DNS

Domain Name Service

Hostnames

• IP Addresses are great for computers

                IP address includes information used for routing.

• IP addresses are tough for humans to remember.
• IP addresses are impossible to guess.

              

DNS History

• Advanced Research Projects Agency Network (ARPANET) utilized a central file HOSTS.TXT

            Contains names to addresses mapping

            Maintained by SRI’s NIC (Stanford-Research-Institute:               

            Network-Information-Center)

• Administrators email changes to NIC

             NIC updates HOSTS.TXT periodically

• Administrators FTP (download) HOSTS.TXT

DNS History

• As the system grew, HOSTS.TXT had problems with

           Scalability (traffic and load)

                Name collisions

                Consistency

• n 1984, Paul Mockapetris released the first version (RFCs 882 and 883, superseded by 1034 and 1035 …)
• RFC: Request for Comments
• RFC 882 - Domain names: Concepts and facilities

The “Domain Name System”

• A service that maps between hostnames and IP addresses
• The mechanism by which Internet software translates names to addresses and vice versa
• Mechanism to store and retrieve information in a global data store
• A hierarchical distributed caching database with delegated authority.
• DNS is one of the core Internet Protocols required for operation of the Internet
• Routing and DNS are the most important infrastructure protocols as without them nothing else will work

• Uses port 53

                  UDP for the queries and responses

                  TCP for the zone transfer

• A globally distributed, scalable, reliable database
• Comprised of three components

                   A “name space”

                   Servers making that name space available

                   Resolvers (clients) which query the servers about the name space

DNS as a Database

• Keys to the database are “domain names”

                 www.foo.com, 

• Over 200,000,000 domain names stored
• Each domain name contains one or more attributes

                Known as “resource records”

• Each attribute individually retrievable

Global Distribution

• Data is maintained locally, but retrievable globally

             No single computer has all DNS data

• DNS lookups can be performed by any device
• Remote DNS data is locally cachable to improve performance

Loose Coherency

• Each version of a subset of the database (a zone) has a serial number

               The serial number is incremented on each database change

• Changes to the master copy of the database are propagated to replicas according to timing set           by the zone administrator
• Cached data expires according to timeout set by zone administrator

Scalability

• No limit to the size of the database
• No limit to the number of queries

               Tens of thousands of queries handled easily every second

• Queries distributed among masters, slaves, and caches

Reliability

• Data is replicated

                Data from master is copied to multiple slaves

• Clients can query

               Master server

               Any of the copies at slave servers

• Clients will typically query local caches
• DNS protocols can use either UDP or TCP

               If UDP, DNS protocol handles retransmission, sequencing, etc.

Dynamicity

• Database can be updated dynamically

              Add/delete/modify of any record

              Only master can be dynamically updated

• Modification of the master database triggers replication

DNS Components

2. The name space
3. The servers
4. The resolvers

By : mogtaba altyib 

Modification by : Mohammed Bakry PhD