Module 2.2: Domain Name System

K. Salah 1

Module 2.2: Domain Name System

• IP assigns 32-bit addresses to hosts (interfaces) – Binary addresses easy for computers to manage – All applications use IP addresses through the TCP/IP

protocol software – Difficult for humans to remember:

% telnet 134.82.11.70

•The Domain Name System (DNS) provides translation between symbolic names and IP addresses

•DNS runs over UDP and uses port 53 of messages less than 512 bytes; otherwise, it uses TCP port 53

K. Salah 2

Structure of DNS names

• Each name consists of a sequence of alphanumeric components separated by periods

• Examples:

www.eg.bucknell.edu

www.netbook.cs.purdue.edu

charcoal.eg.bucknell.edu

• Names are hierarchical, with most-significant component on the right

• Left-most component is computer name

K. Salah 3

DNS naming structure

• Top level domains (right-most components; also known as TLDs) defined by global authority

com Commercial organization

edu Educational institution

gov Government organization

mil Military organization

• Organizations apply for names in a top-level domain:

kfupm.edu

macdonalds.com

• Organizations determine own internal structure

ccse.kfupm.edu

cs.purdue.edu

K. Salah 4

Geographic structure

• Top-level domains are US-centric

• Geographic TLDs used for organizations in other countries:

• Countries define their own internal hierarchy: ac.uk and .edu.au are used for academic organizations in the United Kingdom and Australia. In SA, it is edu.sa.

K. Salah 5

Domain names within an organization

• Organizations can create any internal DNS hierarchy

• Uniqueness of TLD and organization name guarantee uniqueness of any internal name (much like file names in your directories)

• All but the left-most component of a domain name is called the domain for that name:

• Authority for creating new subdomains is delegated to each domain

• Administrator of kfupm.edu has authority to create eg.kfupm.edu and need not contact any central naming authority

• DNS domains are logical concepts and need not correspond to physical location of organizations

• DNS domain for an organization can span multiple networks

K. Salah 6

Domain name space

• Names are defined in an inverted-tree structure with the root at the top.

• Can have 128 levels: level 0 (root) to level 127.

• Label: – Each node in the tree has a level– Maximum of 63 characters.– Root label is a null string (empty string).– Children of a node have different labels.

K. Salah 7

Domain names and labels

• Full domain name is a sequence of labels separated by dots.

• Domain names are always read from the node up to the root. Last label is the label of root (null). So, full domain name always ends in a null label [means dot].

K. Salah 8

FQDN and PQDN

• Fully Qualified Domain Name (FQDN) or Absolute Domain Name– Label is terminated by a null string.– Contains the full name of a host.

• Partially Qualified Domain Name (PQDN) or Relative Domain Name– Not terminated by a null string.– Used when the name to be resolved belongs to the same

site as the client.– Resolver supplies the missing part called as suffix.

• Why Absolute and Relative? Convenience.

K. Salah 9

Domains

• Domain: – Subtree of the domain name space.– Name of the domain is the domain name of the node

at the top of the subtree.– A domain can be divided into subdomains.

K. Salah 10

DNS and client-server computing

•DNS names are managed by a hierarchy of DNS servers

•Root server at top of tree knows about next level servers.

•Next level servers, in turn, know about lower level servers

•Some Jargon– Each DNS server is the authoritative server for the names it

manages – What a server is responsible for or has authority over is called a

zone. A domain can span multiple servers.– Primary server is also called authoritative server– Second server has a copy

K. Salah 11

Root Name Servers

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

e NASA Mt View, CAf Internet Software C. Palo Alto, CA

i NORDUnet Stockholm

k RIPE London

m WIDE Tokyo

a NSI Herndon, VAc PSInet Herndon, VAd U Maryland College Park, MDg DISA Vienna, VAh ARL Aberdeen, MDj NSI (TBD) Herndon, VA

13 root name servers worldwide, according to www.dnso.org 2005

Root Server

K. Salah 12

Choosing DNS server architecture

• Small organizations can use a single server

– Easy to administer

– Inexpensive

• Large organizations often use multiple servers

– Reliability through redundancy

– Improved response time through load-sharing

– Delegation of naming authority

• Locality of reference applies - users will most often look up names of computers within same organization

• All DNS servers are linked together to form a unified system. Each server knows how to reach a root server and how to reach servers that are authorities for names further down the hierarchy.

