TUN inbound: Add autoSystemDNS on Linux (to TUN's gateway) (#6773)

https://github.com/XTLS/Xray-core/issues/6454#issuecomment-4931311976
https://github.com/XTLS/Xray-core/pull/6773#issuecomment-5755516423
https://github.com/XTLS/Xray-core/pull/6807#issuecomment-5807844210

app/dns/dns.go

@@ -212,6 +212,28 @@ func (s *DNS) IsOwnLink(ctx context.Context) bool {
 	return false
 }
 
+// MayUseSystemResolver reports whether any name server configured here could
+// still resolve through the system resolver. That is what happens when no name
+// server is configured at all, and it is also what a name server pointed at
+// "localhost" does. Callers that are about to redirect the system resolver need
+// to know, because a resolution path that reaches it would then loop back to
+// them.
+//
+// Any such server is enough: name servers can be selected per domain, so a
+// single local one makes some query reach the system resolver even when
+// independent upstreams are configured alongside it.
+func (s *DNS) MayUseSystemResolver() bool {
+	if len(s.clients) == 0 {
+		return true
+	}
+	for _, client := range s.clients {
+		if _, isLocal := client.server.(*LocalNameServer); isLocal {
+			return true
+		}
+	}
+	return false
+}
+
 // LookupIP implements dns.Client.
 func (s *DNS) LookupIP(domain string, option dns.IPOption) ([]net.IP, uint32, error) {
 	// Normalize the FQDN form query

app/dns/dns_internal_test.go

@@ -0,0 +1,59 @@
+package dns
+
+import (
+	"context"
+	"testing"
+
+	"github.com/xtls/xray-core/common/net"
+	feature_dns "github.com/xtls/xray-core/features/dns"
+)
+
+// fakeServer stands in for any name server that is not the system resolver.
+type fakeServer struct{}
+
+func (fakeServer) Name() string         { return "fake" }
+func (fakeServer) IsDisableCache() bool { return false }
+func (fakeServer) QueryIP(context.Context, string, feature_dns.IPOption) ([]net.IP, uint32, error) {
+	return nil, 0, nil
+}
+
+// Callers that are about to redirect the system resolver rely on this to tell
+// whether any resolution path could still reach the system resolver, so the
+// mixed shape has to be reported as reachable: a domain-specific rule can
+// select the system resolver even when an independent upstream also exists.
+func TestMayUseSystemResolver(t *testing.T) {
+	tests := []struct {
+		name    string
+		clients []*Client
+		want    bool
+	}{
+		{
+			name: "no clients at all",
+			want: true,
+		},
+		{
+			name:    "only the system resolver",
+			clients: []*Client{{server: NewLocalNameServer()}},
+			want:    true,
+		},
+		{
+			name:    "the system resolver alongside an independent name server",
+			clients: []*Client{{server: fakeServer{}}, {server: NewLocalNameServer()}},
+			want:    true,
+		},
+		{
+			name:    "only independent name servers",
+			clients: []*Client{{server: fakeServer{}}},
+			want:    false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			server := &DNS{clients: tt.clients}
+			if got := server.MayUseSystemResolver(); got != tt.want {
+				t.Errorf("MayUseSystemResolver() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}