[client] Add system DNS fallback for Windows, systemd-resolved, NetworkManager (#6000)

client/internal/dns/host_android.go

@@ -1,28 +1,43 @@
 package dns
 
 import (
+	"net/netip"
+
 	"github.com/netbirdio/netbird/client/internal/statemanager"
 )
 
+// androidHostManager is a noop on the OS side (Android's VPN service handles
+// DNS for us) but tracks the OS-reported resolver list pushed via
+// OnUpdatedHostDNSServer so it can serve as the fallback nameserver source.
 type androidHostManager struct {
+	holder *hostsDNSHolder
 }
 
-func newHostManager() (*androidHostManager, error) {
-	return &androidHostManager{}, nil
+func newHostManager(holder *hostsDNSHolder) (*androidHostManager, error) {
+	return &androidHostManager{holder: holder}, nil
 }
 
-func (a androidHostManager) applyDNSConfig(HostDNSConfig, *statemanager.Manager) error {
+func (a *androidHostManager) applyDNSConfig(HostDNSConfig, *statemanager.Manager) error {
 	return nil
 }
 
-func (a androidHostManager) restoreHostDNS() error {
+func (a *androidHostManager) restoreHostDNS() error {
 	return nil
 }
 
-func (a androidHostManager) supportCustomPort() bool {
+func (a *androidHostManager) supportCustomPort() bool {
 	return false
 }
 
-func (a androidHostManager) string() string {
+func (a *androidHostManager) string() string {
 	return "none"
 }
+
+func (a *androidHostManager) getOriginalNameservers() []netip.Addr {
+	hosts := a.holder.get()
+	out := make([]netip.Addr, 0, len(hosts))
+	for ap := range hosts {
+		out = append(out, ap.Addr())
+	}
+	return out
+}