DNS

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DNS
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Outline

• Brief history
• What is DNS?
• How does DNS work?

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History

• In 1970s ARPANet consisted of few hundred hosts
• Name address bindings in local hosts.txt
• Central bindings on SRI-NIC host.
• ARPANet moved to TCP/IP
• . . .

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Problems of initial name binding

• Traffic and load
• Name collisions
• Consistency
• Scaling ARPANet broke working solution

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Goals and solutions of new system

• Single bottlneck elimination
• Administration takes place locally
• Data should be available globally
• Uniqueness of host names
• Hierarchical name space to name hosts
• 1984 several RFCs were proposed
• Current specification -- RFC 1035
• RFCs 1034 and 1035 have now been augmented by
  many other RFCs, which describe potential DNS
  security problems, implementation problems,
  administrative gotchas, mechanisms for
  dynamically updating name servers and for
  securing domain data, and more.

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DNS, what it is?

• The Domain Name System is a distributed database.
• Local control of segments
• Data globallyavailable through client-server
  scheme
• Robustness and adequate performance are achieved
  through replication and caching

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DNS, what it is? cont.

• System contains two sides(programs)
• Name server
• Resolver
• Structure of DNS database...
• Every (sub)domain has a unique name

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Database structure

• Domain names are used as indexes into the DNS
  database
• Each host on a network has a domain name, which
  points to information about the host
• This information may include IP addresses,
  information about mail routing, etc.
• Hosts may have one or more domain name aliases,
  which are simply pointers from one domain name
  (the alias) to another (the official or canonical
  domain name)
• Max depth 127

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Figures of structure
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Server system structure

• Top level domains com, edu, gov, mil, net, org,
  ...
• Name space is divided into zones
• Each zone has authoritative name server
• zone contains the domain names that the domain
  with the same domain name contains,
• except for domain names in delegated subdomains

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Server system structure

• Redundancy with master and slave
• Master and slave servers are both authoritative
  for their zone
• Each serverhasits own data files

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Resolvers

• Querying a name server
• Interpreting responses (which may be resource
  records or an error)
• Returning the information to the programs that
  requested it
• Smarter resolvers also use cache

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Resolving process

• Option one Ask root server
• Query is directed step by step to subzones
• Results in clogged root servers
• For this there are many root servers across the
  world

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Example
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Query types

• Recursive query
• Server is obliged to respond with the requested
  data or with an error
• Asks closest known name server
• May askeither recursively or iteratively
• Iterative query
• Server simply gives the best answer it already
  knows
• Usually these are the domain names and addresses
  of the closest known name servers
• Minimal load

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IP to name binding

• Throughout search is too time consuming
• in-addr.arpa domain
• Data is indexed by address numbers
• (sub)Domain can have up to 256 subdomains

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IP to name example
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Cacheing

• Name servers remember authoritative servers
• Newer versions of DNS server software alsouse
  negative cacheing
• caching prevents us from having to query the root
  name servers again.
• Original data holder gives TTL

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Cacheing cont.

• Small TTL ensures consistent view, but increase
  the load on your name servers
• Bigger TTL shortens the average time queries take

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References

• OReilly DNS and BIND
• Google.