Lab - Observe DNS Resolution

Lab - Observe DNS Resolution Objectives Part 1: Observe the DNS Conversion of a URL to an IP Address Part 2: Observe DNS Lookup Using the nslookup Command on a Web Site Part 3: Observe DNS Lookup Using the nslookup Command on Mail Servers Background / Scenario The Domain Name System (DNS) is invoked when you type a Uniform Resource Locator (URL), such as http://www.cisco.com, into a web browser. The first part of the URL describes which protocol is used. Common protocols are Hypertext Transfer Protocol (HTTP), Hypertext Transfer Protocol over Secure Socket Layer (HTTPS), and File Transfer Protocol (FTP). DNS uses the second part of the URL, which in this example is www.cisco.com. DNS translates the domain name (www.cisco.com) to an IP address to allow the source host to reach the destination server. In this lab, you will observe DNS in action and use the nslookup (name server lookup) command to obtain additional DNS information. Required Resources 1 PC (Windows with internet and command prompt access) Part 1: Observe the DNS Conversion of a URL to an IP Address a. Open a Windows command prompt. Open a Windows command prompt. b. At the command prompt, ping the URL for the Internet Corporation for Assigned Names and Numbers (ICANN) at www.icann.org. ICANN coordinates the DNS, IP addresses, top-level domain name system management, and root server system management functions. The computer must translate www.icann.org into an IP address to know where to send the Internet Control Message Protocol (ICMP) packets. The first line of the output displays www.icann.org converted to an IP address by DNS. You should be able to see the effect of DNS, even if your institution has a firewall that prevents pinging, or if the destination server has prevented you from pinging its web server. Note: If the domain name is resolved to an IPv6 address, use the command ping -4 www.icann.org to translate into an IPv4 address if desired. Record the IP addresses for . 192.0.32.7 and 2620:0:2d0:200::7 Close the Windows command prompt c. Type the IPv4 addresses from step b into a web browser, instead of the URL. Enter https://192.0.32.7 in the web browser. If your computer has an IPv6 address you can enter the IPv6 address. https:// [2620:0:2d0:200::7] in the web browser. d. Notice that the ICANN home web page is displayed without using DNS. Most humans find it easier to remember words, rather than numbers. If you tell someone to go to www.icann.org, they can probably remember that. If you told them to go to 192.0.32.7, they would have a difficult time remembering an IP address. Computers process in numbers. DNS is the process of Page 1 of 4 Lab - Observe DNS Resolution translating words into numbers. Additionally, there is a second translation that takes place. Humans think in Base 10 numbers. Computers process in Base 2 numbers. The Base 10 IP address 192.0.32.7 in Base 2 numbers is 11000000.00000000.00100000.00000111. What happens if you cut and paste these Base 2 numbers into a browser? The web site does not display. The software code used in web browsers recognizes Base 10 numbers. It does not recognize Base 2 numbers our answers here. e. At a command prompt, ping www.cisco.com. Note: If the domain name is resolved to an IPv6 address, use the command ping -4 www.cisco.com to translate into an IPv4 address if desired. Open a windows command prompt C:\> ping www.cisco.com C:\> ping -4 www.cisco.com Questions: When you ping www.cisco.com, do you get the same IP address as the example? Explain. Answer will vary depending upon where you are geographically. Cisco hosts its web content on a series of mirror servers. This means that Cisco uploads the exact same content to geographically diverse (spread out all over the world) servers. When someone tries to reach www.cisco.com, the traffic is directed to the closest mirror server.. Type the IP address that you obtained when you pinged www.cisco.com into a browser. Does the web site display? Explain. The Cisco web site does not display. There are at least two possible explanations for this: 1. Some web servers are configured to accept IP addresses sent from a browser and some are not. 2. It may be a firewall rule in the Cisco security system that prohibits an IP address from being sent via a browser. Close the windows command prompt Part 2: Observe DNS Lookup Using the nslookup Command on a Web Site a. At the command prompt, type the nslookup command. Your result will be different than the example. Open a windows command prompt C:\> nslookup Question: What is the default DNS server used? 196.168.1.1 Type your answers here. b. Notice how the command prompt changed to a greater than (>) symbol. This is the nslookup prompt. From this prompt, you can enter commands related to DNS. At the prompt, type ? to see a list of all the available commands that you can use in nslookup mode. c. At the nslookup prompt, type www.cisco.com. Page 2 of 4 Lab - Observe DNS Resolution > www.cisco.com Default Server: one.one.one.one Address: 1.1.1.1 Non-authoritative answer: Name: e2867.dsca.akamaiedge.net Addresses: 2600:1404:a:395::b33 2600:1404:a:38e:::b33 172.230.155.162 Aliases: www.cisco.com www.cisco.com.akadns.net wwwds.cisco.com.edgekey.net wwwds.cisco.com.edgekey.net.globalredir.akadns.net Questions: What is the translated IPv4 address? 172.230.155.162 Type your answers here. Note: The IP address from your location will most likely be different because Cisco uses mirrored servers in various locations around the world. Is it the same as the IP address shown with the ping command? Type you’re an Yes Under addresses, in addition to the 172.230.155.162 IP address, there are the following numbers: 2600:1404:a:395::b33 and 2600:1404:a:38e:::b33. What are these? Type your answers here. IPv6 (IP version 6) IP addresses at which the web site is reachable. d. At the nslookup prompt, type the IP address of the Cisco web server that you just found. You can use nslookup to get the domain name of an IP address if you do not know the URL. > 172.230.155.162 Default Server: one.one.one.one Address: 1.1.1.1 Name: a172-230-155-162.deploy.static.akamaitechnologies.com Address: 172.230.155.162 You can use the nslookup tool to translate domain names into IP addresses. You can also use it to translate IP addresses into domain names. Question: Using the nslookup tool, record the IP addresses associated with . 2607:f8b0:4000:80f::2004 and 172.217.9.132. Page 3 of 4 Lab - Observe DNS Resolution Part 3: Observe DNS Lookup Using the nslookup Command on Mail Servers a. At the nslookup prompt, type set type=mx to use nslookup to identify mail servers. > set type=mx b. At the nslookup prompt, type cisco.com. > cisco.com Server: one.one.one.one Address: 1.1.1.1 Non-authoritative answer: cisco.com MX preference = 20, mail exchanger = rcdn-mx-01.cisco.com cisco.com MX preference = 30, mail exchanger = aer-mx-01.cisco.com cisco.com MX preference = 10, mail exchanger = alln-mx-01.cisco.com A fundamental principle of network design is redundancy (more than one mail server is configured). In this way, if one of the mail servers is unreachable, then the computer making the query tries the second mail server. Email administrators determine which mail server is contacted first by using MX preference. The mail server with the lowest MX preference is contacted first. Based upon the output above, which mail server will be contacted first when the email is sent to cisco.com? rcdn-mx-01.cisco.comrs here. c. At the nslookup prompt, type exit to return to the regular PC command prompt. d. At the PC command prompt, type ipconfig /all. Question: Write the IP addresses of all the DNS servers that your school uses. Type Site dependent Reflection Question What is the fundamental purpose of DNS? Type your answers here. DNS acts like the phonebook for the Internet. So DNS translates names to numbers. The numbers can be either IPv4 or IPv6 End of document Page 4 of 4