DNS -- Domain Name System

or Domain Name Server Domain nameCollege of Electrical an Communication Engineering *.en.yzu.edu.twDepartment of Electrical Engineering *.ee.yzu.edu.twDepartment of Electro-Optical Engineering *.eo.yzu.edu.twDepartment of Communications Engineering *.comm.yzu.edu.tw

References

• Computer Networking: A Top Down Approach ,Chapter 2, 4th edition. Jim Kurose, Keith Ross, Addison-Wesley, July 2007.

• http://en.wikipedia.org/wiki/Domain_Name_System

• YouTube: DNS "Domain Name System" Basics

• RFC 1034,1035, 2136 …

YZU DNS Setting

140.138.181.125

router

ADSLTANET

DNS server

140.138.138.250

140.138.2.104 140.138.2.109140.138.2.107

7 Building DNS Mech DNS

dns.eo.yzu.edu.twdns.eed.yzu.edu.tw

Outline

• Introduction to Domain Names

• Management of Domain Servers

• DNS Protocol

5

Part 1Introduction to Domain

Name

Address vs. Name

• Internet hosts, routers:– IP address (32 bit), e.g., 140.138.8.110 -

used for addressing datagrams– “name”, e.g., www.yzu.edu.tw - used by

humans

Q1: How to get a name for a specific IP address?

Q2: How to map between IP addresses and domain name on the network?

Way 1 -- Host Table

• Mapping between IP addresses and host names.

• Host table is the file /etc/hosts.• Static information• You had better have a host table which

contains the information about local host, gateways and other servers. If the DNS does not work, you only have your host table.

Way 2 -- DNS

• Domain Name System

• DNS translates domain names meaningful to humans into the IP addresses.– Address resolution ： translation of a

domain name into an IP address 。– Reverse lookup ： a query of the DNS for

domain names when the IP address is known.

Get Domain Name

• The right to use a domain suffix is delegated by domain name registrars which are accredited by ICANN. – Internet Corporation for Assigned Names and

Numbers– Network Information Centers (NIC)

• TWNIC （ http://www.twinc.net.tw ）– Assign IP address & Maintain DNS servers

Hierarchy of Domain Name

root

yzu

commmech

Top Level Domain (TLD)

Root

Second Level Domain (SLD)

Root Name Servers

• Contact authoritative name server if name mapping not known

• Gets mapping

• Returns mapping to local name server

13 root name servers worldwide

b USC-ISI Marina del Rey, CAl ICANN Los Angeles, CA

e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 36 other locations)

i Autonomica, Stockholm (plus 28 other locations)

k RIPE London (also 16 other locations)

m WIDE Tokyo (also Seoul, Paris, SF)

a Verisign, Dulles, VAc Cogent, Herndon, VA (also LA)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 21 locations)

Domain Name Space

• The management organization for Internet domain name and IP address （ ICANN ） separates domain names into 3 sections ：– Generic Domains (gTLD)– Country Domains (ccTLD)– Inverse Domains

Inverse domain

country domain

generic domain

root

Original Generic (gTLD)

Domain Name Assigned To

com Commercial organization

edu Educational institution

gov Government organization

mil Military group

net Major network support center

org Organization other than those above

arpa Temporary ARPA domain

int International organization

New Generic (gTLD)

• New root domain on Dec, 2000Domain Name Assigned To

info Commercial organization

biz Educational institution

name Government organization

pro Military group

museum Major network support center

coop Organization other than those above

aero Temporary ARPA domain

Country Code (ccTLD)

• country code: A country

• au Australia

• ca Canada

• cn Mainland China

• de Germany

• fr France

• jp Japan

• nz New Zealand

• tw Taiwan

• uk United Kingdom

Inverse Domains

Inverse domain

root

121.45.34.132.in-addr.arpa.

arpa

in-addr

132

34

45

121

Index to names

Search the domain name of 132.34.45.121

17

Part 2Management of Name

Servers

What Is Domain

• DNS ： a tree structure

• Domain ： a subtree

• The host magi.rd.hgiga.com belongs to domain rd.hgiga.com and domain hgiga.com.

domain

subdomainAuthorized by parent domain

Written Types of Domain Name

• Full Qualified Domain Name (FQDN)– Ex: pcs.comm.yzu.edu.tw.– Unique

• Partially Qualified Domain Name (PQDN)– Ex: pcs.comm– Without the ".", DNS Client will automatically

add the suffix yzu.edu.tw to form the FQDN.

Query Process

• Some OS’s resolver asks host table first.

• Resolver asks default DNS servers (local DNS)– DNS searches its database for domain names

in its domain (authoritative answer due to authoritative DNS server).

