Web Server Administration
Chapter 4
Name Resolution
Overview
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Understand the domain name service (DNS)
Identify the components of DNS
Configure zone files
Install and configure DNS in Linux
Understand name resolution in Windows
Install and configure DNS in Windows 2000
and 2003
Troubleshoot DNS
Use WINS to resolve computer names in
Windows
Understanding the DNS
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DNS is used to map host names to IP addresses
on the Internet
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Also called name resolution or address resolution
Whenever a host is added, a configuration file has to be manually
changed
A host represents a service on a server such as FTP or a Web
server
There can be many hosts on a single computer
A Microsoft Windows 2000 or Windows 2003
network uses DNS to resolve computer names on
a LAN
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DNS in Windows is designed to be dynamic - as computers are
added to the network, DNS automatically changes
Clients
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On your PC, the TCP/IP configuration
contains the address(es) of your DNS
server(s)
Whenever you use a URL, whether in a
browser, or a utility such as ping, DNS
servers are used
Domain Namespaces
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The root level domain is "."
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Top-level domains include com, org, fr
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More have been added in 2000
Second-level domains are often owned by
companies and individuals
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Significant in creating DNS files
microsoft.com, devry.edu
A subdomain is a further division of a secondlevel domain
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For devry.edu, there is phx.devry.edu
Not common
Domain Namespaces
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Second-level domains, such as devry.edu
have control over naming within their domain
Create hosts such as www, ftp, bb
A name such as www.devry.edu is a fully
qualified domain name (FQDN)
We could create subdomains such as phx
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www.phx.devry.edu
New Top-Level Domains
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.biz - businesses
.info - anyone can register
.name - must register first and last name
.pro - for professionals only
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must provide proof
.aero, .museum, .coop are controlled by
organizations
Host Names
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The first portion of a URL is typically a host name
Typically different from the name of the computer
Many hosts can be associated with the same Web server
How DNS Works
DNS Components
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Name server – also known as DNS
server
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supports name-to-address and address-toname resolution
Name resolver – also called DNS client
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Can contact DNS server to lookup name
Used by browsers, e-mail clients, and client
utilities such as ping and tracert
DNS Servers that Define the
Internet
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Primary and secondary servers store the
host names used on the Internet
Caching and forwarding servers search
the Internet for host names
Primary and Secondary Servers
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Primary Server
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Defines the hosts for the domain
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Maintains the database for the domain
It has authority for the domain
Secondary Server
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Gets data from primary server
Provides fault tolerance and load distribution
Required for Internet domains
Primary and Secondary Servers
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If you use DNS, you will often work with your
ISP
In a simple environment, the ISP will have
the primary and secondary DNS servers
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You contact them for changes
You can also split the servers
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ISP has primary, you have secondary
You have primary, ISP has secondary
Primary and Secondary Servers
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ISP maintains DNS
You have to send changes to ISP
You have the secondary server which
gets updates from the primary server
Your users reference your secondary
server which is faster
Primary and Secondary Servers
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You have complete control over DNS
You can make changes whenever you
want
If your primary DNS goes down, the
secondary will continue to function (but
not indefinitely)
Resolve Host Names
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Caching Server
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Resolves host names
Caches (saves) the results
Automatically installed when DNS is installed
No configuration necessary
Forwarding Server
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Caching server that has access to the Internet and
forwards traffic from other caching servers
Caching and Forwarding Servers
Zones
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A zone is a part of the domain namespace
For a domain as small as technowidgets.com,
the domain name represents a single zone
For large organizations (such as IBM),
subdomains can be divided into separately
maintained zones
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Each zone typically has a separate DNS
Zones
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Zones must be contiguous
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admin.devry.edu can be combined with devry.edu
admin.devry.edu cannot be combined with
student.devry.edu
There must be one primary DNS server in
each zone (plus a secondary server)
Each zone can have multiple secondary DNS
servers
Zone File Configuration
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Forward Lookup
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These zones contain entries that map
names to IP addresses
Reverse Lookup
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These zones contain entries that map IP
addresses to names
Common DNS Records
DNS record
Function
Address (A)
Associates a host to an IP address.
