Network Protocols Domain Name System DNS largely based on slides from D Comer TDC375 Autumn 03 04 John Kristoff DePaul University 1 Names Internet communication requires an IP address Humans prefer to use easy to remember names Need an automated system to translate IP to name This is the Domain Name System A static database e g etc hosts can also be used Scaling issues with these static databases TDC375 Autumn 03 04 John Kristoff DePaul University 2 Basic DNS functionality Given a name of a computer host Return the IP address associated with the name Method used Distributed lookup of names in a hierarchy Client contacts server s as necessary TDC375 Autumn 03 04 John Kristoff DePaul University 3 Domain name syntax Alphanumber segments separated by dots www depaul edu www cs depaul edu Top level hierarchy starts from the right edu top level domain TLD TDC375 Autumn 03 04 John Kristoff DePaul University 4 Obtaining a domain name An organization chooses a name it must be unique and available register name with a central authority placed under a top level domain Names subject to international law trademark copyright TDC375 Autumn 03 04 John Kristoff DePaul University 5 Top Level Domains TDC375 Autumn 03 04 John Kristoff DePaul University 6 Hierarchy with a organization Subdivision possible Arbitrary levels allowed Not standardized Controlled locally by the organization TDC375 Autumn 03 04 John Kristoff DePaul University 7 Example name structure First level is com Second level is company name Third level is a division within a company Fourth level is either company subdivision host TDC375 Autumn 03 04 John Kristoff DePaul University 8 A domain name example Company foobal with cany and soap divisions Candy has subdivisions soap does not Names in soap division take the form host soap foobar com Names in candy division take the form host subdivision candy foobar com TDC375 Autumn 03 04 John Kristoff DePaul University 9 DNS example illustrated TDC375 Autumn 03 04 John Kristoff DePaul University 10 DNS client server interaction Client is known as a resolver Multiple DNS servers are typically used Arranged in a hierarchy Each server corresponds to its part in the hierarchy TDC375 Autumn 03 04 John Kristoff DePaul University 11 Two possible DNS hierarchies TDC375 Autumn 03 04 John Kristoff DePaul University 12 Inter server links Servers know reach parent domain server Each server knows how to reach the root Servers know child domain servers TDC375 Autumn 03 04 John Kristoff DePaul University 13 DNS deployment in practice Redundant servers are used Root servers A M are geographically dispersed often each root is redundant dispersed generally run by very clueful people ISPs organizations offer DNS services to its users Small organizations can get DNS from an upstream TDC375 Autumn 03 04 John Kristoff DePaul University 14 DNS lookup Application triggers request to local name server If local server knows the answers it returns it Otherwise the local server go finds it for the client Starts at top level domain and follows links Iterative lookups force client to follow links Recursive lookups result in server following links TDC375 Autumn 03 04 John Kristoff DePaul University 15 DNS caching Servers cache answers for some period of time Usually controlled by the TTL in an answer Hosts can also cache previous answers Caching improves efficiency eliminates unnecessary searching works well because of high locality reference Cache poisoning attacks can be a problem TDC375 Autumn 03 04 John Kristoff DePaul University 16 DNS types Each entry in a server consists of domain name DNS type for the name value to with the name corresponds Client asks for the name and specifies the type Server matches name and type and returns answer TDC375 Autumn 03 04 John Kristoff DePaul University 17 Example DNS types Type A address value is an IP address assigned to a host Type MX mail exchanger value is an IP address that handles mail for host Type CNAME canonical name aka alias value is another domain name Type PTR pointer value is a name this is used for reverse lookups TDC375 Autumn 03 04 John Kristoff DePaul University 18 Domain name abbreviations DNS uses fully qualified domain names FQDN Users apps sometimes do not specify the full name Configure resolver with a list of common suffixes e g depaul edu cs depaul edu User enters www resolver tries until match www www depaul edu www cs depaul edu TDC375 Autumn 03 04 John Kristoff DePaul University 19
View Full Document
Unlocking...