TDC375 Autumn 03/04
John Kristoff - DePaul University
1
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 databasesTDC375 Autumn 03/04
John Kristoff - DePaul University
1
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 necessaryTDC375 Autumn 03/04
John Kristoff - DePaul University
1
Domain name syntax
Alphanumber segments separated by dots www.depaul.edu www.cs.depaul.edu Top level hierarchy starts from the right .edu 00top level domain (TLD)TDC375 Autumn 03/04
John Kristoff - DePaul University
1
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 copyrightTDC375 Autumn 03/04
John Kristoff - DePaul University
1
Top Level Domains
TDC375 Autumn 03/04
John Kristoff - DePaul University
1
Hierarchy with a organization
Subdivision possible Arbitrary levels allowed Not standardized Controlled locally by the organizationTDC375 Autumn 03/04
John Kristoff - DePaul University
1
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 hostTDC375 Autumn 03/04
John Kristoff - DePaul University
1
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.comTDC375 Autumn 03/04
John Kristoff - DePaul University
1
DNS example illustrated
TDC375 Autumn 03/04
John Kristoff - DePaul University
1
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 hierarchyTDC375 Autumn 03/04
John Kristoff - DePaul University
1
Two possible DNS hierarchies
TDC375 Autumn 03/04
John Kristoff - DePaul University
1
Inter-server links
Servers know reach parent domain server Each server knows how to reach the root (.) Servers know child domain serversTDC375 Autumn 03/04
John Kristoff - DePaul University
1
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 upstreamTDC375 Autumn 03/04
John Kristoff - DePaul University
1
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 linksTDC375 Autumn 03/04
John Kristoff - DePaul University
1
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 problemTDC375 Autumn 03/04
John Kristoff - DePaul University
1
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 answerTDC375 Autumn 03/04
John Kristoff - DePaul University
1
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 lookupsTDC375 Autumn 03/04
John Kristoff - DePaul University
1
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
