If you are having problems communicating with your DNS server, you might think that the Internet connection is down because you don’t get a response when you try to access sites by name. One way to confirm this issue is trying to ping a public IP address. If that works but you cannot ping any domain names, it means that you have a DNS issue (i.e., translating domain names into IP addresses) and not a general communication problem.
If your applications are not working but you have Layer 3 connectivity within the network, this might also mean that you have a DNS issue, as some applications are communicating using names and not IP addresses.
The first thing you should do when you suspect DNS issues is verify the DNS server configuration on your devices. You should make sure that you have the correct IP address of the DNS server on every device that might need a name-to-IP translation, as the DNS server could have been statically or automatically assigned (DHCP). A good way to check this basic functionality is to perform an “nslookup” test. This is a utility built into the operating system that allows you to perform a DNS query from the command line interface to see if the DNS server responds.
The screenshot in Figure 12.3 below shows an nslookup query performed on a Windows machine by simply typing the nslookup command followed by the domain you want to translate:
Figure 12.3 – Nslookup Test
In the screenshot above, you can see the configured DNS server (8.8.8.8) and the DNS query result (in this case, an IPv6 address and multiple IPv4 addresses were returned). This example confirms that the DNS server is reachable and is working properly. If the DNS server could not find the domain you requested, the following message would have been returned: *** 8.8.8.8 can’t find <requested domain name>: Non-existing domain. On the other hand, if you don’t receive a response, this means the DNS server is not reachable, so you can start investigating standard network connectivity issues that would prevent you from reaching the configured DNS server.
If your preferred server does not respond, you can try using a different DNS server, even a public one, to validate the issue.