>
 
  
 
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 databases
 
 
 
 
  
 
TDC375 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 necessary
 
 
 
 
  
 
TDC375 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
     copyright
 
 
 
 
  
 
TDC375 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 
   organization
 
 
 
 
  
 
TDC375 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
     host
 
 
 
 
  
 
TDC375 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.com
 
 
 
 
  
 
TDC375 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 hierarchy
 
 
 
 
  
 
TDC375 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 
   servers
 
 
 
 
  
 
TDC375 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 upstream
 
 
 
 
  
 
TDC375 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 links
 
 
 
 
  
 
TDC375 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 problem
 
 
 
 
  
 
TDC375 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 answer
 
 
 
 
  
 
TDC375 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 lookups
 
 
 
 
  
 
TDC375 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