– Then search cache. – If no answer exists, it asks the root server.

• Discussed later…

Management Based on Domain

Q: Problem ?

Zone• magi.rd.hgiga.com belongs to the zone

rd.hgiga.com but not to hgiga.com.

Graphical representation• walnut.candy.foobar.com

Examples of Partitions

Distributed Architecture

• Many name servers are implemented in hierarchical architecture.

• Application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation)

How to Get magi.rd.hgiga.com IP

TLD and Authoritative Servers

• Top-level domain (TLD) servers– Responsible for com, org, net, edu, etc, and

all top-level country domains uk, fr, ca, jp.

• Authoritative DNS servers– Organization’s DNS servers, providing

authoritative hostname to IP mappings for organization’s servers (e.g., Web, mail).

– Can be maintained by organization or service provider

– Master and Slave DNS servers

Local Name Server

• Each ISP (residential ISP, company, university) has one.– Close to the hosts– Also called “default name server”

• When host makes DNS query, query is sent to its local DNS server– Acts as proxy, forwards query into hierarchy

• Does not belong to hierarchy

How to Get IP Addresses

• Resolver queries local name server

• Local name server that can not resolve name contacts to root server

• Example: Host at cis.poly.edu wants IP address for gaia.cs.umass.edu– Iterated query– Recursive query

requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

23

4

5

6

authoritative DNS serverdns.cs.umass.edu

78

TLD DNS server

Iterated Query

• Contacted server replies with name of server to contact

• “I don’t know this domain name, but ask this server”

requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

2

45

6

authoritative DNS serverdns.cs.umass.edu

7

8

TLD DNS server

3

Recursive Query

• Puts burden of name resolution on contacted name server

• Heavy load?

Caching and Updating Records

• Once name server learns mapping, it caches mapping– Cache entries timeout (disappear) after

some time– TLD servers typically cached in local name

servers• Thus root name servers not often visited

• Update/notify mechanisms– RFC 2136– http://www.ietf.org/html.charters/dnsind-charter.html

Why Not Centralization DNS?

• Distant centralized database

• Traffic volume

• Maintenance

• Doesn’t scale!

• A single point of failure

• More secure!

Services Provided by DNS

• Hostname to IP address translation

• Host aliasing– Canonical, alias names

• Mail server aliasing

• Load distribution– replicated Web servers: set of IP

addresses for one canonical name

Optimization of DNS performance

• Replicating– Each root server is replicated– Many copies of the server exist around the world– In practice, the geographically closest server usually

responds best

• Caching– Each server maintains a cache of names– Caching work well because name resolution shows a

strong tendency toward temporal locality of reference

36

Part 3DNS Protocol

DNS Records: Resource Record

RR format: (name, value, type, ttl)

• Register name networkuptopia.com at DNS registrar – registrar inserts two RRs into com TLD server

(networkutopia.com, dns1.networkutopia.com, NS)(dns1.networkutopia.com, 212.212.212.1, A)

– Provide authoritative server (master and slave) for networkutopia.com

– Create IP addresses of authoritative server for www.networkuptopia.com

Domain / Question Type

Type Mnemonic Description

1 A Domain name →IPv4 Address.

2 NS Name Server. Find the authoritative

name server for the zone.

12 PTR Pointer. IP → domain name.

15 MX Mail Exchange. Find the mail

server of the zone.

255ANY A request for all records.

(networkutopia.com, dns1.networkutopia.com, NS)(dns1.networkutopia.com, 212.212.212.1, A)

Resource Record Format

Domain name

Domain Type Domain Class

TTL (Time to Live)

Resource data length

Resource Data

DNS Messages

• DNS protocol : query and reply messages, both with same message format– Query ： Client → Server

• Header+Question section

– Response ： Server → Client

• Header+Question +Answer +Authoritative+Additional

Header

Question section

Answer section

Authoritative section

Additional section

Header

Question section

Header

Opcode

Identification (16 bits)

Number of question records

Flags (16 bits)

Number of authoritative records

Number of answer recodes

Number of additional records

QR

0 1 5 6 7 8 9 10 11 12 15

AA TC RD RA rCode

Flag

Z AD CD

QR=0: query, QR=1: response

Opcode=0: standard, Opcode=1: inverse, Opcode=2: server status request

12 bytes

Flags

• AA=1: authoritative answer (for response)

• TC=1: truncated to 512 bytes (for UDP)

• RD=1: recursive desired (set by query)

• RA=1: recursive available (for response)

• AD=1: authenticated data

• CD: checking disable

• rCode: status of error in response

Question Record Format

Query name

Query Type Query Class

Query name format

5 a d m i n 3 a t c 4 f h d a 3 e d u 0

Query Type: which Resource Data (ex: A, PTR, Mx, or NS)

