15.4.8 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.
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.
C:\> ping www.icann.org
Pinging www.vip.icann.org [2620:0:2d0:200::7] with 32 bytes of data:
Reply from 2620:0:2d0:200::7: time=43ms
Reply from 2620:0:2d0:200::7: time=41ms
Reply from 2620:0:2d0:200::7: time=44ms
Reply from 2620:0:2d0:200::7: time=39ms
Ping statistics for 2620:0:2d0:200::7:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 39ms, Maximum = 44ms, Average = 41ms
C:\> ping -4 www.icann.org
Pinging www.vip.icann.org [192.0.32.7] with 32 bytes of data:
Reply from 192.0.32.7: bytes=32 time=41ms TTL=241
Reply from 192.0.32.7: bytes=32 time=42ms TTL=241
Reply from 192.0.32.7: bytes=32 time=42ms TTL=241
Reply from 192.0.32.7: bytes=32 time=43ms TTL=241
Ping statistics for 192.0.32.7:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 41ms, Maximum = 43ms, Average = 42ms
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 browserd.
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 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.
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.
C:\> ping www.cisco.com
Pinging origin-www.cisco.com [2600:1408:7:1:9300::90] with 32 bytes of data:
Reply from 2600:1408:7:1:9300::90: time=70ms
Reply from 2600:1408:7:1:9300::90: time=74ms
Reply from 2600:1408:7:1:9300::90: time=72ms
Reply from 2600:1408:7:1:9300::90: time=71ms
Ping statistics for 2600:1408:7:1:9300::90:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 70ms, Maximum = 74ms, Average = 71ms
C:\> ping -4 www.cisco.com
Pinging e2867.dsca.akamaiedge.net [172.230.155.162] with 32 bytes of data:
Reply from 172.230.155.162: bytes=32 time=7ms TTL=54
Reply from 172.230.155.162: bytes=32 time=6ms TTL=54
Reply from 172.230.155.162: bytes=32 time=7ms TTL=54
Reply from 172.230.155.162: bytes=32 time=6ms TTL=54
Ping statistics for 172.230.155.162:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 6ms, Maximum = 7ms, Average = 6ms
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. Depending on the Web Browser you can also get a message saying the connection is not secure or there is a certificate error.
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.
C:\> nslookup Default Server: one.one.one.one Address: 1.1.1.1 >
What is the default DNS server used?
Site dependent
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.
> 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
What is the translated IPv4 address?
From a specific location, 172.230.155.162.
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?
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?
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.
Using the nslookup tool, record the IP addresses associated with www.google.com.
Answers may vary. At the time of writing, the IP addresses are 2607:f8b0:4000:80f::2004 and 172.217.9.132.
> www.google.com Server: one.one.one.one Address: 1.1.1.1 Non-authoritative answer: Name: www.google.com Addresses: 2607:f8b0:4000:80f::2004 172.217.9.132
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.com
c. At the nslookup prompt, type exit to return to the regular PC command prompt.
d. At the PC command prompt, type ipconfig /all.
Write the IP addresses of all the DNS servers that your school uses.
Site-dependent
Reflection Question
What is the fundamental purpose of DNS?
DNS basically acts like the phonebook for the Internet. So DNS translates names to numbers. The numbers can be either IPv4 or IPv6.
At part 3 Q b, shouldn’t the mail server be selected based on the lowest preference and the answer be “alln-mx-01.cisco.com”? Thanks!