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Computer Networks - DNS

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DHIVYADEVAKI

The document discusses the Domain Name System (DNS) which maps human-readable domain names to IP addresses. DNS uses a hierarchical domain name space and resource records stored in name servers. When an application needs to resolve a name to an IP address, it queries a local DNS server which communicates with other name servers until the correct IP address is found. This recursive query process uses the DNS protocol over UDP port 53. DNS was developed to make managing Internet addresses easier as the number of hosts grew.

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By Ms.A.Dhivya Assistant Professor, V.V.Vanniaperumal College for Women, Virudhunagar
The Application Layer Uses transport services to build distributed applications Physical Link Network Transport Application
DNS – Domain Name System The DNS resolves high-level human readable names for computers to low-level IP addresses  DNS name space »  Domain Resource records »  Name servers »
Why do we need DNS?  Too difficult to remember IP addresses and if a server is changed everyone would need to be notified of the new IP address  Originally one file with names and IP addresses – became too large  Host name conflicts began to occur  DNS was developed to solve these problems  DNS is a hierarchical domain based naming scheme and a database system for implementing it.  It is used primarily for mapping host names to IP addresses
How DNS works To map a name to an IP address:  Application program calls a library procedure called a resolver, for example gethostname, passing it the name as a parameter.  The resolver sends a query with the name to a local DNS server, which looks up the name and returns the IP address.  The query and response are sent as UDP packets.  Once it has the IP address the host can now establish a TCP connection or send UDP packets.
DNS Name Space  ICANN – (Internet Corporation for Assigned Names and Numbers)  Established in 1998 to manage naming hierarchy  Internet is divided into 250 top level domains, each partitioned into subdomains  Top level domains are Generic ( originals from 1980’s as well as newer additions ) or Countries (one entry per country)  Original 1985 domains were com, edu, gov, int, mil net, org  Top level domains are run by registrars appointed by ICANN  New domains have been added, but often with disagreements ( xxx, pro, etc.) as to their use.  Domain names are NOT case sensitive  Registering a domain with the intent to sell it is called cybersquatting
The DNS Name Space DNS namespace is hierarchical from the root down  Different parts delegated to different organizations The computer robot.cs.washington.edu
The DNS Name Space Generic top-level domains are controlled by ICANN who appoints registrars to run them This one was controversial
Domain Resource Records  Domains have a set of resource records associates with them  A resource record is a five tuple: domain_name time_to_live class type value  TTL – large number indicates that this is a stable record  Class – for internet information – always IN  Type – what kind of record (see next slide)  Most important type is the Address type  Value – a number, domain name or string
Domain Resource Records The key resource records in the namespace are IP addresses (A/AAAA) and name servers (NS), but there are others too (e.g., MX)
Domain Resource Records  A portion of a possible DNS database for cs.vu.nl. IP addresses of computers Name server Mail gateways
Name Servers Name servers contain data for portions of the name space called zones (circled). One zone
Name Servers Finding the IP address for a given hostname is called resolution and is done with the DNS protocol. ( Remember ARP?) Resolution:  Computer requests local name server to resolve  Local name server asks the root name server  Root returns the name server for a lower zone  Continue down zones until name server can answer DNS protocol:  Runs on UDP port 53, retransmits lost messages  Caches name server answers for better performance
Name Servers Example of a computer looking up the IP for a name
Computer Networks - DNS

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Computer Networks - DNS

  • 2. The Application Layer Uses transport services to build distributed applications Physical Link Network Transport Application
  • 3. DNS – Domain Name System The DNS resolves high-level human readable names for computers to low-level IP addresses  DNS name space »  Domain Resource records »  Name servers »
  • 4. Why do we need DNS?  Too difficult to remember IP addresses and if a server is changed everyone would need to be notified of the new IP address  Originally one file with names and IP addresses – became too large  Host name conflicts began to occur  DNS was developed to solve these problems  DNS is a hierarchical domain based naming scheme and a database system for implementing it.  It is used primarily for mapping host names to IP addresses
  • 5. How DNS works To map a name to an IP address:  Application program calls a library procedure called a resolver, for example gethostname, passing it the name as a parameter.  The resolver sends a query with the name to a local DNS server, which looks up the name and returns the IP address.  The query and response are sent as UDP packets.  Once it has the IP address the host can now establish a TCP connection or send UDP packets.
  • 6. DNS Name Space  ICANN – (Internet Corporation for Assigned Names and Numbers)  Established in 1998 to manage naming hierarchy  Internet is divided into 250 top level domains, each partitioned into subdomains  Top level domains are Generic ( originals from 1980’s as well as newer additions ) or Countries (one entry per country)  Original 1985 domains were com, edu, gov, int, mil net, org  Top level domains are run by registrars appointed by ICANN  New domains have been added, but often with disagreements ( xxx, pro, etc.) as to their use.  Domain names are NOT case sensitive  Registering a domain with the intent to sell it is called cybersquatting
  • 7. The DNS Name Space DNS namespace is hierarchical from the root down  Different parts delegated to different organizations The computer robot.cs.washington.edu
  • 8. The DNS Name Space Generic top-level domains are controlled by ICANN who appoints registrars to run them This one was controversial
  • 9. Domain Resource Records  Domains have a set of resource records associates with them  A resource record is a five tuple: domain_name time_to_live class type value  TTL – large number indicates that this is a stable record  Class – for internet information – always IN  Type – what kind of record (see next slide)  Most important type is the Address type  Value – a number, domain name or string
  • 10. Domain Resource Records The key resource records in the namespace are IP addresses (A/AAAA) and name servers (NS), but there are others too (e.g., MX)
  • 11. Domain Resource Records  A portion of a possible DNS database for cs.vu.nl. IP addresses of computers Name server Mail gateways
  • 12. Name Servers Name servers contain data for portions of the name space called zones (circled). One zone
  • 13. Name Servers Finding the IP address for a given hostname is called resolution and is done with the DNS protocol. ( Remember ARP?) Resolution:  Computer requests local name server to resolve  Local name server asks the root name server  Root returns the name server for a lower zone  Continue down zones until name server can answer DNS protocol:  Runs on UDP port 53, retransmits lost messages  Caches name server answers for better performance
  • 14. Name Servers Example of a computer looking up the IP for a name