>
 
  
 
Case 
 Studies: 
 DNS, X.500, and NARS 
 EEE465 1999 
 Lecture 38 
 
Major Greg Phillips
 
Royal Military College 
 of Canada
 
Electrical and Computer 
 Engineering
 
greg.phillips@rmc.ca
 
+01-613-541-6000 ext. 
 6190
 
 
 
  
 
Context 
 
 
   We have looked at the 
   design of a Simple Name Service, discussed some of its deficiencies, 
   and examined some additional design issues for name services
   Today we will look at two 
   of the most successful name services: the Internet Domain Name System 
   (DNS) and ITU(C) recommendation X.500
 
 
 
 
  
 
The Problem 
 
 
dial
 
1234 
 
 
Y
 
1234 
 
 
1234? 
 
 
X
 
 
 
  
 
Inefficient Solution 
 I 
 
dial
 
1234 
 
 
Y
 
1234 
 
 
X 
 
 
1234? 
 
 
1234? 
 
 
1234? 
 
 
1234? 
 
 
1234? 
 
 
1234? 
 
 
It00 
 ME!!!
 
 
 
  
 
Inefficient Solution 
 II 
 
dial
 
1234 
 
 
Y
 
1234 
 
 
X
 
1234 : Y 
 
 
1234 : Y 
 
 
1234 : Y 
 
 
1234 : Y 
 
 
1234 : Y 
 
 
1234 : Y 
 
 
1234 : Y 
 
 
1234 : Y 
 
 
1234 : Y 
 
 
5555
 
 
 
  
 
The Usual Solution 
 
 
dial
 
1234 
 
 
Y
 
1234 
 
 
X
 
ask A 
 
 
A
 
1234 : Y 
 
 
1234? 
 
 
1234 
 : Y
 
 
 
  
 
Internet Domain Name 
 System 
 
   pre-DNS (1987), Internet 
   relied on a central hostname database downloaded by FTP to all clients. 
   This did not:
   
     scale 
     to large numbers of computers
     allow distributed 
     administration
     allow lookups 
     of entities other than hosts
     
   key design goal: scalability 
   resolves domain names (e.g., 
   tarpit.rmc.ca) into numeric IP addresses (e.g., 137.94.178.161), and 
   vice versa
   also responsible for resolving 
   service names (e.g., mail addresses: greg.phillips@rmc.ca)
   based on a distributed database
   
     the hierarchical 
     namespace is divided into zones
     the responsibility 
     and authority for the names in each zone assigned to a name 
     authority
     zones can 
     be further divided and authority and responsibility delegated downward
 
 
 
 
  
 
DNS Name Hierarchy 
 
 
root 
 
 
edu 
 
 
gov 
 
 
net 
 
 
org 
 
 
ch 
 
 
us 
 
 
au 
 
 
su 
 
 
at 
 
 
rmc 
 
 
tarpit 
 
 
An administrative (vice 
 physical) hierarchy. The name itself tells you nothing about IP addresses, 
 routing, or physical location of the named entity.  
 
 
00eneric00/font> 
 
 
ISO-3166 
 
 
int 
 
 
mil 
 
 
com 
 
 
ca
 
 
 
  
 
DNS Name Servers 
 
 
   Each zone will have one or 
   more name servers
   Each server is either a primary 
   or a secondary 
   server
   
     primary 
     servers read zone data directly from a local master file
     secondary 
     servers periodically download data from a primary server
     
   Servers typically cache frequently 
   used data from other servers
   
     when cached 
     data is supplied, it must be marked as non-authoritative
     cached data 
     has an associated time to live value, 
     and eventually 00ges out00/font>
 
 
 
 
  
 
DNS Lookup 
 
 
   DNS requests are typically 
   directed to a local name server
   Resolves names within the 
   local domain, plus any cached names
   Maintains references to other 
   domains at various levels, including the root 
   
     when a 
     non-resolvable query comes in, it is forwarded to the lowest level appropriate 
     server known by the forwarding server
     lookup can 
     be either iterative or recursive
     
   Also 00everse resolves00
   IP addresses to host names using the special in-addr.arpa domain
 
 
 
 
  
 
X.500 
 
 
   A directory 
   service or attribute-based 
   name service
   
     can store 
     arbitrary attributes
     allows lookup 
     by arbitrary (and partially-known) attributes, not just known names
     
   All information contained 
   in a single global hierarchy called the Directory 
   Information Tree (DIT) stored 
   in the Directory 
   Information Base (DIB)
   The DIB is organizationally 
   partitioned
   Actual information partitioned 
   among the distributed servers which provide the service
   Data structure is object-based; 
   entries have objectClasses
 
 
 
 
  
 
X.500 Lookup 
 
 
   can perform lookup (read) 
   based on absolute or relative names
   
     relative 
     to a defined context, including a base node
     
   can search on a base 
   name and a filter 
   expression
   
     base name 
     specifies search start; filter expression specifies success criteria
     search returns 
     all names for which the filter evaluates true
     searches 
     can be extremely costly
     
       possible 
       to supply additional arguments which restrict search scope
       
   X.500 does not specify implementation
   
     most systems 
     expected to include replication and caching
     
   Lightweight Directory Access 
   Protocol (LDAP) is a small and widely used protocol for accessing 
   X.500 conformant name databases; including RMC00
   
     implemented 
     by many internet directory systems; most web browsers can act as clients
 
 
 
 
  
 
Tactical Use of DNS 
 and X.500 
 
   DNS and X.500 both require 
   each computer to know of at least one available name server
   In the tactical domain, the 
   presence of such a server cannot be guaranteed
 
 
 
 
  
 
The Iris Solution 
 (NARS) 
 
dial
 
1234 
 
 
Y
 
1234 
 
 
X
 
ask ? 
 
 
A
 
1234 : Y 
 
 
1234? 
 
 
1234 
 : Y 
 
A
 
 
 
  
 
To     $     
 Port
 
A      
 5 M
 
.....
 
F      
 12 R
 
X      
 7 L
 
Y      
 0     -
 
.... 
 
To     $     
 Port
 
A      
 17 Q
 
.....
 
F      
 1 F
 
X      
 0      -
 
Y     7 B
 
.... 
 
Finding the Name 
 Server 
 
dial
 
1234 
 
 
Y
 
1234 
 
 
X
 
ask ? 
 
 
A
 
1234 : Y
 
 
 
  
 
The ADB Calculation 
 
 
Intermediate
 
Hash Space 
 
 
Name
 
Space 
 
 
Address
 
Space 
 
 
0 
 
 
n 
 
 
0000 
 
 
9999 
 
 
0.0.0.0 
 
 
255.255.255.255 
 
 
1234 
 
 
A 
 
 
F 
 
 
X 
 
 
Y
 
 
 
  
 
Summary 
 
 
   Name services attempt to 
   solve the name or attribute lookup problem
   
     name services 
     search by well-known name
     directory 
     services search by partially-defined attributes
     
   DNS is the name service of 
   the Internet
   
     has proven 
     very flexible and scalable
     can also 
     be used in non-Internet contexts
     
   X.500 is the most widely 
   implemented directory service standard
   
     large and 
     complex; the LDAP subset has enjoyed considerable success
     
   NARS is a special purpose 
   name service designed for the tactical environment, where it is unwise 
   to rely on the availability of any particular set of name servers
   
     NARS can 
     implement a service which looks like DNS from the client perspective; 
     this is likely to become a requirement 
 
 
 
 
  
 
Next 
 Class: 
 Course Summary and Exam Study Hints