# get network interfaces(Wi-Fi....)
$ networksetup -listallnetworkservices
#set
$ networksetup -setdnsservers Wi-Fi x.x.x.x
#get
$ networksetup -getdnsservers Wi-Fi
# clear
$ networksetup -getdnsservers Wi-Fi Empty

Reference

• https://osxdaily.com/2015/06/02/change-dns-command-line-mac-os-x/