Transcript No Slide Title

Telecommunications Networking
II
Lecture 33
The Domain Name System (DNS)
Ref: Tanenbaum 622-630
Copyright 1999, S.D. Personick. All Rights Reserved.
The Domain Name System
(DNS)
• The principal purpose of the Domain Dame
System (DNS) is to allow Internet hosts,
and other entities that have IP addresses, to
refer to each other by “names” that are
easier to remember than 32-bit IP addresses
• The concept is very similar to telephone
numbers like 1-888-ProfStu
Copyright 1999, S.D. Personick. All Rights Reserved.
The Domain Name System
(DNS)
• The Domain Name System is based on a
hierarchical naming scheme
Root
us
(etc.)
gov
mil
edu
com
org net
Copyright 1999, S.D. Personick. All Rights Reserved.
The Domain Name System
(DNS)
Examples:
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
Copyright 1999, S.D. Personick. All Rights Reserved.
The Domain Name System
(DNS)
Domain names are case-insensitive
[email protected] = [email protected]
but….some Web page names, e.g.,
WWW.Drexel.edu/… have case-sensitive
fields after the “/” that follows the top level
domain name (e.g., .edu, .com, .org, .gov, ...)
Copyright 1999, S.D. Personick. All Rights Reserved.
The Domain Name System
• Top level domains are either “generic” :
.com, .edu, .gov, .mil, .net, .org
or “geographic”: .us, .jp, .it, ….
• The domain naming structure is about to be
modified in order to introduce competition
into the process of assigning domain names
Copyright 1999, S.D. Personick. All Rights Reserved.
The Domain Name System
• Domains are managed by assigned entities
• .edu is managed by Network Solutions Inc
(a publicly traded subsidiary of SAIC Inc),
under contract from the U.S. Government
(NSF). If you want to establish a new .edu
domain name (example: ucphila.edu), you
ask Network Solutions Inc to assign this
name to you (if available)
Copyright 1999, S.D. Personick. All Rights Reserved.
The Domain Name System
• .gov and .mil are also managed by Network
Solutions Inc under contract with the U.S.
Government (last I looked)
• .jp is managed by an an entity appointed by
the japanese government
• .com is currently managed by Network
Solutions Inc, but …..
Copyright 1999, S.D. Personick. All Rights Reserved.
The Domain Name System
• …there is a plan being implemented, as we
meet, to open up the assignment of domain
names to competition
• There will be registries and registrars; and
.com will be supplemented with other
commercial and non-commercial top level
domain names (e.g., .firm, .bus, .nom)
Copyright 1999, S.D. Personick. All Rights Reserved.
The Domain Name System
• The US Postal Service has suggested that it
assign “.us” Internet names to everyone in
the U.S., and that it administer the .us
domain
• Note: Assignment of names is a very
contentious and litigious process
Copyright 1999, S.D. Personick. All Rights Reserved.
DNS Queries
• The naming hierarchy allows for distributed
administration of the assignment of domain
names…and the tables that link domain
names to IP addressses:
-Network Solutions Inc administers .edu
-Drexel Univ administers drexel.edu
-The Drexel ECE Department can
administer ece.drexel.edu
Copyright 1999, S.D. Personick. All Rights Reserved.
DNS Queries
• Each entity that administers a part of the
domain name space must maintain a server
at a known address (or more than one
server) which can be queried to translate
domain names into IP addresses.
• Translations can be cached, but are typically
not cached for long (at least not at the lower
levels of the hierarchy)
Copyright 1999, S.D. Personick. All Rights Reserved.
DNS Queries
• If you need to translate a domain name into
an IP address, you ask your (“local”)
domain name server. If it doesn’t have the
translation (authoritative or cached), it will
send a DNS query to a higher level server
and/or another server at its own level in the
hierarchy
Copyright 1999, S.D. Personick. All Rights Reserved.
DNS Queries
Root
DNS
.edu
DNS
1
.mil
DNS
2
Drexel
DNS
3
IP address of:
[email protected]
Copyright 1999, S.D. Personick. All Rights Reserved.
DARPA
DNS
DNS Resource Records
Domain_name Time_to_live Class Type Value
See Tanenbuam p 625:
Time_to_live = integer (seconds, e.g., 86400)
Class= IN (Internet information)
Example Type: A (IP address of a host)
Value: 32-bit integer
Copyright 1999, S.D. Personick. All Rights Reserved.