CITA 310 Section 1 Name Resolution (Textbook Chapter 4)

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 changed A host represents a service on a server such as a Web server There can be many hosts on a single computer

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 is used

Domain Namespaces The root level domain is "." Significant in creating DNS files Top-level domains include com, org, fr 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

Domain Namespaces 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 is a fully qualified domain name (FQDN) We could create subdomains such as phx

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 Can be 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, 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 recordFunction 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 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.

Forward Lookup Zone Example IN SOA web1.technowidgets.com. admn.technowidgets.com. ( ; Serial ; Refresh ; Retry ; Expire ) ; Minimum IN NS web1 IN A IN MX 10 mail.technowidgets.com. web1 INA www INCNAMEweb1 research INA INMX 10mail mail INA

Reverse Lookup Zone Example IN SOA web1.technowidgets.com. admn.technowidgets.com. ( ; Serial ; Refresh ; Retry ; Expire ) ; Minimum IN NS web1 100 IN PTR web1.technowidgets.com. 150IN PTR research.technowidgets.com. 200IN PTR mail.technowidgets.com.

Finished DNS Configuration in Windows

Troubleshooting DNS ping ping displays name resolution even if the computer cannot be contacted

Troubleshooting DNS nslookup nslookup can display information from the DNS server

Troubleshooting DNS dig – available on Linux