Spring 2006 CS 332 1

Name Service (DNS)

OutlineTerminology Domain Naming System

Spring 2006 CS 332 2

Why A Name Service?

• Fundamental property of name: identify object

• Fundamental property of an address: locate object

• Using name service allows us to decouple these two different functions (though often times naming conventions can help with location, and vice versa)– Name can help locate objects, define membership in a group,

specify a role, etc.

• Differences (related to Internet):– Names more user friendly than IP addresses

– Names can have variable length

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Name Properties

• Location transparent or location-dependent • Flat or hierarchical • Global versus local • Absolute versus relative

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Terminology

• Name space: defines set of possible names– Flat: names not divisible into components– Hierarchical (e.g. Unix file names, Internet domain names)

• Naming system maintains a collection of bindings of names to values– Value can be anything we want naming system to return

when presented with a name

• Resolution mechanism: a procedure that, when invoked with a name, returns the corresponding value

• Name server: specific implementation of a resolution mechanism that can be queried via a network

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In The Beginning…

• Network Information Center (NIC) maintained a flat table of name-to-address bindings (called hosts.txt)

• To add host to Internet, email NIC with new name/address pair, which NIC manually added to table

• Modified table was mailed to every site every few days; sysadmins installed hosts.txt on every host

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Domain Name System (DNS)• Went online in mid 1980s• Hierarchical name space

– Names processed from right to left (though humans read them from left to right) with periods as field separators

– Ex. cleopatra.richmond.edu

edu com

princeton … mit

cs ee

ux01 ux04

physics

cisco … yahoo nasa … nsf arpa … navy acm … ieee

gov mil org net uk fr

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Domain Name System (DNS)• Distributed database of domain name bindings

– Each site (university department, campus, company, etc.) maintains its own part of database, along with a server

– Mapping is not necessarily host name to address

• Apps presented with host names query DNS to find address– Accessed through a resolver, which in Unix is primarily

reached via gethostbyname() and gethostbyaddress()

– resolver opens UDP connection to pass name service packets (IP address of server(s) in /etc/resolve.conf)

– resolver is not part of TCP implementation (or kernel)

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Example

Nameserver

Mailprogram

User

TCP

IP

2cs.princeton.edu

192.12.69.53

user @ cs.princeton.edu1

192.12.69.5 4

192.12.69.5 5

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DNS Implementation• Partition hierarchy into zones

edu com

princeton … mit

cs ee

ux01 ux04

physics

cisco … yahoo nasa … nsf arpa … navy acm … ieee

gov mil org net uk fr

• Zones correspond to administrative authority (NIC gets top)– Each zone implemented by two or more name servers

• Primary: get its info from disk files• Secondary: get their info from the primary (called a zone-transfer)

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DNS Implementation (cont.)

• DNS is really a hierarchy of name servers, rather than hierarchy of domains

• Servers respond either with info

requested or with IP address of

another name server• All primary servers must

know IP addresses of

all root servers

Rootname server

Princetonname server

Cisconame server

CSname server

EEname server

…

…

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Resource Records • Each name server maintains a collection of resource records

(Name, Value, Type, Class, TTL)

• Name/Value: not necessarily host names to IP addresses• Type: tells how value should be interpreted

– A: Value is an IP address (gives the name-to-address mapping)– NS: Value gives domain name for host running name server that knows

how to resolve names within specified domain.– CNAME: Value gives canonical name for a particular host; used to

define aliases (e.g. indirection for web server).– MX: Value gives domain name for host running mail server that accepts

messages for specified domain.

• Class: allow other entities to define types (Internet class is IN)• TTL: how long the resource record is valid (when TTL expires,

this record must be removed from cache)

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Root Server

(princeton.edu, cit.princeton.edu, NS, IN)

(cit.princeton.edu, 128.196.128.233, A, IN)

(cisco.com, thumper.cisco.com, NS, IN)

(thumper.ciscoe.com, 128.96.32.20, A, IN)

…

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Princeton Server

(cs.princeton.edu, optima.cs.princeton.edu, NS, IN)

(optima.cs.princeton.edu, 192.12.69.5, A, IN)

(ee.princeton.edu, helios.ee.princeton.edu, NS, IN)

(helios.ee.princeton.edu, 128.196.28.166, A, IN)

(jupiter.physics.princeton.edu, 128.196.4.1, A, IN)

(saturn.physics.princeton.edu, 128.196.4.2, A, IN)

(mars.physics.princeton.edu, 128.196.4.3, A, IN)

(venus.physics.princeton.edu, 128.196.4.4, A, IN)

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CS Server

(cs.princeton.edu, optima.cs.princeton.edu, MX, IN)

(cheltenham.cs.princeton.edu, 192.12.69.60, A, IN)

(che.cs.princeton.edu, cheltenham.cs.princeton.edu, CNAME, IN)

(optima.cs.princeton.edu, 192.12.69.5, A, IN)

(www.cs.princeton.edu, optima.cs.princeton.edu, CNAME, IN)

(baskerville.cs.princeton.edu, 192.12.69.35, A, IN)

(bas.cs.princeton.edu, baskerville.cs.princeton.edu, CNAME, IN)

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Name Resolution• Client sends request to a root server for cleopatra.mathcs.richmond.edu

• Root server returns the best info it has, which includes all information relating to richmond.edu (which should contain the name and address of a richmond.edu name server)

• Client queries richmond.edu name server, which returns name and address of mathcs.richmond.edu name server

• Client queries mathcs.richmond.edu name server and gets IP address of cleopatra.mathcs.richmond.edu

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How It Really Works• Client queries local name server, and name server

goes through resolution process on client’s behalf– Hosts must be configured with address of local name

server– Only local name server needs to know addresses of root

server(s)– Local server sees all DNS responses, and can thus

cache these (removing record when TTL expires)

• Partial names (e.g. cleopatra): client program is configured with local domain and appends this to incomplete names before query

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Example

Rootnameserver

Princetonnameserver

CSnameserver

Localnameserver

Client

1cicada.cs.princeton.edu

192.12.69.608

cicada.cs.princeton.edu

princeton.edu, 128.196.128.233

cicada.cs.princeton.edu

cicada.cs.princeton.edu,

192.12.69.60

cicada.cs.princeton.edu

cs.princeton.edu, 192.12.69.5

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