dnsx: opt-in default transport as fallback

Author: ignoramous

intra/backend/netstat.go

@@ -158,6 +158,7 @@ type RDNSInfo struct {
 	PanicTest               bool
 	FatalTest               bool
 	SystemDNSForUndelegated bool
+	DefaultDNSAsFallback    bool
 	SetUserAgent            bool
 	OwnTunFd                bool
 	PortForward             bool

intra/dnsx/transport.go

@@ -791,11 +791,12 @@ func (r *resolver) determineTransport(id string) Transport {
 	tf = r.transports[Default]
 	r.RUnlock()
 
-	if t0 != nil && activeTransport(t0) {
+	mayusedefault := canUseDefaultDNS(id0)
+	if t0 != nil && (t1 == nil || !mayusedefault || activeTransport(t0)) {
 		return t0
-	} else if t1 != nil {
+	} else if t1 != nil && (!mayusedefault || activeTransport(t1)) {
 		return t1
-	} else if canUseDefaultDNS(id0) {
+	} else if tf != nil && mayusedefault {
 		log.W("dns: fwd: %s is missing; using default", id0)
 		return tf // todo: assert tf != nil?
 	}
@@ -1309,9 +1310,9 @@ func canUseDefaultDNS(id string) bool {
 	case Local, CT + Local:
 		return false
 	case Alg, Preferred, Plus, BlockFree:
-		return true
+		return settings.DefaultDNSAsFallback.Load()
 	case CT + Alg, CT + Preferred, CT + Plus, CT + BlockFree:
-		return true
+		return settings.DefaultDNSAsFallback.Load()
 	}
 	return false
 }

intra/settings/config.go

@@ -54,6 +54,10 @@ var EndpointIndependentFiltering = atomic.Bool{}
 // for undelegated domains.
 var SystemDNSForUndelegatedDomains = atomic.Bool{}
 
+// DefaultDNSAsFallback is a global flag to allow using the Default transport
+// as a fallback when the Preferred transport is missing or paused or ended.
+var DefaultDNSAsFallback = atomic.Bool{}
+
 // SetUserAgent is a global flag to set User-Agent for DoH requests
 // to "Intra" and for HTTP "Reaches" checks to the Android default.
 var SetUserAgent = atomic.Bool{}

intra/tun2socks.go

@@ -192,6 +192,13 @@ func UndelegatedDomains(useSystemDNS bool) {
 	log.I("tun: resolve undelegated with system DNS? %t / ok? %t", useSystemDNS, ok)
 }
 
+// DefaultDNSAsFallback allows using the Default transport as a fallback when
+// the Preferred transport is missing or paused or ended.
+func DefaultDNSAsFallback(y bool) {
+	ok := settings.DefaultDNSAsFallback.CompareAndSwap(!y, y)
+	log.I("tun: allow default DNS as fallback? %t / ok? %t", y, ok)
+}
+
 // Transparency enables/disables endpoint-independent mapping/filtering.
 // Currently applies only for UDP (RFC 4787).
 func Transparency(eim, eif bool) {

intra/tunnel.go

@@ -490,6 +490,7 @@ func (t *rtunnel) stat() (*x.NetStat, error) {
 	out.RDNSIn.FatalTest = settings.FatalAtRandom.Load()
 	out.RDNSIn.SetUserAgent = settings.SetUserAgent.Load()
 	out.RDNSIn.SystemDNSForUndelegated = settings.SystemDNSForUndelegatedDomains.Load()
+	out.RDNSIn.DefaultDNSAsFallback = settings.DefaultDNSAsFallback.Load()
 	out.RDNSIn.Dialer4 = dialers.Use4()
 	out.RDNSIn.Dialer6 = dialers.Use6()
 	out.RDNSIn.DialerOpts = csv2ssv(settings.GetDialerOpts().String())