Chapter 23 The Domain Name System (DNS)

Chapter 23

The Domain Name System (DNS)

吳俊興

國立高雄大學 資訊工程學系

CSC521 – Communication Protocols

Outline

2. Names For Machines 3. Flat Namespace

4. Hierarchical Names

5. Delegation Of Authority For Names

6. Subset Authority

7. Internet Domain Names 8. Top-Level Domains

9. Name Syntax And Type

10. Mapping Domain Names To Addresses

11. Domain Name Resolution 12. Efficient Translation

13. Caching: The Key To Efficiency 14. Domain Name System Message

Format

15. Compressed Name Format

16. Abbreviation Of Domain Names 17. Inverse Mappings

18. Pointer Queries

19. Object Types And Resource Record Contents

20. Obtaining Authority For A Subdomain 21. Dynamic DNS Update And

Notification

22. DNS Security Extensions (DNSSEC) 23. Summary

Names For Computers

• Humans prefer pronounceable names rather than numeric

addresses

• Two possibilities of names

– Flat namespace

– Hierarchical namespace

• Two possibilities of naming hierarchy

– According to network topology

– By organizational structure (independent of physical networks) Internet uses the latter

Internet Domain Names

• Internet Hierarchy

– In the Internet, hierarchical machine names are assigned according to the structure of organizations that obtain authority for parts of the namespace, not necessarily according to the structure of the physical network interconnections

• Flexible hierarchy

– Universal naming scheme (same everywhere)

– Each organization determines internal naming structure

• Mechanism known as Domain Name System (DNS)

– Name assigned to a computer known as domain name

• Domain Name Syntax

– Set of labels separated by delimiter character (period) – Example: cs . purdue . edu

Original Top-Level Domains

• Meaning assigned to each

– ccTLD: Country Code TLD – gTLD: Generic TLD

• Three domains considered generic

– .com – .net – .org

• Proponents argued (incorrectly) that DNS would collapse

without additional TLDs

• New TLDs created legal nightmare

• Authority delegated down the tree

• Example

– Purdue University registers under top level domain .edu and receives authority for domain purdue.edu

– Computer Science Department at Purdue registers with the Purdue authority, and becomes the authority for domain cs.purdue.edu

– Owner of a lab in the CS Department registers with the departmental authority, and becomes the authority for domain xinu.cs.purdue.edu

Mapping Domain Names To Addresses

• DNS uses a set of on-line servers

• Servers arranged in tree

• Given server can handle entire subtree

– Example: ISP manages domain names for its clients

• DNS Database: Record has ( name, class )

– Class specifies type of object (e.g., computer, email exchanger)

• Consequence:

– A given name may map to more than one item in the domain system. The client specifies the type of object desired when resolving a name, and the server returns objects of that type

• DNS server known as name server

– Single server can handle multiple levels of the naming tree – Example: root server handles all top-level domains

• DNS client software known as resolver

• Conceptually, must search from root of tree downward

• In practice

– Every name server knows location of a root server – Only contacts root if no subdomain known

– Lookup always starts with local server first (host can learn address of DNS server from DHCP)

• Efficient Translation

– Most lookups refer to local names

– Name-to-address bindings change infrequently – User is likely to repeat same lookup

• To increase efficiency

– Initial contact begins with local name server

Resolution Example

requesting host

pc1.csie.nuk.edu.tw www.google.com

local DNS server

ccdns.nuk.edu.tw

(140.127.198.1)

1

root DNS server

2

3 4

5

6

authoritative DNS server

ns1.google.com 7 8 .com TLD DNS server Host pc1.csie.nuk.edu.tw wants IP address for

www.google.com

Later:

• host pc1.csie.nuk.edu.tw wants IP address for mail.google.com • host pc2.csie.nuk.edu.tw wants

DNS Records

DNS: distributed database storing resource records (RR)

• Type=NS

– name is domain (e.g. foo.com)

– value is IP address of

authoritative name server for

RR format:

• Type=A

– name is hostname

– value is IP address

• Type=CNAME

– name is alias name for some

“cannonical” (the real) name

www.ibm.com is really

servereast.backup2.ibm.com

– value is cannonical name

• Type=MX

DNS Message Format

TYPE meaning

A 1 a host address

NS 2 an authoritative name server CNAME 5 canonical name for an alias SOA 6 start of a zone of authority PTR 12 a domain name pointer MX 15 mail exchange

Specifying Domain Names with Labels

• Domain name

– a sequence of labels

– A maximum length of 255 characters is allowed

• Label

– Must start with a letter and can only consist of letters, digits and hyphens – Can be up to 63 characters long and are case insensitive (00xx xxxx)

• Label encoding

– Begins with an octet that specifies its length; 0 marks the end

– i.e. www.google.com = 0x03 w w w 0x06 g o o g l e 0x03 c o m 0x00

• Label compression with pointer

– Begins with two ones (11xx xxxx xxxx xxxx) – Followed with the offset field

DNS

Query

Example

Looking up www.google.com.tw

– Parameter=0x0100=0000 0001 0000 0000: Recursion desired – Class=INternet: 0x01

DNS

Reply

Domain Name Abbreviation

• Abbreviation Of Domain Names

– DNS only recognizes full domain names – Client software allows abbreviation

• Example of Abbreviation

– Client configured with suffix list • . cs . purdue . edu

• . cc . purdue . edu • . purdue . edu • null

– User enters abbreviation xinu – Client tries the following in order

• xinu. cs . purdue . edu • xinu. cc . purdue . edu • xinu. purdue . edu • xinu

Pointer Query

• Inverse Query

– Map in reverse direction – Excessive overhead

– May not have unique answer – Not used in practice

• Pointer Query

– Special case of inverse mapping – Convert IP address to domain name

– Trick: write IP address as a string and look up as a name

• Example

– Start with dotted decimal address such as – aaa . bbb . ccc . ddd

Discussions

• Internationalized Domain Name

– Label extension?

• TTL Issues

– DNS Caching – Dynamic Update

• DNS Security Issues

– Authentication – Data integrity

• Performance Issues

– ARP: cache / ? – IP: routing table /?

Summary

• Domain Name System provides mapping from pronounceable

names to IP addresses

• Domain names are hierarchical; top-level domains are dictated

by a central authority

• Organizations can choose how to structure their domain names

• DNS uses on-line servers to answer queries