all: use local resolver for ADDC

For normal OS resolver, ctrld does not use local addresses as nameserver
to avoid possible looping. However, on AD environment with local DNS
running, AD queries must be sent to the local DNS server for proper
resolving.

resolver.go

@@ -30,8 +30,10 @@ const (
 	ResolverTypeOS = "os"
 	// ResolverTypeLegacy specifies legacy resolver.
 	ResolverTypeLegacy = "legacy"
-	// ResolverTypePrivate is like ResolverTypeOS, but use for local resolver only.
+	// ResolverTypePrivate is like ResolverTypeOS, but use for private resolver only.
 	ResolverTypePrivate = "private"
+	// ResolverTypeLocal is like ResolverTypeOS, but use for local resolver only.
+	ResolverTypeLocal = "local"
 	// ResolverTypeSDNS specifies resolver with information encoded using DNS Stamps.
 	// See: https://dnscrypt.info/stamps-specifications/
 	ResolverTypeSDNS = "sdns"
@@ -47,6 +49,16 @@ var controldPublicDnsWithPort = net.JoinHostPort(controldPublicDns, "53")
 // or is the Resolver used for ResolverTypeOS.
 var or = newResolverWithNameserver(defaultNameservers())
 
+var localResolver = newLocalResolver()
+
+func newLocalResolver() Resolver {
+	var nss []string
+	for _, addr := range Rfc1918Addresses() {
+		nss = append(nss, net.JoinHostPort(addr, "53"))
+	}
+	return NewResolverWithNameserver(nss)
+}
+
 // LanQueryCtxKey is the context.Context key to indicate that the request is for LAN network.
 type LanQueryCtxKey struct{}
 
@@ -89,7 +101,8 @@ 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 {
-	return initializeOsResolver(availableNameservers())
+	ns := initializeOsResolver(availableNameservers())
+	return ns
 }
 
 // initializeOsResolver performs logic for choosing OS resolver nameserver.
@@ -301,6 +314,8 @@ func NewResolver(uc *UpstreamConfig) (Resolver, error) {
 		return &legacyResolver{uc: uc}, nil
 	case ResolverTypePrivate:
 		return NewPrivateResolver(), nil
+	case ResolverTypeLocal:
+		return localResolver, nil
 	}
 	return nil, fmt.Errorf("%w: %s", errUnknownResolver, typ)
 }