DNS

Question 1

Name Service (DNS)

Answer 1

• We have been using addresses (IP) to identify
  hosts
• While perfectly suited for processing by routers,
  addresses are not exactly user friendly
• So, unique name is also typically assigned to
  each host in a network
• Host names differ from host addresses:– they are usually of variable length and mnemonic,
  thereby making them easier for humans to remember
• In contrast, fixed-length numeric addresses are
  easier for routers to process

Question 2

Some useful information about DNS

Answer 2

– a name space defines the set of possible names
* A name space can be either flat (names are not
divisible into components) or hierarchical
– the naming system maintains a collection of bindings of
names to values
* The value can be anything we want the naming system
to return when presented with a name; in many cases it
is an address
– a resolution mechanism is a procedure that, when invoked
with a name, returns the corresponding value
* A name server is a specific implementation of a
resolution mechanism that is available on a network
and that can be queried by sending it a message

Question 3

DNS Host Entries
* Different kinds of mappings are possible:–

Answer 3

• Simple case: 1-1 mapping between domain name and IP
  address:
  *kittyhawk.cmcl.cs.cmu.edu maps to
  128.2.194.242
  – Multiple domain names map to the same IP address:
  *eecs.mit.edu and cs.mit.edu both map to
  18.62.1.6
  – Single domain name maps to multiple IP addresses:
  *aol.com and www.aol.com map to multiple IP
  addrs.
  – Some valid domain names don’t map to any IP address:
• for example: cmcl.cs.cmu.edu

Question 4

Domain Hierarchy

Answer 4

• DNS implements a hierarchical name space for Internet
  objects
• DNS names are processed from right to left and use
  periods as the separator
  – Still, humans “read” domain names from left to right
• The DNS hierarchy can be visualized as a tree, where
  – each intermediate node in the tree corresponds to a domain and
  – the leave nodes in the tree correspond to the hosts being named
• How this hierarchy is actually implemented?
  – The first step is to partition the hierarchy into subtrees called
  zones
  – Each zone can be thought of as corresponding to some
  administrative authority that is responsible for that portion of the
  hierarchy

Question 5

Hierarchical Name Space

Answer 5

• In a hierarchical name space, each name is
  made of several parts:
  – The first part can define the nature of the organization
  – The second part can define the name of an
  organization
  – The third part can define departments in the
  organization, and so on

Question 6

Domain Name Space

Answer 6

• To have a hierarchical name space, a domain name
  space was designed
  – The names are defined in an inverted-tree structure with the root
  at the top
• Label:
  – Each node in the tree has a label except the root node
  – The root label is a null string
  – DNS requires that children of a node have different labels
  which guarantee the uniqueness of the domain names
• Domain Name
  – Each node in the tree has a domain name
  – A full domain name is a sequence of labels separated by dots (.)
  – The domain names are always read from the node up to the root
  – A full domain name always ends with a null label.

Question 7

Name Resolution

Answer 7

• How a client (host) engages DNS servers to resolve a domain
  name?
• Suppose the client wants to resolve the name
  cicada.cs.princeton.edu
• The client first sends a query containing this name to its local
  server (or the root server)
• The root server, unable to match the entire name, returns the
  best match it has—the NS record for princeton.edu
• The server also returns all records that are related to this
  record, in this case, the A record for cit.princeton.edu
• Resolving a name actually involves a client querying the local
  server, which in turn acts as a client that queries the remote
  servers on the original client’s behalf.