Domain Name System ( DNS )  DNS is the system that provides name to address mapping for the internet.

Name Space  The names assigned to machines must be unique because the addresses are unique. A name space that maps each address to a unique name can be organized in two ways:  flat or  hierarchical

Domain Name Space  The domain name space is hierarchical in design. The names are defined in an inverted- tree structure with the root at the top. The tree can have 128 levels: level 0 (root) to level 127.

Domain name space

Domain names and labels

Fully qualified domain name  If a label is terminated by a null string is called fully qualified domain name  An FQDN is a domain name that contains the full name of a host.  It contains all labels, from the most specific to the most general, that uniquely define the name of the host.

Domain  A domain is a subtree of the domain name space  The name of the domain is the domain name of the node at the top of the structure

Domains

Distribution of Name Space  The information contained in the domain name space is distributed among many computers called DNS servers.

Hierarchy of name servers

Zone  Domain name servers store information about part of the domain name space called a zone

Figure 17.6 Zones and domains

Root Server  A root server is a server whose zone consists of the whole tree.  A root server usually does not store any information about domains but delegates its authority to other servers.

DNS server  Defines two types  Primary server  Secondary server  A primary server is a server that stores a file about the zone for which it is an authority.  It is responsible for creating, maintaining and updating the zone file.  It stores the zone file on a local disk

Secondary server  A secondary server is a server that transfers the complete information about a zone from another server and stores the file on its local disk.

 A primary server loads all information from the disk file; the secondary server loads all information from the primary server. When the secondary downloads information from the primary, it is called zone transfer

DNS in the internet  The domain name space (tree) is divided into three different sections: generic domains, country domains, and the inverse domain.

DNS used in the Internet

Generic domains

Generic domain labels

Generic domain labels (Continued)

Country domains

Resolution  Mapping a name to an address or an address to a name is called name-address resolution  Resolver A host that needs to map an address to a name or a name to an address calls a DNS client called resolver

Types  Recursive Resolution  Iterative Resolution  Caching

DNS Messages types

 The DNS query message consists of a header and question records; the DNS response message consists of a header, question records, answer records, authoritative records, and additional records. header, question records, answer records, authoritative records, and additional records.

Query and response messages

Header format

 Identification: 16 bit field used match the response with the query. Use different identification number for each query  Flags

Flag: 16 bit  QR(query/response) : 0 query 1 response  Opcode: 4 bit: type of query or response  AA(authoritative answer) 1 means the name server is authoritative server  TC(truncated) truncated to 512(if more)  RD(recursion desired)  RA(recursion available)  rCode: shows the status error.

Types of record  Question Record Consists of 3 fields 1) Query name 2) Query Type 3) Query class

Query name format

Types

Classes

Resource record format

 The Dynamic Domain Name System (DDNS) updates the DNS master file dynamically.

Encapsulation  DNS uses UDP as the transport protocol when the size of the response message is less than 512 bytes. If the size of the response message is more than 512 bytes, a TCP connection is used.  DNS can use the services of UDP or TCP using the well-known port 53.