CCNA 5.2: Troubleshoot client connectivity issues involving DNS

Overview:

Troubleshooting client connectivity issues involving DNS typically comes down to whether or not a client can resolve a domain name to an IP address

 

Study Notes:

  • Troubleshooting client connectivity issues involving DNS typically comes down to whether or not a client can resolve a domain name to an IP address
  • What does it look like when DNS resolution is not working on a client?  If you ping a domain name and DNS isn't working, you'll see something like this:
  • C:\Users\Joe>ping www.google.com
    Ping request could not find host www.google.com. Please check the name and try again.
    
  • There are really only a couple of reasons that DNS resolution would not work from the client's perspective
    • The DNS Server IP address is wrong in the client's Network Configuration Settings
    • The DNS Server is unreachable from the client.  Either the client can't route to it properly or the DNS Server is down.
  • On a Windows client, open the Command Prompt and use ipconfig /all to help troubleshoot
  • Find the DNS Server IP.  In this case, the DNS Server IP is a private address and it is different than the Default Gateway address.  That is most likely the issue since the Default Gateway would realistically be the same as the DHCP Server and DNS Server on a private network.  Change 192.168.100.1 to 192.168.0.1 and that fixes issue #1 where the DNS Server IP is wrong.  If you don't have access to change the setting on the router you could statically set the DNS Server IP on the client machine to the IP address of the Default Gateway or to a known public DNS Server IP.  Then test it using the same ping command to verify.
ipconfig /all

Wireless LAN adapter Wi-Fi:

Connection-specific DNS Suffix . : myrouter.home
Description . . . . . . . . . . . : My awesome Wireless Network Adapter
Physical Address. . . . . . . . . : 93-B3-BC-5F-2A-5D
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
IPv4 Address. . . . . . . . . . . : 192.168.0.10(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Lease Obtained. . . . . . . . . . : Wednesday, October 10, 2018 1:49:59 AM
Lease Expires . . . . . . . . . . : Saturday, November 3, 2018 7:28:23 PM
Default Gateway . . . . . . . . . : 192.168.0.1
DHCP Server . . . . . . . . . . . : 192.168.0.1
DNS Servers . . . . . . . . . . . : 192.168.100.1
NetBIOS over Tcpip. . . . . . . . : Enabled
Connection-specific DNS Suffix Search List : myrouter.home
  • In the second case, the Client cannot reach the DNS Server IP.
  • The DNS Server IP address in our example is now set correctly at 192.168.0.1.  Simply ping that IP address from the client to verify connectivity.
ping 192.168.0.1

C:\Users\Joe>ping google.com

Pinging 192.168.0.1 with 32 bytes of data:
Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 192.168.0.1:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
  • This is a layer3 connectivity issue between the client and the Default Gateway.
  • If the pings responded when pinging the DNS Server IP address, but not when pinging a domain name, it could be that the DNS service is not working properly on the Default Gateway.
  • On the client, statically change the DNS Server IP to another DNS Server IP address, most likely a public DNS Server IP, and use ping and traceroute commands to the IP address and to the domain name to determine whether it's a networking or service issue.
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