, .edu, and all top-level country domains
Examples: Network Solutions (authoritative registry for .com and .net), Educause (authoritative registry for .edu)

Authoritative DNS Servers

Organization's own DNS server(s) that provide authoritative hostname to IP address mappings for the organization's named hosts
Can be maintained by the organization or service provider

Caching DNS Information

Once a name server learns a mapping, it caches the mapping and returns a cached mapping in response to a query
Caching improves response time
Cache entries timeout (disappear) after some time (TTL)
TLD servers are typically cached in local name servers

A DNS client (resolver) is implemented as library software
The resolver accepts queries, formats them into messages, and communicates with one or more name servers
A simple request-reply protocol is used, typically using UDP packets on the Internet
The resolver can be configured to contact a list of initial name servers in order of preference

Resource Records

Zone data are stored by name servers in files in one of several fixed types of resource records
Examples: A records (store IPv4 addresses), AAAA records (store IPv6 addresses), TXT records (store arbitrary other information)

DNS Performance

The DNS Internet implementation achieves relatively short average response times for lookups
This is achieved by a combination of partitioning, replicating, and caching naming data

Discussion of DNS

The DNS stores a limited variety of naming data, but this is sufficient for applications such as electronic mail
The DNS database represents the lowest common denominator of what would be considered useful by the many user communities on the Internet
The DNS coexists with local name and directory services that store data most pertinent to local needs

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Description

This quiz covers the concepts of name services and the Domain Name System (DNS) in the context of distributed systems. It reviews the background of Sun Network File System and explores the role of directory services.