hotfix(dns): always run and use built-in DNS server

- start DNS server before healthcheck
- do not fallback to plaintext anymore
- allow to use plain addresses with a port different than 53, system-wide
- do not wait for the DNS server and rely on healtcheck only

internal/vpn/tunnelup.go

@@ -76,6 +76,8 @@ func (l *Loop) onTunnelUp(ctx, loopCtx context.Context, data tunnelUpData) {
 		}
 	}
 
+	_, _ = l.dnsLooper.ApplyStatus(ctx, constants.Running)
+
 	icmpTargetIPs := l.healthSettings.ICMPTargetIPs
 	if len(icmpTargetIPs) == 1 && icmpTargetIPs[0].IsUnspecified() {
 		icmpTargetIPs = []netip.Addr{data.serverIP}
@@ -101,8 +103,6 @@ func (l *Loop) onTunnelUp(ctx, loopCtx context.Context, data tunnelUpData) {
 	// to start monitoring health and auto-healing.
 	go l.collectHealthErrors(ctx, loopCtx, healthErrCh)
 
-	_, _ = l.dnsLooper.ApplyStatus(ctx, constants.Running)
-
 	err = l.publicip.RunOnce(ctx)
 	if err != nil {
 		l.logger.Error("getting public IP address information: " + err.Error())