dnsx/plus: disable use of preferred and system dns

In some cases, Plus may be used as a fallback/Default, which means that
it couldn't on its own rely on other DNS transports which may require
use of a fallback / Default (for instance, if the current Preferred transport
is doh, then when it queries for ECH records, it does so over fallback/Default)
which will infinitely loop if Plus is acting as fallback/Default and ends up
selecting Preferred for the query.

Author: ignoramous

intra/dnsx/plus.go

@@ -22,6 +22,8 @@ import (
 )
 
 const plusSupportsCachedTransports = false
+const plusUsesPreferred = false
+const plusUsesSystem = false
 
 const plusMaxTries = 6
 
@@ -126,10 +128,16 @@ func (t *plus) defaultdns() (Transport, error) {
 }
 
 func (t *plus) systemdns() (Transport, error) {
+	if !plusUsesSystem {
+		return nil, errNoSuchTransport
+	}
 	return t.r.GetInternal(System) // may return Goos or Default
 }
 
 func (t *plus) preferreddns() (Transport, error) {
+	if !plusUsesPreferred {
+		return nil, errNoSuchTransport
+	}
 	return t.r.GetInternal(Preferred) // may return Default
 }