Under Servers, you can specify up to four DNS servers, which will be automatically assigned to DHCP clients. If you do not specify DNS servers here, pfSense will automatically assign DNS servers to clients in the following ways:

• If DNS Forwarder is enabled, then the IP address of the interface is used. This is because the DNS Forwarder turns the pfSense machine itself into a DNS server, so the IP address of the pfSense machine is assigned to each client.
• If DNS Forwarder is not enabled, then the DNS servers configured on the General Setup page are used. And if Allow DNS server list to be overridden by DHCP/PPP on WAN is enabled in General Setup, then the DNS servers obtained through the WAN will be used instead: