DNS

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DNS

• Redes de Datos 2002
• etapia_at_eie.fceia.unr.edu.ar

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Accessing Resources Applications

• The Domain Name System
• It is described in RFC 1034 and RFC 1035
• Symbolic host names
• The early Internet use only numeric IP addresses
• Problem of maintaining the mappings between IP
• Initially, host names to address mappings were
  maintained by the NIC in a single file
  (HOSTS.TXT), which was fetched by all hosts using
  FTP. This is called a flat namespace

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The problem

• Mapping hostnames
• foobar.cs.colorado.edu to an IP address like
  128.138.241.71

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The Solution

• The Domain Name System (DNS)
• Unreliable database with distributed data and
  management
• A resolver maps names to IP addresses using name
  servers

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DNS Hierarchical Name Space
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Naming

• A host has a domain name specified using a
  sequence of names, each of which may be up to 63
  characters long, separated by periods.
• Names are case insensitive
• A domain is an absolute domain name or a fully
  qualifieddomain name (FQDN) if it ends with a
  period.
• E.g., foobar.cs.colorado.edu.
• Most generic domains (.com, .edu, etc) are
  international, but .gov and .mil are US-specific.

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Hierarchical Administration
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Top Level Domains
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Administration Zones

• A zone is a subtree of the DNS tree that is
  independently managed
• Second-level domains (colorado.edu) are usually
  an independent zone
• Most sub-domains (cs.colorado.edu) are
  independent.
• E.g., most universities have departmental domains
  that are then independently administered

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Administration Zones contd

• A zone must provide multiple name servers This
  server records the members in the domain
• You typically need a primary name server and one
  or more secondary name servers.
• Secondary retrieves information from primary
  using a zone transfer

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Domain Name Resolution

• A user program issues a request such as the
  gethostbyname() system call
• This particular call is used to ask for the IP
  address of a host by passing the host name.
• The resolver formulates a query to the name
  server.
• Full resolvers have a local name cache to consult
  first
• Stub resolvers do not

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Domain Name Resolution

• The name server checks to see if the answer is in
  its local authoritative database or cache, and
  if so, returns it to the client
• Otherwise, it will query other available name
  server(s), starting down from the root of the DNS
  tree or as high up the tree as possible.
• The user program will finally be given a
  corresponding IP address (or host name, depending
  on the query) or an error if the query could not
  be answered

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Domain Name Resolution

• Normally, the program will not be given a list of
  all the name servers that have been consulted to
  process the query.
• The query/reply messages are transported by
  either UDP or TCP

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Domain Name Resolution

• Domain name resolution is a client/server process
• The client function (called the resolver or name
  resolver) is transparent to the user
• Called by an application to resolve symbolic
  high-level names into real IP addresses or vice
  versa.
• The name server (also called a domain name
  server) is a server application

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Full Resolver
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Stub Resolver
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Domain Name Queries

• Queries can be one of two types recursive or
  iterative (also termed non-recursive)
• A flag bit in the domain name query specifies
  whether the client desires a recursive query. A
  flag bit in the response specifies whether the
  server supports recursive queries
• The server receives a request
• A recursive query requests that the server should
  issue a query itself
• An iterative query means that server should
  return what information it has available and a
  list of additional servers

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Domain Name Responses

• Authoritative and Non-authoritative
• A flag bit in the response indicates which type a
  response is.
• A name server receives a query for a domain in a
  zone over which it has authority
• It returns all of the requested information in a
  response with the authoritative answer flag set
• When it receives a query for a domain over which
  it does not have authority
• Actions depend upon the setting of the recursion
  desired flag in the query.

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Domain Name Responses

• If the recursion desired flag is set and the
  server supports recursive queries
• It will direct its query to another name server
• This will either be a name server with authority
  for the domain given in the query, or it will be
  one of the root name servers
• If the second server does not return an
  authoritative answer (for example, if it has
  delegated authority to another server), the
  process is repeated

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Domain Name Responses

• Upon a response, the server will cache it.
  Improve the performance of repeat queries. The
  cache entry is stored for a maximum length of
  time specified by the originator in a 32-bit
  time-to-live (TTL) field contained in the
  response. 172,800 seconds (two days) is a typical
  TTL value
• If the recursion desired flag is not set or the
  server does not support recursive queries
• It will return whatever information it has in its
  cache and also a list of additional name servers
  to be contacted for authoritative information.

