Distributed Systems II Lecture 6: Name Services & DNS

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

Navigation and Query Processing

• 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