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What is DNS?

What is DNS?. DNS = Domain Name System Used to map names (www.luther.edu) to IP addresses (192.203.196.18) and vice versa

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What is DNS?

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  1. What is DNS? • DNS = Domain Name System • Used to map names (www.luther.edu) to IP addresses (192.203.196.18) and vice versa • Domain Name Servers maintain lists of domain names/IP addresses and map the domain names in Internet requests to other servers on the Internet until a specified web site is found • * From www.easynet.com/investorinfo/investorinfo_glossary.asp

  2. Basic Concepts of DNS • DNS works hierarchically and rely on a few name servers (servers running DNS software such as Bind, DJBDNS, etc) world-wide that know what name servers are authoritive for the top-level domains (.com, .edu, org, .net, etc). • These nameservers can either resolve hostnames themselves or know which nameservers are responsible for second-level domains (google.com, luther.edu, slashdot.org, etc). • This continues through nameservers until you reach one that is authoritive for the domain and can resolve the hostname you are seeking (www.google.com, mail.luther.edu, games.slashdot.org, etc).

  3. Basic Concepts of DNS – Part Two • Registrars maintain the top level domains (Verisign/Network Solutions, Go Daddy, Register.com, etc). • ISPs (AOL, MSN, NetINS, etc) maintain individual DNS servers. • DNS system is redundant so that if one network is inaccessible, traffic can continue to route.

  4. DNS Terminology –Resource Records • SOA = Start of Authority • Marks beginning of record data • NS = Name Server • Identifies domain’s name servers • A = Address • Coverts host names to IP addresses • PTR = Pointer • Coverts IP addresses to host names • MX = Mail Exchange • Identifies where to send mail for a domain name • CNAME = Canonical Name • Defines a host alias

  5. DNS Tools – Host & NSlookup • Host = Utility for performing DNS lookups (Linux) • Ex: # host www.luther.edu • www.luther.edu is an alias for ada.luther.edu. • ada.luther.edu has address 192.203.196.18 • NSlookup = Utility for performing DNS lookups (legacy, Windows) • Ex: C:\>nslookup www.luther.edu • Server: martin.luther.edu • Address: 192.203.196.20 • Name: ada.luther.edu • Address: 192.203.196.18 • Aliases: www.luther.edu

  6. DNS Tools – Dig • Dig = Utility for interrogating DNS name servers • Ex: # dig A www.luther.edu • ; <<>> DiG 9.2.3rc2 <<>> A www.luther.edu • ;; global options: printcmd • ;; Got answer: • ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 43972 • ;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 3, ADDITIONAL: 2 • ;; QUESTION SECTION: • ;www.luther.edu. IN A • ;; ANSWER SECTION: • www.luther.edu. 86400 IN CNAME ada.luther.edu. • ada.luther.edu. 86400 IN A 192.203.196.18 • ;; AUTHORITY SECTION: • luther.edu. 86400 IN NS DNS.UNI.edu. • luther.edu. 86400 IN NS NS-3.IASTATE.edu. • luther.edu. 86400 IN NS martin.luther.edu.

  7. DNS Tools - WHOIS • WHOIS = Utility for looking up records in the Network Information (NIC) database (http://www.networksolutions.com/en_US/whois/index.jhtml) • Domain Name: LUTHER.EDURegistrant:Luther CollegeComputer Center 700 College DriveDecorah, IA 52101UNITED STATESContacts: Administrative Contact:Adam R. ForsythLuther College700 College DriveDecorah, IA 52101UNITED STATES(319) 387-1402forsytad@luther.eduTechnical Contact:Same as aboveName Servers: MARTIN.LUTHER.EDU 192.203.196.20NS-3.IASTATE.EDU 129.186.142.200DNS.UNI.EDU 134.161.1.32Domain record activated: 26-Apr-1993Domain record last updated: 06-Jan-2002

  8. How DNS Works – Workstation Configuration Workstation IP Address: 172.17.12.84 DNS Servers: 192.203.196.20 Personal Workstation that is attached to the Luther College network. The IP Address is assigned via DHCP (found through ipconfig /all) which also assigns a pair of DNS Servers (one being a backup for the other).

  9. Initial Request (Web) I want to go to http://www.google.com. Luther College DNS Server IP Address: 192.203.196.20 Luther College Workstation

  10. DNS Server Searches for Domain I want to go to http://www.google.com. Tell me where www.google.com is. Luther College DNS Server Workstation I do! The Internet .com Registry Database Who controls google.com domain? .org Registry Database .edu Registry Database

  11. DNS Server Finds Out Where To Go Who handles the A record for www.google.com? .com Registry Database I handle A records. Web Server www.google.com IP Address: 216.239.39.104 Google DNS Server IP Address: 216.239.32.10 I handle MX records. Mail Server smtp1.google.com IP Address: 216.239.33.25

  12. Response Returns to DNS Server The Internet Web Server Google DNS Server Luther College DNS Server Luther College Workstation

  13. Workstation Requests Additional Data I need more information from google.com. Luther College DNS Server 1 Luther College Workstation 2 Is google.com in my cache? Yes! Here’s the IP! 216.239.39.104 Great! I’ll go straight there and cut out the middle men! 3 Web Server www.google.com

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