Domain name system

PRESENTATION
ON
“DOMAIN NAME
SYSTEM”
SUBMITTED BY :
VIVEK GAUTAM
MCA

3
1. What is the IP
address of
udel.edu ?
Introduction
1. What is the
host name of
128.175.13.74
It is 128.175.13.92
It is strauss.udel.edu

4
Real Life Analogy: Telephone Example
Source:
Child
Newark, DE
Destination: Dad
Udel-Newark, DE
Telephone connection
Information Child Needs: Dad’s Phone #

5
Dials 302-731-1212:
(What is University # ?)
operator
2
Dials 0: (what is
Newark’s area code?)
3
(Newark’s area code is
302)
Directory
assist
45
(University number: 302-831-
4567)
University
operator
6
Dials 302-831-4567:
(what is Dad phone #?)
7
Dad’s phone is
302-831-1949
Calls dad
(What is Dad’s Phone#?)
1
Older sister
Child
Dad’s phone is
302-831-1949
8

6
DNS
Components
There are 3 components:
• Name Space:
Specifications for a structured name space and data
associated with the names
• Resolvers:
Client programs that extract information from Name
Servers.
• Name Servers:
Server programs which hold information about the
structure and the names.

8
Resolvers
A Resolver maps a name to an address and vice
versa.
Query
Response
Resolver Name Server

9
iterative response (referral)
“I don't know. Try a3.nstld.com.”
2
a3.nstl
d.com
3
Iterative Resolution
client
iterative request
“What is the IP address of
www.google.com?”
udel
server
1
“I don't know. Try a.root-servers.net.”
4
a.root
server
5
“I don't know. Try a.gtld-servers.net.”
6
a.gtld-
server
7
“I don't know. Try ns1.google.com.”
8
ns1.goo
gle.com
9
iterative response
“The IP address of www.google.com
is 216.239.37.99.”10

10
Recursive Resolution
client
recursive request
“What is the IP address of
www.google.com?”
udel
serve
r
1
edu
server
2
root
server
3 com
server
4
google
server
5
recursive response
“The IP address of www.google.com is
216.239.37.99.”
6
10
7
8
9

11
Master
serverZone transfer
Zone
data
file
From
disk
Authoritative Data
(primary master and
slave zones)
Agent
(looks up queries
on behalf of resolvers)
Cache Data
(responses from
other name servers)
Name Server Process
Name Server
Architecture:

12
Resolver
Query
Response
Authoritative Data
(primary master and
slave zones)
Agent
(looks up queries
Cache Data
(responses from
other name servers)
Name Server Process
Authoritative Data:
Name Server (cont’d)

13
Arbitrary
name
server
Response
Resolver
Query
Query
Authoritative Data
(primary master and
slave zones)
Agent
(looks up queries
Cache Data
(responses from
other name servers)
Name Server Process
Response
Using Other Name Servers:

14
Cached Data :
Query
Response
Authoritative Data
(primary master and
slave zones)
Agent
(looks up queries
Cache Data
(responses from
other name servers)
Name Server Process
Resolver

15
Block Diagram
User
Program
Foreign
Name
Server
Cache
Resolver
Query Query
Reference
Response
Addition
Response

16
DNS Messages
Messages
ResponseQuery

17
DNS Message Format
Header (12 bytes)
Question section
Header (12 bytes)
Question section
Answer section
Authoritative section
Additional section
Identification Flags
2 bytes 2 bytes
Number of
Question Records
Number of
Answer Records
(zeroed in query)
Number of Auth-
oritative Records
(Zeroed in query)
Number of
Additional Records
(zeroed in query)
0 = query,
1 = response
0 = standard, 1 = inverse,
2 = server status request
Authoritative
Answer flag Truncated flag Recursion Desired flag
Recursion Available flag
QR OpCode AA TC RD RA 0 0 0 rCode
no error
format error
problem at name server
domain reference problem
query type not supported
administratively prohibited
reserved
0
1
2
3
4
5
6-15

18
Question Record Format
class of network (1 = Internet)
12
5
2
1 Address – IPv4
Name Server (authoritative)
Canonical Name (alias)
Pointer – reverse lookup
15 Mail Exchange
28 Address - IPv6
252 Zone Transfer
PTR
CNAME
NS
A
MX
AAAA
AXFR
3 r e n 5 e e c i s 4 u d e l 3 e d u 0
counts
sent in query;
repeated in response
Query type
(16 bits)
Query name
(variable length)
Query class
(16 bits)

19
Resource Record Format
answer, authoritative, and
additional sections in response
Resource data
(variable length)
Domain type
(16 bits)
Domain Name
(variable length)
Domain class
(16 bits)
Time to Live
(32 bits)
data length
(16 bits)
name of host/domain that this
record provides information for
type of data in resource record
(same types as used in question record)
number of seconds this
record may be cached
length of resource data
the “payload” of the
resource record
same as in question record