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Domain name server operation

• Primary
• A primary name server loads a zone's information
  from disk, and has authority over the zone.
• Secondary
• A secondary name server has authority for a zone,
  but obtains its zone information from a primary
  server using a process called zone transfer.
• To remain synchronized, the secondary name
  servers query the primary on a regular basis
  (typically every three hours) and re-execute the
  zone transfer if the primary has been updated.

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Domain name server operation

• A name server can operate as a primary or a
  secondary name server for multiple domains, or a
  primary for some domains and as a secondary for
  others.
• A primary or secondary name server performs all
  of the functions of a caching only name server.
• Caching-only
• A name server that does not have authority for
  any zone is called a caching-only name server. A
  caching-only name server obtains all of its data
  from primary or secondary name servers as required

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Registering Domains

• When a domain is registered with the root and a
  separate zone of authority established, the
  following rules apply
• The domain must be registered with the root
  administrator
• There must be an identified administrator for the
  domain
• There must be at least two name servers with
  authority for the zone that are accessible from
  outside and inside the domain to ensure no single
  point of failure.

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Domain Name System resource records

• The Domain Name System's distributed database is
  composed of resource records (RRs)
• which are divided into classes for different
  kinds of networks.We only discuss the Internet
  class of records.
• Resource records provide a mapping between domain
  names and network objects.
• The most common network objects are the addresses
  of Internet hosts, but the Domain Name System is
  designed to accommodate a wide range of different
  objects.

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Domain Name System resource records

• A zone consists of a group of resource records,
  beginning with a Start of Authority (SOA) record.
  
• The SOA record identifies the domain name of the
  zone.
• There will be a name server (NS) record for the
  primary name server for this zone
• There may also be NS record(s) for the secondary
  name server(s) for this zone. The NS records are
  used to identify which of the name servers are
  authoritative or in charge of the zone

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Domain Name System resource records

• Then come all the other resource records, which
  might map names to IP addresses, or aliases to
  names

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DNS General resource record format
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DNS General resource record format

• Name The domain name to be defined
• The Domain Name System is very general in its
  rules for the composition of domain names.
• It will consist of a series of labels consisting
  of alphanumeric characters or hyphens, each label
  having a length of between 1 and 63 characters,
  starting with an alphabetic character. Each pair
  of labels is separated by a dot (period) in human
  readable form, but not in the form used within
  DNS messages. Domain names are not
  case-sensitive.

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DNS General resource record format

• ttl
• Time in seconds that this resource record will be
  valid in a name server cache. This is stored in
  the DNS as an unsigned 32-bit value. 86400 (one
  day) is a typical value for records pointing to
  IP addresses.
• Class
• Identifies the protocol family. The only commonly
  used value is IN (the Internet system)
• Type
• Identifies the type of the resource in this
  resource record

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DNS General resource record format

• Rdata
• The value depends on the type, for example
• A A 32-bit IP address (if the class is IN)
• CNAME A domain name
• MX A 16-bit preference value (low values being
  preferred) followed by a domain name
• NS A host name
• PTR A domain name

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DNS General resource record format
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Transport

• Domain Name System messages are transmitted
  either as datagrams (UDP) or via stream
  connection (TCP).
• UDP usage Server port 53 (decimal).
• Messages carried by UDP are restricted to 512
  bytes. Longer messages are truncated and the TC
  bit is set in the header.
• Since UDP frames can be lost, a retransmission
  strategy is required.
• TCP usage Server port 53 (decimal).
• In this case, the message is preceded by a 2-byte
  field indicating the total message frame length

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DNS Message Format
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DNS Message Format

• Identification - set by client, returned by
  server. Used to let clients match questions
  answers
• Questions - a hostname you want resolved - send
  by a query, returned by responses
• Answers - answers to the questions, only in a
  response. You may get multiple answers per
  question.
• Authority - what host is the authoritative name
  server for this query?

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DNS Message Format
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Flags
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Flags
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Flags