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Web Server Administration

Web Server Administration

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Web Server Administration

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  1. Web Server Administration Chapter 4 Name Resolution

  2. 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 2000 and 2003 • Troubleshoot DNS • Use WINS to resolve computer names in Windows

  3. 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 2000 or 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

  4. 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

  5. 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 •, • A subdomain is a further division of a second-level domain • For, there is • Not common

  6. Domain Namespaces • Second-level domains, such as have control over naming within their domain • Create hosts such as www, ftp, bb • A name such as is a fully qualified domain name (FQDN) • We could create subdomains such as phx •

  7. 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

  8. 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

  9. How DNS Works

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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)

  16. 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

  17. Caching and Forwarding Servers

  18. Zones • A zone is a part of the domain namespace • For a domain as small as, 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

  19. Zones • Zones must be contiguous • can be combined with • cannot be combined with • There must be one primary DNS server in each zone (plus a secondary server) • Each zone can have multiple secondary DNS servers

  20. 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

  21. Common DNS Records

  22. 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 • 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

  23. /etc/named.confCreating a DNS for the domain • Default setup is for localhost • In named.conf add the following line zone "" { type master; file “”; }; • This allows to be resolved by /var/named/ • There can be multiple domains in a single named.conf file

  24. /etc/named.conf • Also, we can add the following line zone “” IN { type master; file “named.0.168.192”; }; • This allows for reverse lookup for the domain • It uses all or part of the network

  25. /var/ $TTL 86400 @ IN SOA ( 2002072100 ; Serial 28800 ; Refresh 14400 ; Retry 3600000 ; Expire 86400 ) ; Minimum IN NS web1 IN A IN MX 10 web1 IN A www IN CNAME web1 research IN A IN MX 10 mail mail IN A

  26. named.0.168.192 $TTL 86400 @ IN SOA ( 2002072100 ; Serial 28800 ; Refresh 14400 ; Retry 3600000 ; Expire 86400 ) ; Minimum IN NS web1 100 IN PTR 150 IN PTR 200 IN PTR

  27. 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

  28. Configuring Client DNS in Linux • Modify /etc/resolv.config • The following line directs the client to use the DNS server at • nameserver • The following line associates this computer with the domain • domain

  29. 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 • Reboot and ping

  30. 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

  31. Finished DNS Configuration in Windows

  32. Troubleshooting DNSping • ping displays name resolution even if the computer cannot be contacted

  33. Troubleshooting DNSnslookup • nslookup can display information from the DNS server

  34. Troubleshooting DNSdig – available on Linux

  35. 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