Transcript Why Names? - ECSE - Rensselaer Polytechnic Institute

Domain Name System (DNS)
NA M
E
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
[email protected]
http://www.ecse.rpi.edu/Homepages/shivkuma
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Overview
Naming hierarchy
 Server hierarchy
 Name resolution
 Other information in name servers
 Ref: Chap 14, RFC 1034

Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Why Names?
Computers use addresses
 Humans cannot remember IP addresses
 Need names
Example, “shiva” for 128.113.50.56
 Simplest Solution: Each computer has a unique
name and has a built in table of name to address
translation (mapping)
 Problem: Not scalable
 Solution: DNS (Adopted in 1983)
 Hierarchical Names: shiv.ecse.rpi.edu
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Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Name Hierarchy
Unnamed root
com
edu
gov
au
dec
rpi
nsf
co
ecse
ipl
hr
...
us
va
reston
cnri
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Name Hierarchy
Unique domain suffix is assigned by Internet
Authority
 The domain administrator has complete control
over the domain
 No limit on number of sub-domains or number of
levels
 computer.site.division.company.com
 Domains within an organization do not have to
be uniform in number of subdomains or levels
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Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Name Hierarchy (Continued)
Name space is not related to physical
interconnection, e.g., ecse.rpi.edu and ipl.rpi.edu
could be on the same floor or in different cities
 Geographical hierarchy is also allowed, e.g.,
cnri.reston.va.us
 A name could be a subdomain (eg: ecse.rpi.edu)
or an individual object (eg: cortez.rpi.edu)
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Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Top Level Domains
Domain Name/Assignment
com
Commercial
edu
Educational
gov
Government
mil
Military
net
Network
org
Other organizations
arpa
Advanced Research Project Agency
country code au, uk, ca
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Server Hierarchy
Servers are organized in a hierarchy
 Each server has an authority over a part of the
naming hierarchy
 It needs to know other servers who are
responsible for other subdomains
 A single node in the naming tree cannot be split
among multiple servers
 A given level of hierarchy can be partitioned into
multiple servers
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Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Server Hierarchy (Example)
com
com
foobar
foobar
soap
soap
candy
candy
peanut almond walnut
peanut almond
walnut
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Server Hierarchy (Continued)
Authority  has the name to address translation
table
 Responsible  Either has the name to address
translation table or knows the server who has
 But such a reply is called “non-authoritative”
reply
 Root server knows about servers for top-level
domains, e.g., com
 Each server knows the root server
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Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Server Hierarchy: Example
Root
Server
Server
for com
Server
for edu
Server
for gov
Server
for au
Server
for us
Server for
dec.com
Server
for rpi.edu
Server for
nsf.gov
server for
co.au
Server for
va.us
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Server Hierarchy: Better
Root
Server
Server for
dec.com
Server
for rpi.edu
Server for
nsf.gov
server for
co.au
Server for
va.us
Fewer servers
 More entries/links per server
 Fewer levels to traverse before resolving a name
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Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Name Resolution
Each computer has a name resolver routine,
e.g., gethostbyname & gethostbyaddr in UNIX
 Each resolver knows the IP address of a local
DNS server
 Resolver sends a DNS request to the server
 DNS server either gives the answer, forwards the
request to another server, or gives a referral
 Referral = Next server to whom request should
be sent
 Recursive Query:
Give me an answer (Don't give me a referral)
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Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Recursion!
Cache
Cache
Query
Query
Response
Response
Name
Server
Query
Database
Name
Server
Database
Query
Name
User
Response Resolver Response
Cache
Name
Server
Cache
Database
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Name Resolution (Continued)
Name
Server
Name
Server
Query
Response
Query
Database
Query
Name
User
Response Resolver Response
Rensselaer Polytechnic Institute
Referral
Cache
Query
Database
Cache
Name
Server
Cache
Cache
Database
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DNS Optimization
Spatial Locality: Local computers referenced
more often than remote
 Temporal Locality: Same set of domains
referenced repeatedly  Caching
 Each entry has a time to live (TTL)
 Replication: Multiple servers. Multiple roots.
Ask the geographically closest server.
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Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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DNS Message Format
Identification
Parameter
Number of Questions
Number of Answers
Number of Authority
Number of Additional
Question Section
...
Answer Section
...
Authority Section
...
Additional Information Section
...
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Rensselaer Polytechnic Institute
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Format (Continued)

Format of the query section entries:
Query Domain Name
...
Query Type
Query Class
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Format of other section entries:
Resource Domain Name
Type
Class
Time to live
Resource Data Length Resource Data
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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DNS Message Format (Continued)
Bit
0
1-4
Meaning
Operation: 0=Query, 1=Response
Query type: 0=Standard, 1=Inverse, 2,3
obsolete
5
Set if answer authoritative
6
Set if message truncated
7
Set if recursion desired
8
Set if recursion available
9-11 Reserved
12-15 Response type: 0=No error, 1=Format error,
2=Server Failure, 3=Name does not exist
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Types of DNS Entries
DNS used other types of resolution
 Eg: also for finding mail server, pop server,
responsible person, etc for a computer
 DNS database has multiple “types”
 Record type A  Address of X
 Record type MX  Mail exchanger of X
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Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Types of DNS Entries (Continued)
DNS database may also have multiple “classes”
 Can support name resolution for multiple
protocols eg: IP, SNA, DECbit etc
 Pointer queries: given IP address find name
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Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Resource Record Types
Type
A
CNAME
HINFO
MINFO
MX
NS
PTR
RP
SOA
TXT
Meaning
Host Address
Canonical Name (alias)
CPU and O/S
Mailbox Info
Mail Exchanger
Authoritative name server for a domain
Pointer to a domain name (link)
Responsible person
Start of zone authority (Which part of
naming hierarchy implemented)
Arbitrary Text
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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Summary
DNS: Maps names to addresses
 Names are hierarchical. Administration is also
hierarchical.
 No standard for number of levels
 Replication and caching is used for performance
optimization.
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Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
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