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ITEC350 Networks ILecture 3
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DNS (Domain Name Service)
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Address “translation”
Domain Name Service (DNS) Given a Domain Name
(e.g., yahoo.com), lookup the IP address.
Command nslookup <somedomain.com> returns:
• DNS Server name & IP addr
• IP address(es) of the domain
Microsoft Windows 2000 [Version 5.00.2195](C) Copyright 1985-2000 Microsoft Corp.
H:\>nslookup yahoo.comServer: newriver.radford.eduAddress: 137.45.26.19
Non-authoritative answer:Name: yahoo.comAddresses: 64.58.79.230, 66.218.71.198
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U.S. Top Level Domains
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Address Resolution Early Internet had no Domain Name System – Just HOSTS
file Win2K: C:\WINNT\system32\drivers\etc\HOSTS Unix\Linux: /etc/hosts
Ancient History: Before DNS, The master HOSTS file was maintained by SRI International Periodically, every computer in the internet reloaded HOSTS file
~1984 someone realized that millions of computers and domains needed a central database – DNS was born
Still, when a host needs to translate yahoo.com, First, HOSTS file is scanned Then DNS is used.
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Domain Name Service (DNS)Developed by Postel & Mockapetriswww.internic.net is a good site to browseThe phone book of the internet
But more restrictive … Each entry must be• Unique• Authentic
Universal Resolvability is ensured Distributed Database
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Figure 1.27: Domain Name System (DNS)
DNS Request Message“The host name is Voyager.cba.hawaii.edu”
OriginatingHost
DNSServer
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Figure 1.27: Domain Name System (DNS)
DNS Table
Host Name……Voyager.cba.hawaii.edu…
IP Address……128.171.17.13…
DNS Response Message“The IP address is 128.171.17.13”Originating
HostDNS
Server
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DNS
13 Identical Root Servers All Top Level Domain (TLD) Registries Database for each TLD
• .com .org .edu .biz …. (gTLDs)
• .fr .ca etc. country-specific TLDs, or ccTLDs
Root Servers are Authoritative Maintained by ICANN, www.icann.org (International Corporation for Assigned Names and Numbers)
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Structure of DNSStructure of DNS
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The DNS TreeThe DNS Tree
jp uk com org edu
acco
keio
sfcmed
Root Zone File
icann
TLDs
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Map of the Root ServersMap of the Root Servers
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DNS
Why does Radford have a DNS server called newriver.radford.edu ? The 13 authoritative servers could not handle
the load of billions of name resolutions Faster to have a “non-authoritative” local server If there is a domain name that
newriver.radford.edu does not have• Newriver can contact one of the authoritative root
servers
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DNS NamesHierarchical, right to left
nth level ……. Top Level Domain (Label) yadda.yadda.yadda.yadda……….edu
TLD, or Label may be up to 63 chars longTotal length of name must be <= 255 charsTotal length <= 127 labelsDNS Names are either
Relative (newriver) Fully qualified (newriver.radford.edu, an actual host or
server)
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Sidebar on Domain Names in USA
ISO 3166 is a standard that defines a unique TLD for each country (i.e., .fr = France)
Why don’t we see radford.edu.us? Because the USA, alone among all other countries,
decided not to bother using the ISO’s TLD schemeOther variations adopted by some countries
include “sub-domains” e.g.: In the UK, ac.uk, co.uk, and sch.uk are reserved for
academic, company and schools In Australia, a blend of the US and ISO is used: .edu.au,
.com.au
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The root zone, AKA
DNS Zones: Any collection of hosts
.edu
radford.edu
Network lab in DA214
.
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DNS ServersEach DNS zone has
A domain name At least a primary server Probably a secondary server as well
A computer that maintains a single master list of DNS Names and IP Addresses for a zone Has Authority for that Zone Is known as the primary server for the zone
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Application (Host Process) Interfaces
Given a domain Name there are several methods of resolution Host table lookup (on Unix, /etc/hosts is a
text file ) Local name server process (on Unix, named) Send msg to a DNS primary or secondary
server
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NSlookup – DNS Translation
Manual Page for nslookup can be found at
http://www.stopspam.org/usenet/mmf/man/nslookup.html
http://www.kloth.net/services/nslookup-man.php It is for Unix shell, but most operation is same for WIN2K
or up. The command nslookup allows DNS translation to a
DOS cmd window.
