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6. Name Services and DNS

24/02/23

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Name Services and Directory Services​

Names​

  • Name uniquely distinguishes one thing from other like it
  • Identifiers are names intended for machine use
  • Descriptions are statements about some aspect of a thing

Names in distributed systems​

  • Pure name is just an array of bits
  • Address specifically locates a resource
  • Different services may use different name

URIs, URNs, URLs​

  • Uniform Resource Identifiers (URI) - service-independent names
  • Uniform resource Names (URNs) - pure names
  • Uniform Resource Locators (URLs) - include information to locate and access a resource

Name Services​

  • Name Service - Names are bound to attributes of the object/resource
  • Name Space - Is the set of possible names in a name service. May be flat or hierarchical. Given name may be bound or unbound. Can be subdivided into name contexts
  • Can be independent of the distributed system

Example: JavaRMI Registry​

  • The rmiregistry application provides the name service

Directory Services allow clients to look up names or address using more flexible queries. SQL-like queries over attribute values

Summary​

  • A name distinguishes one particular thing from others like it
    • Identifiers are names typically intended for machine use
  • A name service allows a (pure) name to be resolved to an address or other attributes bound to that name
  • WWW URI includes URLs and URNs
  • A directory service allows resources to be looked up flexibly than a name service

DNS - Domain Name System​

  • Name space is hierarchical, comprising
    • Name components or labels
  • Absolute but not relative. Default domains are often configured to try as a possible suffix for unqualified names
  • Attributes are encoded as Resource Records of various types for various purposes
  • Host name resolution, i.e. mapping a domain name to an IP address
    • DNS A and AAAA records - IP addresses
    • CNAME records specify an aliases. Automatically checked when resolving a domain name to an IP address

Lesser used recrods​

  • Mail host location - finding mail server to send internet mail to (MX records)
  • Reverse resolution - IP address to domain name. Uses PTR records
  • Host Information - OS. Uses HINFO records
  • Service Lookup - Bonjour/DNS-SD. Uses dns SRV records Lecture 15
  • Arbitrary Information - TXT records

Multiple Records/values​

  • Single domain name can have multiple records of the same type
  • Multiple NS/MX/A records.

Administrative Domains​

  • Administrative domain - Subset of namespace with a single administrative authority
  • Can correspond to a domain name suffix
  • Authorities look after that root domain e.g .uk, .ac.uk, .com. Should be non-profit organisations.

Name resolution​

  • DNS name resolution is performed by a resolver
  • First name server may not know the answer, but knows the address of a different name server to ask
  • Steps can be handled
    • Iteratively - Resolver asks each name server in turn, or
    • Recursively - the first server ask is the second and so on

Cacheing​

  • Every response has a time to live (ttl)
  • This determines how long things are cached for
  • This is critical to the performance of DNS
  • Cached responses are non-authoritative

Other Notes on DNS​

  • Dynamic DNS (DDNS) - Records may be updated automatically
  • Rather insecure, secure extensions for DNS (DNSSEC) are not universally available/used
  • DNS over HTTPS (DOH) is a proposed standard for browsers to do DNS over HTTPS to trusted DoH servers
  • DNS can be done by multicast without a server
  • DNS is also used to exfiltrate data or as a hidden control channel

Summary​

  • DNS is a global name service used in the internet
    • Supports host name resolution (A records), service aliases (CNAME)
  • DNS is divided into administrative domains or zones
    • Each is independently controlled and partially deligated
  • DNS clients use a resolver to resolve domain names.
  • DNS includes caching in resolvers and servers for scalability and performance