DNS - The Domain Name System

Sirak Kaewjamnong

Computer Network Systems

2

Outline

• DNS basic

• name space

• resolver

• protocol

• configuration

3

Why need DNS?

• host table /etc/hosts – simple text file with has IP address to name

mapping

• problems– name collision– consistency

• A hierarchical name with distributed control is needed

4

DNS basic

• DNS is a distributed database

• TCP/IP applications use DNS to – map hostname to IP address– map IP address to hostname– provide e-mail routing information

• mail ksirak@ratree.psu.ac.th => ratree.psu.ac.th

– handle aliases• www.cs.psu.ac.th is actually www2.cs.psu.ac.th

5

Naming Scheme

• name space is a tree of domain• names are case-insensitive

www.cs.psu.ac.thwww.cs.psu.ac.th

more specific

th

ac

psu

cs

www

building tree from top to bottom

www.cs.psu.ac.th

6

Domain Name Space

edu gov int mil net org au th

ac

psu

cs eng

www

ku

www.cs.psu.ac.th

usu

cc cc.usu.edu

com

root

generic domains country domains

arpa

in-addr

7

DNS Management

• ICANN manages root and top level domain name

• local admins manage 3rd level or more

root

ac

psu

cs

ku

or

com

managed by ICANN

managed bythnic Thailand

managed by psu

manage by ku

th

eng

8

Domain Name Concept

label label every node has a label (except root)

domain name domain name

absolute domain nameabsolute domain name

the list of labels, starting at that node, working up to the root, using a “.” to separate

domain name that ends with a period

relative domain namerelative domain name name to be completed

www.cs.psu.ac.th.

www

psu.ac.th, cs.psu.ac.th

th

ac

psu

cs

www

9

Domains

domain domain subtree of the domain name spaceth

ac

psu

cs eng

www

ku

www.cs.psu.ac.th node

ac.th domain

psu.ac.th domain

10

Domains and Zones

• Zone is a subtree for which naming authority has been delegated

psu.ac.th domainpsu.ac.th zone

case 1 : single DNS administration

psu.ac.th domain

case 2 : cs and cc have authority for their zones

psu

sci ccpn

clibmgt

cs

engpsu

sci ccpn

clibmgt cs

eng

psu.ac.th zone

11

psu

sci ccpn

clibmgt cs

eng

Name Servers

• Name server : Server that store information about the zone

ns.psu.ac.th

responsibility for psu.ac.th zone

ns.cc.psu.ac.th

responsibility for cc.psu.ac.th zone

responsibility for Cs.psu.ac.th zone

ns.cs.psu.ac.th

12

Type of Name Servers

• Primary Name server gets the data for zones from files on the host it runs on

• Secondary Name server gets its zone data from the primary for redundancy and workload distribution

13

Zone Transfer

• Secondary Name Server pulls zone data over from the primary called zone transfer.

primary for ku.ac.thsecondary for cpe.ku.ac.th

primary for eng.ku.ac.thsecondary for ku.ac.thsecondary for cpe.ku.ac.th

primary for cpe.ku.ac.thsecondary for ku.ac.thsecondary for eng.ku.ac.th

secondary for ku.ac.thsecondary for cpe.ku.ac.th

secondary for eng.ku.ac.th

secondary for cpe.ku.ac.th

ns.eng.ku.ac.th

ns.ku.ac.th

nontri.ku.ac.th

ns2.eng.ku.ac.th

ns.cpe.ku.ac.th

cc2.cpe.ku.ac.th

14

Root Name Server

• name server must contact other name servers for non local IP

• it has to know IP address of the top most server called root name server

• root name server - provide the names and address of the name server authoritative for top level domain name