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H:\>nslookupDefault Server: newriver.radford.eduAddress: 137.45.26.19> ?Commands: (identifiers are shown in uppercase, [] means optional)NAME - print info about the host/domain NAME using default serverNAME1 NAME2 - as above, but use NAME2 as serverhelp or ? - print info on common commandsset OPTION - set an option all - print options, current server and host [no]debug - print debugging information [no]d2 - print exhaustive debugging information [no]defname - append domain name to each query [no]recurse - ask for recursive answer to query [no]search - use domain search list [no]vc - always use a virtual circuit domain=NAME - set default domain name to NAME srchlist=N1[/N2/.../N6] - set domain to N1 and search list to N1,N2, etc. root=NAME - set root server to NAME retry=X - set number of retries to X timeout=X - set initial time-out interval to X seconds type=X - set query type (ex. A,ANY,CNAME,MX,NS,PTR,SOA,SRV) querytype=X - same as type class=X - set query class (ex. IN (Internet), ANY) [no]msxfr - use MS fast zone transfer ixfrver=X - current version to use in IXFR transfer requestserver NAME - set default server to NAME, using current default serverlserver NAME - set default server to NAME, using initial serverfinger [USER] - finger the optional NAME at the current default hostroot - set current default server to the rootls [opt] DOMAIN [> FILE] - list addresses in DOMAIN (optional: output to FILE) -a - list canonical names and aliases -d - list all records -t TYPE - list records of the given type (e.g. A,CNAME,MX,NS,PTR etc.)view FILE - sort an 'ls' output file and view it with pgexit - exit the program
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(Example) Nslookup
Server newriver.radford.edu Not authoritative for
yahoo.com Authoritative for
neelix• Relative form used
C:\>nslookup yahoo.comServer: newriver.radford.eduAddress: 137.45.26.19
Non-authoritative answer:Name: yahoo.comAddresses: 64.58.79.230, 66.218.71.198
C:\>nslookup neelixServer: newriver.radford.eduAddress: 137.45.26.19
Name: neelix.RADFORD.EDUAddresses: 137.45.192.213, 137.45.192.215
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DNS records
Domain Name Servers get their information from a database maintained by the domain administrator.
A client sends a message to the DNS using the UDP protocol.
A server has different types of entries
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DNS Query types
A, CNAME, NS, MX, PTR, SOA, TXT http://www.kloth.net/services/nslookup-man.php
Each query type returns slightly different subset of the total information.
Following slides were the output from http://www.kloth.net/services/nslookup.php Using radford.edu as the domain Each different query type selected in turn
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DNS Resource Records (RR)
Start of Authority (SOA) – denotes the primary DNS and time limits.
Address (A) – supplies a host name's IP address Canonical Name (CNAME) – provides alias host
names Mail Exchanger (MX) – defines a domain's mail
systems Name Server (NS) – defines a domain's name
servers
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DNS uses two request flavors
A recursive request will respond with the answer or an error message if the host is not known. This is the type of request made by a client when the user program executes a “gethostbyname” function.
An Interactive request will respond with the answer or the name of a DNS that may be able to answer the question. This type of request is usually used between Domain Name Servers. http://cr.yp.to/djbdns/intro-dns.html
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DNS Search Example
. (ROOT)
Newriver.radford.edu
Cist_hp_da.radford.edu
.COM DNS zone Server
DNS.yahoo.com
yahoo.com
Cist_hp_da.radford.edu needs the address of a web server on yahoo.com
1
234
5
67
8
Radford University
Yahoo
One of the 13 root servers
Each blue oval is a network operated by a distinct organization. (ISP networks not shown)
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Second DNS Search Example
. (ROOT)
Newriver.radford.edu
Cist_da_hp.radford.edu
.COM DNS
DNS.yahoo.com
ftp.yahoo.com
cist_da_hp.radford.edu needs the address of ftp.yahoo.com after
finding yahoo.com
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.arpa
An explicit way to signal for reverse translation
http://cr.yp.to/djbdns/dot-arpa.html
Reverse lookups
http://cr.yp.to/djbdns/intro-dns.html#reverse