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Web Server Administration
Chapter 4Name Resolution
Overview 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
2003 Troubleshoot DNS
Understanding the DNS DNS is used to map host names to IP
addresses on the Internet 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 2003 network uses DNS to resolve computer names on a LAN
DNS in Windows is designed to be dynamic - as computers are added to the network, DNS automatically changes
Clients 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 The root level domain is "."
Significant in creating DNS files Top-level domains include com, org, fr
More have been added in 2000 Second-level domains are often owned by
companies and individuals microsoft.com, devry.edu
A subdomain is a further division of a second-level domain For devry.edu, there is phx.devry.edu Not common
Domain Namespaces Second-level domains, such as
devry.edu have control over naming within their domain
Create hosts such as www, ftp A name such as www.devry.edu is a
fully qualified domain name (FQDN) We could create subdomains such as
phx www.phx.devry.edu
New Top-Level Domains .biz - businesses .info - anyone can register .name - must register first and last
name .pro - for professionals only
must provide proof .aero, .museum, .coop are controlled by
organizations
Host Names 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 Name server – also known as DNS
server supports name-to-address and address-to-
name resolution Name resolver – also called DNS
client 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
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 Primary Server
Defines the hosts for the domain Maintains the database for the domain
It has authority for the domain Secondary Server
Gets data from primary server Provides fault tolerance and load
distribution Required for Internet domains
Primary and Secondary Servers 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 You contact them for changes
You can also split the servers ISP has primary, you have secondary You have primary, ISP has secondary
Primary and Secondary Servers
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
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 Caching Server
Resolves host names Caches (saves) the results Automatically installed when DNS is installed No configuration necessary
Forwarding Server Caching server that has access to the
Internet and forwards traffic from other caching servers
Caching and Forwarding Servers
Zones 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 Each zone typically has a separate DNS
Zones Zones must be contiguous
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 Forward Lookup
These zones contain entries that map names to IP addresses
Reverse Lookup 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 /etc/named.conf describes the files that
configure the zones There are two primary files that it describes
Forward lookup is described by named.technowidgets.com
It has the host names and how to handle e-mail Reverse lookup is described by
named.0.168.192 Can be necessary for e-mail (SMTP) and security
programs
/etc/named.confCreating a DNS for the technowidgets.com domain
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”;};
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 Also, we can add the following line
zone “0.168.192.in-addr.arpa” IN {type master;file “named.0.168.192”;};
This allows for reverse lookup for the domain
It uses all or part of the 192.168.0.0 network
/var/named.technowidgets.com
$TTL 86400@ IN SOA web1.technowidgets.com.
admn.technowidgets.com. ( 2002072100 ; Serial 28800 ; Refresh 14400 ; Retry 3600000 ; Expire 86400 ) ; Minimum IN NS web1
IN A 192.168.0.100 IN MX 10 mail.technowidgets.com.
web1 IN A 192.168.0.100www IN CNAME web1research IN A 192.168.0.150
IN MX 10 mailmail IN A 192.168.0.200
named.0.168.192$TTL 86400@ IN SOA web1.technowidgets.com.
admn.technowidgets.com. ( 2002072100 ; Serial 28800 ; Refresh 14400 ; Retry 3600000 ; Expire 86400 ) ; Minimum IN NS web1
100 IN PTR web1.technowidgets.com.150 IN PTR research.technowidgets.com.200 IN PTR mail.technowidgets.com.
Starting DNS in Linux To start DNS
/etc/rc.d/init.d/named start To restart DNS
/etc/rc.d/init.d/named restart To stop DNS
/etc/rc.d/init.d/named stop Make DNS start when you boot Linux
Add the command to start DNS to /etc/rc.d/rc.local
Configuring Client DNS in Linux Modify /etc/resolv.config The following line directs the client
to use the DNS server at 192.168.0.100 nameserver 192.168.0.100
The following line associates this computer with the technowidgets.com domain domain technowidgets.com
Test the DNS Configure a Windows PC to use the DNS
server 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 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 DNS is required for Active Directory Services
DNS as described for Linux can also be configured Wizards guide you through the configuration
Finished DNS Configuration in Windows
Troubleshooting DNSping
ping displays name resolution even if the computer cannot be contacted
Troubleshooting DNSnslookup nslookup can display information from the DNS server
Troubleshooting DNSdig – available on Linux
Summary 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