K. Salah 13

Name Resolution

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server

local name serverdns.eurecom.fr

1

23

4 5

6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

host surf.eurecom.fr wants IP address of gaia.cs.umass.edu

1. contacts its local DNS server, dns.eurecom.fr

2. dns.eurecom.fr contacts root name server, if necessary

3. root name server eventually contacts authoritative name server, dns.cs.umass.edu, if necessary

• This is called “Recursive Resolution”

K. Salah 14

Types of Queries

recursive query:

puts burden of name resolution on contacted name server

heavy load?

iterated query:

contacted server replies with name of server to contact

“I don’t know this name, but ask the following server(s)”

Gives more control to client

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server

local name serverdns.eurecom.fr

1

23

4

5 6

authoritative name serverdns.cs.umass.edu

7

8

iterated query

K. Salah 15

DNS caching

•DNS resolution can be very inefficient

• Every host referenced by name triggers a DNS request

• Every DNS request for the address of a host in a different organization goes through the root server

•Servers and hosts use caching to reduce the number of DNS requests

• Cache is a list of recently resolved names and IP addresses

• Authoritative server include time-to-live with each reply

•DDNS (Dynamic Domain Name System)

• When a new node is added/deleted authoritative DNS server is updated accordingly.

• When a new domain is added, update message is sent to upper servers

• Servers query other servers periodically for latest changes update/notify mechanisms under design by IETF

RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.html

K. Salah 16

DNS records

DNS: distributed db storing resource records (RR)

•Type=NS– name is domain (e.g.

foo.com)– value is IP address of

authoritative name server for this domain

RR format: (name, TTL, class, type, value)

•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– value is name of

mailserver associated with name

Class is typically “IN” information, and TTL is in seconds

K. Salah 17

DNS protocol, messages

DNS protocol : query and reply messages, both with same message format

msg header

•identification: 16 bit # for query, reply to query uses same #

•flags:– query or reply– recursion desired – recursion available– reply is authoritative

K. Salah 18

DNS protocol, messages

Name, type fields for a query

RRs in reponseto query

records forauthoritative servers

additional “helpful”info that may be used

K. Salah 19

Example using “dig”

The number next to MX is the preference value in case you get multiple replies.

K. Salah 20

Content distribution networks (CDNs)

• A different business model than web caching

• The content providers(CNN, Yahoo, Lycos, Google, etc) are the CDN customers.

Content replication

• CDN company (e.g. Akamai.com) installs hundreds of CDN servers throughout Internet– in lower-tier ISPs, close to

users

• CDN replicates its customers’ content in CDN servers. When provider updates content, CDN updates servers

• But how it works?

origin server in North America

CDN distribution node

CDN serverin S. America CDN server

in Europe

CDN serverin Asia

K. Salah 21

CDN example

Origin server

• www.foo.com

• Distributes only GIF files to CDN

• In each HTML page, it replaces: http://www.foo.com/sports.ruth.gif with

http://www.cdn.com/www.foo.com/sports/ruth.gif

HTTP request for

www.foo.com/sports/sports.html

DNS query for www.cdn.com

HTTP request for

www.cdn.com/www.foo.com/sports/ruth.gif

1

2

3

Origin server

CDNs authoritative DNS server

NearbyCDN server

CDN company

• cdn.com

• distributes gif files

• uses its authoritative DNS server to return the IP of the nearby CDN server

Note: No changes is required to DNS, HTTP, or browser to implement this.

K. Salah 22

More about CDNs

routing requests

•CDN creates a “map”, indicating distances from leaf ISPs and CDN nodes

•when query arrives at authoritative DNS server:

– server determines ISP from which query originates

– uses “map” to determine best CDN server

shortest distance Least loaded

not just Web pages

•streaming stored audio/video

•streaming real-time audio/video