The first step involved in troubleshooting DNS related issues is to ensure that the right DNS server settings are configured on the clients. This is critical to ensure that the IP address resolved as part of a DNS query is legitimate and is not spoofed. There are two different types of DNS client configuration, as follows:

• Manually setting the DNS server on the client
• Dynamic DNS

With manual configuration, we need to statically define the DNS server information on the client. Depending on the client, the configuration may vary. For example, in Linux distributions, the DNS server should be configured in the /etc/resolv.conf file.

With Dynamic DNS (DDNS), we leverage the dynamic configuration protocols to advertise the DNS server information. In a large network, it is very common to see DHCP as the address assignment protocol. DHCP can be leveraged to dynamically advertise one or more DNS servers.

As shown in the preceding screenshot, DNS server information is one of the DHCP options that will be included while assigning the IP address.

In an IPv6 environment, it is possible to advertise the DNS server information as part of the IPv6 router advertisement. Any client that is enabled with IPv6 auto configuration will use the DNS server received as part of an IPv6 RA message.