Canonical name
(CNAME)
Creates an alias for a specified host.
Internet (IN)
Identifies Internet records; precedes most DNS record
entries.
Mail Exchanger
(MX)
Identifies a server used for processing and delivering e-mail
for the domain.
Name server (NS)
Identifies DNS servers for the DNS domain.
Pointer (PTR)
Performs reverse DNS lookups. Resolves an IP address to a
host name.
Start of Authority
(SOA)
Identifies the DNS server with the most current information for
the DNS domain.
DNS Configuration in Linux
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/etc/named.conf describes the files that
configure the zones
There are two primary files that it describes
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Forward lookup is described by
named.technowidgets.com
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It has the host names and how to handle e-mail
Reverse lookup is described by named.0.168.192
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Can be necessary for e-mail (SMTP) and security programs
/etc/named.conf
Creating a DNS for the technowidgets.com
domain
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Default setup is for localhost 127.0.0.1
In named.conf add the following line
zone "technowidgets.com" {
type master;
file “named.technowidgets.com”;
};
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This allows technowidgets.com to be resolved
by /var/named/named.technowidgets.com
There can be multiple domains in a single
named.conf file
/etc/named.conf
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Also, we can add the following line
zone “0.168.192.in-addr.arpa” IN {
type master;
file “named.0.168.192”;
};
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This allows for reverse lookup for the domain
It uses all or part of the 192.168.0.0 network
/var/named.technowidgets.com
$TTL
86400
@
IN SOA web1.technowidgets.com. admn.technowidgets.com. (
2002072100 ; Serial
28800
; Refresh
14400
; Retry
3600000 ; Expire
86400 ) ; Minimum
IN
NS
web1
IN
A
192.168.0.100
IN
MX 10 mail.technowidgets.com.
web1
IN
A
192.168.0.100
www
IN
CNAME web1
research IN
A
192.168.0.150
IN
MX 10 mail
mail
IN
A
192.168.0.200
named.0.168.192
$TTL 86400
@
IN SOA web1.technowidgets.com. admn.technowidgets.com. (
2002072100 ; Serial
28800
; Refresh
14400
; Retry
3600000 ; Expire
86400 ) ; Minimum
IN
NS
web1
100
150
200
IN
IN
IN
PTR
PTR
PTR
web1.technowidgets.com.
research.technowidgets.com.
mail.technowidgets.com.
Starting DNS in Linux
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To start DNS
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To restart DNS
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/etc/rc.d/init.d/named restart
To stop DNS
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/etc/rc.d/init.d/named start
/etc/rc.d/init.d/named stop
Make DNS start when you boot Linux
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Add the command to start DNS to
/etc/rc.d/rc.local
Configuring Client DNS in Linux
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Modify /etc/resolv.config
The following line directs the client to
use the DNS server at 192.168.0.100
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nameserver 192.168.0.100
The following line associates this
computer with the technowidgets.com
domain
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domain technowidgets.com
Test the DNS
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Configure a Windows PC to use the DNS server
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Start->Settings->Network and Dial-up Connections
Right-click on Local Area Connection and select
Properties
Select Internet Protocol (TCP/IP) and click on
Properties
Change DNS to 192.168.0.10
Reboot and ping www.technowidgets.com
Name Resolution in Windows
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NetBios (computer) names are broadcast to
the local network
Starting with Windows NT, WINS database
has computer name to IP address resolution
Windows 2000 introduces Dynamic DNS
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DNS is required for Active Directory Services
DNS as described for Linux can also be
configured
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Wizards guide you through the configuration
Finished DNS Configuration in
Windows
Troubleshooting DNS
ping
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ping displays name resolution even if the computer cannot be contacted
Troubleshooting DNS
nslookup
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nslookup can display information from the DNS server
Troubleshooting DNS
dig – available on Linux
Summary

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DNS is an application that translates names
to IP addresses and IP addresses to names
Organized in a hierarchical structure
Servers come in many forms: primary,
secondary, caching, forwarding
To configure DNS, set up a forward and
reverse zone
Use ping, nslookup, and dig to troubleshoot
DNS