20
Compression
3 r e n 5 e e
4 u d e l 3 e d u 0
c i s
bytes 0-11
Query type
(16 bits)
Query Class
(16 bits)
Domain Name
(variable length)
Header (12 bytes)
Query name
(variable length)
Question
Section
Answer
Section
byte 12
C0 0C
1100000000001100 = 1210

21
Example forward query/response
ident 0x0100
0x0001 0x0000
0x0000 0x0000
3 'w' 'w' 'w'
4 'u' 'd' 'e'
'l' 3 'e' 'd'
'u' 0
0x0001(IN)
0x0001(A)
flags: recursion desired (RD)
“What is the IP address
of www.udel.edu?”
Hdr
Qry 3 'w' 'w' 'w'
4 'u' 'd' 'e'
'l' 3 'e' 'd'
'u' 0
0x0004
same ident 0x8180
0x0001 0x0001
0x0004 0x0004
0x0001(IN)
0x0001(A)
0xC00C 0x0001(A)
0x0001(IN)
0x80AF0D3F
(128.175.13.63)
...0xB2F5
...
flags: query response (QR),
recursion desired (RD),
recursion available (RA)
0x0000...
TTL: 45301 seconds ≈ 12.6 hours
“www.udel.edu's IP address
is 128.175.13.63.”
Hdr
Qry
Ans

22
Example inverse query/response
ident 0x0100
0x0001 0x0000
0x0000 0x0000
0x000C(PTR) 0x0001(IN)
2 '6' '3' 2
'1' '3' 3 '1'
'7' '5' 3 '1'
'2' '8' 7 'i'
'n' '-' 'a' 'd'
'd' 'r' 4 'a'
'r' 'p' 'a' 0
“What is the name of the
host at 128.175.13.63?”
Hdr
Qr
y
3 'w' 'w' 'w'
4 'u' 'd' 'e'
'l' 3 'e' 'd'
'u' 0
same ident 0x8180
0x0001 0x0001
0x0004 0x0004
0x000C(PTR)
0x0001(IN)
...0xB003
0x0000...
2 '6' '3' 2
'1' '3' 3 '1'
'7' '5' 3 '1'
'2' '8' 7 'i'
'n' '-' 'a' 'd'
'd' 'r' 4 'a'
'r' 'p' 'a' 0
0x000C(PTR) 0x0001(IN)
0x000E
0xC00C
...
“The host at 128.175.13.63
is named www.udel.edu.”
Hdr
Qry
Ans
TTL: 45056 seconds ≈ 12.5 hours

23
Resource Record Sections
• Resource Record sections:
– answer = record(s) sent in response to query(s).
– authoritative = DNS servers which are authoritative for answer
record(s).
– additional = any other related information.
• MX records:
– mail exchange (MX) records provide mail addressing info.
– MX query asks “What hosts will accept mail for domain X?”
– MX resource records say “You can send mail for domain X to host
Y.”
delivery priority (lower value = higher priority)preference (2 bytes)
exchange
(variable length)
domain name of host that will accept mail
MX Resource Data

24
0x000A 7 'c'
'o' 'p' 'l' 'a'
'n' 'd' 0xC00C
ident 0x8180
0x0001 0x0002
0x0004 0x0006
0x0001(IN)
0x000F(MX)
0x0001...
4 'u' 'd' 'e'
'l' 3 'e' 'd'
'u' 0 0x000F(MX)
0xC00C
0x000F(MX)
0x0001(IN)
0xC00C
0x000C
0x0014 7 's'
't' 'r' 'a' 'u'
's' 's' 0xC00C
0x000C
0x0001(IN)
0x0001...
...0x28F6
...0x28F6
...
Hdr
Qry
Ans
Ans
4 'D' 'N' 'S'
'1' 0xC00C
0xC00C 0x0002(NS)
0x0001(IN) 0x0000...
4 'D' 'N' 'S'
'2' 0XC00C
...0x19FA
0xC028 0x0001(A)
0x0001(IN) 0x0001...
0x0002(NS)
0x0001(IN) 0x0000...
0x0007
0xC00C
128.175.13.74
...0x2FB4 0x0004
0xC040 0x0001(A)
0x0001(IN) 0x0001...
...0x0D5D 0x0004
...0x19FA
0x0007
128.175.13.92
...
...
...
Auth
Auth
Adtl
Adtl
Example MX response

25
Transport
IP
header
UDP
header
DNS message
max. 512 bytes
no limit (up to max. TCP payload size)
IP
header
TCP
header
DNS message
2-byte
DNS msg.
length
●
DNS messages are encapsulated in UDP by default.
●
If the resolver expects the response to exceed 512 bytes, the
resolver encapsulates the query in TCP instead.
●
If a request is sent over UDP and the response is longer than 512
bytes, the server sends the first 512 bytes of the response using
UDP and sets the TC (truncated) flag. The resolver then re-sends
the query using TCP.

26
Dynamic DNS
Client
DHCP
Server
Primary DNS Server
Zone File
IP Address?
IP Address Update

Domain name system

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