don't overwrite OS resolver nameservers if there arent any

cmd/cli/dns_proxy.go

@@ -1249,8 +1249,12 @@ func (p *prog) reinitializeOSResolver(networkChange bool) {
 	mainLog.Load().Debug().Msg("DNS reset completed")
 
 	mainLog.Load().Debug().Msg("initializing OS resolver")
-	ns := ctrld.InitializeOsResolver()
-	mainLog.Load().Warn().Msgf("re-initialized OS resolver with nameservers: %v", ns)
+	ns := ctrld.InitializeOsResolver(true)
+	if len(ns) == 0 {
+		mainLog.Load().Warn().Msgf("no nameservers found, using existing OS resolver values")
+	} else {
+		mainLog.Load().Warn().Msgf("re-initialized OS resolver with nameservers: %v", ns)
+	}
 
 	// start leaking queries immediately// start leaking queries immediately
 	if networkChange {

cmd/cli/prog.go

@@ -268,7 +268,7 @@ func (p *prog) preRun() {
 func (p *prog) postRun() {
 	if !service.Interactive() {
 		p.resetDNS()
-		ns := ctrld.InitializeOsResolver()
+		ns := ctrld.InitializeOsResolver(false)
 		mainLog.Load().Debug().Msgf("initialized OS resolver with nameservers: %v", ns)
 		p.setDNS()
 		p.csSetDnsDone <- struct{}{}

resolver.go

@@ -130,8 +130,13 @@ func availableNameservers() []string {
 //
 // It's the caller's responsibility to ensure the system DNS is in a clean state before
 // calling this function.
-func InitializeOsResolver() []string {
-	ns := initializeOsResolver(availableNameservers())
+func InitializeOsResolver(guardAgainstNoNameservers bool) []string {
+	nameservers := availableNameservers()
+	// if no nameservers, return empty slice so we dont remove all nameservers
+	if len(nameservers) == 0 && guardAgainstNoNameservers {
+		return []string{}
+	}
+	ns := initializeOsResolver(nameservers)
 	resolverMutex.Lock()
 	defer resolverMutex.Unlock()
 	or = newResolverWithNameserver(ns)

resolver_test.go

@@ -115,7 +115,7 @@ func Test_osResolver_InitializationRace(t *testing.T) {
 	for range n {
 		go func() {
 			defer wg.Done()
-			InitializeOsResolver()
+			InitializeOsResolver(false)
 		}()
 	}
 	wg.Wait()