15

Root Name Server

• 13 root servers are currently available in Internet

A.ROOT-SERVERS.NET 198.41.0.4

B.ROOT-SERVERS.NET 128.9.0.107

C.ROOT-SERVERS.NET 192.33.4.12

D.ROOT-SERVERS.NET 128.8.10.90

E.ROOT-SERVERS.NET 192.203.230.10

F.ROOT-SERVERS.NET 192.5.5.241

G.ROOT-SERVERS.NET 192.112.36.4

H.ROOT-SERVERS.NET 128.63.2.53

I.ROOT-SERVERS.NET 192.36.148.17

J.ROOT-SERVERS.NET 198.41.0.10

K.ROOT-SERVERS.NET 193.0.14.129

L.ROOT-SERVERS.NET 198.32.64.12

M.ROOT-SERVERS.NET 202.12.27.33

http://www.icann.org/committees/dns-root/y2k-statement.htm

16

Name Resolution Process

address of www.psu.ac.th?

address of www.psu.ac.th?

address of www.psu.ac.th?

address of www.psu.ac.th?

resolver

th

ac

psu

www

or

ku

root name server

th name server

ac.th name server

psu.ac.th name server

ask th name servers

ask ac.th name servers

ask psu.ac.th name server

address of www.psu.ac.th?

address is 192.168.100.61

name server

ns.psu.ac.th name server

ask ns.psu.ac.th name server

address of www.psu.ac.th?

address is 192.168.100.61

17

Reverse Resolution

au th

ac

psu

www cs

ku

www. psu.ac.th

arpa

in-addr

root

0

0

0

0

255

255

255

255

61

100

168

192

• in-addr.arpa domainin reverse directionof IP address

61.100.168.192.in-addr.arpa

18

Caching

• all name servers employ a cache to reduce the DNS traffic

• standard UNIX keep cache in name server with time-out

• cache data is non-authoritative

19

DNS message format fixed 12 byte header with 4 variable length fields DNS message format is defined for both queries and answers

0 15 16 31

identification:16 flags:16

number of questions :16 number of answer RRs:16

number of authority RRs :16 number of additional RRs:16

1 or more questions

1 or more answers

1 or more authority

1 or more additional information

==

==

==

==

fixed header

query

reply

20

DNS message format: detail

• set by the client and return by the server

• lets the client match responses to requests

0 15 16 31

Identification : 16 Flags : 16

21

DNS message format: detail

identification:16 flags:16 identification:16 flags:16

QR opcode AA TC RD RA (zero) rcode

QR opcode AA TC RD RA (zero) rcode 1 4 1 1 1 1 3 4

QR 0= query, 1= response

opcode 0= standard query, 1=inverse query, 2=server status request

AA 0= authoritatived answer,1 = non authoritatived answer

TC 1= truncated. using UDP, reply was>512 bytes, return only 512 bytes

RD 1= recursive desired, 0= iterative

RA 1= recursion available (server support recursion)

rcode return code : 0=no error, 3=name error

22

DNS message format: detail

questions :32 questions :32

query name query name

query type query class query type query class

3www2cs3psu2ac2th0 query name is the name being lookup sequence of label begins with 1-byte count

A 1 IP address

NS 2 name server

CNAME 5 canonical name

PTR 12 pointer record

HINFO 13 host info

MX 15 mail exchange record

query type indicates desired response

query class normally =1 meaning Internet address

23

DNS message format: detail

• domain name : corresp. response name, (query name format)

• type : response RR type code (see query type)

• time-to-live : cache life time of RR (often 86400=1 day)

• resource length : specify the size of resource data

• resource data : the answer, e.g. IP address or other type

1 or more answers

1 or more authority

1 or more additional information

===

===

reply

these three fields share a common resource record (RR)

domain name type class

time-to-live

resource data

domain name type class

time-to-live

resource data

resource length

=

= =

=

24

Operation

• use port 53

• typically UDP request and reply• if answer is too big, use TCP

ip hdr UDP hdr DNS hdr query answer#1 answer#2 ip hdr UDP hdr DNS hdr query answer#1 answer#2

25

Resolves file

• resolver must have address for local name server

• /etc/resolv.conf on UNIX

/etc/resolv.conf

#domain

domain cs.psu.ac.th

#list of name server

192.100.77.5

192.100.77.2

26

Setting up DNS

• BIND (Berkeley Internet Name Domain) package

• /usr/somewhere/in.named - BSD named DNS server

• /etc/named.boot - named configuration (tell named where to find database files)