Domain Name System DNS History and Background

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Domain Name System (DNS) History and Background

• Server-based HOST files were the original
  solution to name resolution on the Internet
• Became infeasible for Internet-wide name
  resolution as number of domains and hosts grew
• RFCs 882 and 883
• Original RFCs for DNS

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DNS History and Background (continued)

• DNS combines the following virtues
• Allows local control over domain name database
  segments
• Data from all database segments is available
  everywhere
• Database information is robust and highly
  available
• DNS
• One of the most effective uses of distributed
  database technology in the world today

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DNS Database Structure

• Mirrors structure of the domain namespace itself
• Top-level domains in the U.S.
• .com
• .edu
• .gov
• .mil
• .net
• .org

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The DNS Namespace

• DNS
• Arbitrarily partitions tree and creates subtrees
  for database information
• Domains (such as ibm.com)
• Can be broken into subdomains (such as
  clearlake.ibm.com)
• A host in this subdomain would have at least 4
  parts to the domain name www.clearlake.ibm.com
• However, just because you see a 4-part name,
  e.g. www.is.uwp.edu doesnt mean that a
  subdomain is in use
• Any valid domain name
• Ultimately resides within some specific DNS
  database

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Delegating Zones

• The DNS database is partitioned into zones.
• A zone is a portion of DNS database.
• A zone is authoritative for resource records
  within it
• Authoritative means that updates to the dns
  database are originally made in the authoritative
  zone.
• Ex UW Parksides DNS Server (131.210.1.15) is
  authoritative for all host records in the uwp.edu
  domain.
• A single DNS server can host multiple zones.
• Names within a zone can be delegated to other
  zones.

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Reasons for Delegation

• To subdivide management of a DNS domain
• To improve the name resolution performance and
  provide fault-tolerance
• To allow for hosts organizational affiliations

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Resource Records

• Name servers resource records facilitate
  delegation.
• Appear in forward and reverse lookup zones.
• Refer to the name servers resource records in
  the target zone.
• Zone is a portion of a domain.
• If there are no subdomains, then zone and domain
  are the same.

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The microsoft.com Domain Delegated Across Two
Zones
Mydomain is a subdomain of microsoft.com
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Static and Dynamic Changes

• Static
• Changes edited by an administrator
• Good when the number of changes is small or when
  updates occur infrequently
• Dynamic
• Registers and dynamically updates resource
  records
• Better support for frequent changes
• Generally
• Server entries are static
• Workstation entries should usually be dynamic
  if the DNS server supports dynamic updates
• Workstation IP addresses are dynamic (from DHCP)
  and Server IP addresses usually are static
• Servers are usually up and connected to the
  network, workstations are often not

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DNS Database Records

• Resource records
• Stores data associated with domain names, address
  records, and other specific data
• Most commonly used types
• Address (A) record
• Canonical name (CNAME) record
• Host information (HINFO) record
• Mail exchange (MX) record
• Name server (NS) record

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Resolvers

• Resolvers recognize when names need to be
  resolved into IP addresses (forward lookups)
• Anytime a computer attempts to use a host name
  that is not already resolved
• Can also issue requests to resolve an IP
  addresses into names (reverse lookup)
• Resolvers issue resolution requests to a DNS
  server
• Usually implemented as a library routine
• Uses User Datagram Protocol (UDP) first for
  performance
• Uses Transmission Control Protocol (TCP) if
  return data is truncated

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Zones

• Its possible to subdivide a single domain into
  subdomains.
• Not required, but may make sense for domains with
  a large number of hosts
• Subdomains MAY be put into separate zones (this
  isnt required)
• A zone is a subtree of the DNS database.
• A zone is an administrative unit.
• A zone can contain a domain and/or one or more
  subdomains
• If a DNS Server is authoritative for a zone, it
  contains the mappings of hosts to IP address for
  all hosts within the zone.
• A zone is (ultimately) where a host name is
  mapped into its IP address
• Allows for distributing management and load
  balancing

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Domains Across Multiple Zones
Whats missing from this picture is that with
Windows 2003, Windows Administrative (user
accounts/groups) domains can be subdivided into
subdomains. The DNS domain/subdomain hierarchy
mirrors the Administrative domain structure.
(lets all thank Microsoft for making terminology
confusing by giving the term domain a 2nd and
unrelated meaning!)
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Name Server Roles

• Primary where updates to zone data originally
  is made
• The primary name server is the definitive source
  of DNS information for the domain. All other DNS
  servers for a domain ultimately pull their
  information from the primary name server
• Secondary receive zone data from primary name
  servers, via zone transfers.
• provide redundancy, load balancing, and
  distributed access
• Master - The secondary name server may be
  configured to zone transfers from either the
  primary name server or another secondary name
  server. In either configuration, the server that
  provides the zone is the master
• Caching-only- only stores resolved queries, not
  authoritative for any zone