Query Class=1: Internet (only one)

Other Sections

• Answer Section: a list of resource record

• Authoritative Section: the information of (1 or more) authoritative servers

• Additional Information Section: other response message, ex: IP of authoritative server

45

Part 4DNS Tools

nslookup

• A software for debugging# nslookup

Default Server: dorm-dns.yzu.edu.tw

Address: 140.138.2.107

>www.yzu.edu.tw

Server: dorm-dns.yzu.edu.tw

Address: 140.138.2.107

Name: www.yzu.edu.tw

Address: 140.138.8.10

nslookup -MX （ 1/2 ）> set type=MX

> mail.yzu.edu.tw

Default Server: dorm-dns.yzu.edu.tw

Address: 140.138.2.107

Non-authoritative answer:

mail.yzu.edu.tw mail exchanger = 30 nopam-3.yzu.edu.tw.

mail.yzu.edu.tw mail exchanger = 1 yzu-mx2.yzu.edu.tw.

mail.yzu.edu.tw mail exchanger = 10 mx3.yzu.edu.tw.

mail.yzu.edu.tw mail exchanger = 20 mx2.yzu.edu.tw.

nslookup -MX （ 2/2 ）Authoritative answers can be found from:

yzu.edu.tw nameserver = taurus.yzu.edu.tw.

yzu.edu.tw nameserver = dorm-dns.yzu.edu.tw.

yzu.edu.tw nameserver = dorm-dns2.yzu.edu.tw.

yzu-mx2.yzu.edu.tw internet address = 140.138.36.215

mx3.yzu.edu.tw internet address = 140.138.36.126

mx2.yzu.edu.tw internet address = 140.138.36.13

taurus.yzu.edu.tw internet address = 140.138.2.104

dorm-dns.yzu.edu.tw internet address = 140.138.2.107

dorm-dns2.yzu.edu.tw internet address = 140.138.2.109

>

nslookup-NS> set type=NS

> mail.yzu.edu.tw

Server: dorm-dns.yzu.edu.tw

Address: 140.138.2.107

Authoritative answers can be found from:

yzu.edu.tw

origin = taurus.yzu.edu.tw

mail addr = joejoe.saturn.yzu.edu.tw

serial = 2011032201

refresh = 3600

retry = 3600

expire = 3900

minimum = 3600

>

nslookup-any> server 140.138.2.109

Default server: 140.138.2.109

Address: 140.138.2.109#53

> set type=any

> mail.yzu.edu.tw

Default server: 140.138.2.109

Address: 140.138.2.109

mail.yzu.edu.tw mail exchanger = 10 mx3.yzu.edu.tw.

mail.yzu.edu.tw mail exchanger = 20 mx2.yzu.edu.tw.

mail.yzu.edu.tw mail exchanger = 30 nopam-3.yzu.edu.tw.

mail.yzu.edu.tw mail exchanger = 1 yzu-mx2.yzu.edu.tw.

Name: mail.yzu.edu.tw

Address: 140.138.36.14

>

Who is (1/3)

• http://www.twnic.net.tw/ (English version)• Whois: yzu + .tw( 泛英 / 泛中 )Domain Name: yzu.tw

Registrant: Ministry of Education Computer Center 12th Fl, 106, Hoping E. Road, Sec 2. Taiwan Republic of China, R.O.C

Contact: TANet, Administrator tanet@adm.edu.tw

TEL: tanetadm.edu.tw

Record expires on 2012-10-27 (YYYY-MM-DD)

Record created on 2005-10-27 (YYYY-MM-DD)

Registrar: EDU

Whois (2/3)

• Whois 查詢 yzu.edu + .twMinistry of Education Computer Center 12th Fl, 106,

Hoping E. Road, Sec 2. Taiwan Republic of China, R.O.C TW

Domain Name: edu.tw

Contact: TANet, Administrator tanetadm@moe.edu.tw 886-2-87329007

Whois (3/3)

• Whois 查詢 140.138.137.171 + IP

用戶單位 :140.138.0.0/16

Netname T-YZU.EDU.TW-NET

Registered Date 1992-05-24

Admin. Contact abuse@yzu.edu.tw

Tech. Contact abuse@yzu.edu.tw

IP代理發放單位網段 :140.138.0.0-140.138.255.255

Chinese Name 教育部 Netname TANET-NET

Organization Name Ministry of Education Computer Center

Street Address 12F, No 106, Sec.2,Hoping E. Rd.,

AdminHandle chuang@mail.moe.gov.tw

TechHandle tanetadm@moe.edu.tw

SpamHandle tanetadm@moe.edu.tw