Simple macOS script to quickly switch DNS servers

I travel a lot, so I often connect to public Wi-Fi networks (on the train, for instance) but as a netadmin I also need to use very specific custom DNS servers at my day job (VPN, specific hostname resolutions, etc.). The thing is, when you have custom DNS configured, the DHCP-provided DNS servers aren’t used, so the Wi-Fi portal doesn’t show up. Conclusion: you can’t connect to the public Wi-Fi, and you’re mad at the train company until you figure out that you’re the problem.

So I created a simple script that I put in my .zshrc so I can easily see my current DNS (because I don’t want to go to the settings panel each time I need to change them), and update or remove them altogether if needed.

Here is the script:

dns() {
  local SERVICE="${DNS_SERVICE:-Wi-Fi}"

  case "$1" in
    on)
      sudo networksetup -setdnsservers "$SERVICE" 1.1.1.1 1.0.0.1 9.9.9.9
      ;;
    google)
      sudo networksetup -setdnsservers "$SERVICE" 8.8.8.8 8.8.4.4
      ;;
    quad9)
      sudo networksetup -setdnsservers "$SERVICE" 9.9.9.9 149.112.112.112
      ;;
    off|auto)
      sudo networksetup -setdnsservers "$SERVICE" empty
      ;;
    show|status)
      networksetup -getdnsservers "$SERVICE"
      ;;
    list)
      networksetup -listallnetworkservices
      ;;
    service)
      shift
      export DNS_SERVICE="$*"
      echo "DNS service set to: $DNS_SERVICE"
      ;;
    *)
      echo "Usage:"
      echo "  dns on        # Cloudflare + Quad9"
      echo "  dns google    # Google DNS"
      echo "  dns quad9     # Quad9 DNS"
      echo "  dns off       # automatic DHCP DNS"
      echo "  dns show      # current DNS"
      echo "  dns list      # list services"
      echo "  dns service Wi-Fi"
      ;;
  esac
}

And the usage is pretty self-explanatory:

Usage:
  dns on        # Cloudflare + Quad9
  dns google    # Google DNS
  dns quad9     # Quad9 DNS
  dns off       # automatic DHCP DNS
  dns show      # current DNS
  dns list      # list services
  dns service Wi-Fi

I’ve removed my custom DNS settings for this post, but the script is easy to update, so feel free to set your own DNS presets.

Don’t forget to run source ~/.zshrc to apply and test your changes after updating the file.