wgengine/router: dns: unify on *BSD, multimode on Linux, Magic DNS (#536)

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

wgengine/router/router_freebsd.go

@@ -5,6 +5,8 @@
 package router
 
 import (
+	"fmt"
+
 	"github.com/tailscale/wireguard-go/device"
 	"github.com/tailscale/wireguard-go/tun"
 	"tailscale.com/types/logger"
@@ -18,3 +20,35 @@ import (
 func newUserspaceRouter(logf logger.Logf, _ *device.Device, tundev tun.Device) (Router, error) {
 	return newUserspaceBSDRouter(logf, nil, tundev)
 }
+
+func upDNS(config DNSConfig, interfaceName string) error {
+	if len(config.Nameservers) == 0 {
+		return downDNS(interfaceName)
+	}
+
+	if resolvconfIsActive() {
+		if err := dnsResolvconfUp(config, interfaceName); err != nil {
+			return fmt.Errorf("resolvconf: %w")
+		}
+		return nil
+	}
+
+	if err := dnsDirectUp(config); err != nil {
+		return fmt.Errorf("direct: %w")
+	}
+	return nil
+}
+
+func downDNS(interfaceName string) error {
+	if resolvconfIsActive() {
+		if err := dnsResolvconfDown(interfaceName); err != nil {
+			return fmt.Errorf("resolvconf: %w")
+		}
+		return nil
+	}
+
+	if err := dnsDirectDown(); err != nil {
+		return fmt.Errorf("direct: %w")
+	}
+	return nil
+}