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Recursive Queries
iterative query indicates that the server will
accept a referral to another server in place of a
definitive answer to the query.
A recursive query indicates that the client wants
a definitive answer to its query. The response to
the recursive query must be a valid address or a
message indicating that the address cannot be
found
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Iterative Query Steps

• Recursive query sent to local server.
• Local server checks zone, sends iterative query
  to root server.
• If the zone in the query is one for which the
  local server is authoritative, no need to do an
  Iterative Query . Ex A query for www.uwp.edu
  made from an on-campus host will be sent to UWPs
  local server, it contains the mapping for this
  host.
• Root server returns top-level domain servers IP
  address.
• Local server sends iterative query to top-level
  server.
• Top-level server returns domains name server IP
  address.
• Local server sends iterative query to domain name
  server.
• Domain name server returns IP address of DNS
  query.
• Local server returns address to resolver.

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Root name server list

• A list of current root domain name servers (A
  M) is found at
• ftp//ftp.rs.internic.net/domain/named.root
• DNS Servers issuing iterative requests need to
  have an up-to-date list of root servers as a
  starting point of iterative queries, examples
  entries
• .
  3600000 IN NS A.ROOT-SERVERS.NET.
• A.ROOT-SERVERS.NET. 3600000 IN A
  198.41.0.4
• Entries for B through L root servers
• .
  3600000 IN NS M.ROOT-SERVERS.NET.
• M.ROOT-SERVERS.NET. 3600000 IN A
  202.12.27.33

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Inverse Queries

• A resolver sends a request to a name server.
• The in-addr.arpa domain.
• Pointer (PTR) resource records.

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Caching and Time to Live

• Name server caches all information it receives.
• Saves information for time called Time to Live
  (TTL).

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DNS Configuration Files

• DNS is a hierarchical, distributed database.
• First record is start of authority (SOA).
• Name server (NS) record.
• Host record.
• Canonical name (CNAME) record.

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Start of Authority (SOA)

• Each zone has a start of authority record that
  defines the general parameters of the zone
• Refresh Interval
• How often secondary DNS servers should check if
  changes are made to the zone.
• Retry Interval
• How often secondary DNS server should retry
  checking if changes are made - if the first
  refresh fails.
• Expire Interval
• How long the zone will be valid after a refresh.
• Secondary servers will discard the zone if no
  refresh could be made within this interval.
• Minimum (default) TTL
• TTL property, specifying the amount of time other
  DNS servers and applications are allowed to cache
  the record

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Start of Authority (SOA)
Name of primary DNS server for the zone

• Example of SOA record
• _at_ IN SOA nameserver.example.microsoft.com.
• postmaster.example.microsoft.com. (
• 1 serial number
• 3600 refresh 1h
• 600 retry 10m
• 86400 expire 1d
• 3600 ) min TTL 1h

Email address of the administator
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Name Server Record
The Name Server record identifies an
authoritative name server for a domain Example
of NS record _at_ IN NS nameserver2.microsoft.com
The _at_ means the current domain, in other words
nameserver2.microsoft.com is authoritative for
the domain this name server
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Host Record

• Example of Host Record
• rhino IN A 157.55.200.143
• localhost IN A 127.0.0.1
• remember?
• .
  3600000 IN NS M.ROOT-SERVERS.NET.
• M.ROOT-SERVERS.NET. 3600000 IN A
  202.12.27.33

The name server that is authoritative for the
domain
The domain
A host record
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CNAME Record

• Example of CNAME (canonical name) record
• They refer to a name that needs to be defined in
  a host record
• Used to define aliases to the name in a host
  record
• FileServer1 CNAME rhino
• www CNAME rhino
• ftp CNAME rhino

When there is no period at the name of the host,
domain name of the zone will be appended as the
full name of the host
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Reverse Lookup File

• Allows resolver to request a name.
• Contains SOA, NS, and PTR records.
• Used for some security applications.
• Reverse lookup zones are optional.
• Reverse lookup zones correspond to subnets
• Zone contains mapping of IP addresses to host
  names for hosts on the subnet

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PTR Record
Example of PTR record 51.200.55.157.in-addr.arpa.
IN PTR mailserver1.microsoft.com
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Cache File

• Used by Name servers to identify root name
  servers
• Example of Cache file entry
• . 3600000 IN NS A.ROOT-SERVERS.NET.
• A.ROOT-SERVERS.NET. 3600000 A 198.41.0.4
• Cache file location
• SystemRoot\System32\Dns

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Second-Level Domain

• The Internet Network Information Center
  (InterNIC) must be informed of the domain name.
• The InterNIC must be given the IP addresses of at
  least two DNS servers.
• Additional DNS servers can be used independent of
  the Internet.