diff --git a/src/Type.cc b/src/Type.cc
index f252aea70f..fc0c9ef43a 100644
--- a/src/Type.cc
+++ b/src/Type.cc
@@ -121,9 +121,29 @@ BroType::BroType(TypeTag t, bool arg_base_type)
 
 	}
 
-BroType* BroType::Clone() const
+BroType* BroType::ShallowClone() const
 	{
-	// Fixme: Johanna
+	switch ( tag ) {
+		case TYPE_VOID:
+		case TYPE_BOOL:
+		case TYPE_INT:
+		case TYPE_COUNT:
+		case TYPE_COUNTER:
+		case TYPE_DOUBLE:
+		case TYPE_TIME:
+		case TYPE_INTERVAL:
+		case TYPE_STRING:
+		case TYPE_PATTERN:
+		case TYPE_TIMER:
+		case TYPE_PORT:
+		case TYPE_ADDR:
+		case TYPE_SUBNET:
+		case TYPE_ANY:
+			return new BroType(tag, base_type);
+
+		default:
+			reporter->InternalError("cloning illegal base BroType");
+	}
 	return nullptr;
 	}
 
@@ -381,6 +401,16 @@ TableType::TableType(TypeList* ind, BroType* yield)
 		}
 	}
 
+TableType* TableType::ShallowClone() const
+	{
+	if ( indices )
+		indices->Ref();
+	if ( yield_type )
+		yield_type->Ref();
+
+	return new TableType(indices, yield_type);
+	}
+
 bool TableType::IsUnspecifiedTable() const
 	{
 	// Unspecified types have an empty list of indices.
@@ -488,6 +518,16 @@ FuncType::FuncType(RecordType* arg_args, BroType* arg_yield, function_flavor arg
 		}
 	}
 
+FuncType* FuncType::ShallowClone() const
+	{
+	auto f = new FuncType();
+	f->args = args->Ref()->AsRecordType();
+	f->arg_types = arg_types->Ref()->AsTypeList();
+	f->yield = yield->Ref();
+	f->flavor = flavor;
+	return f;
+	}
+
 string FuncType::FlavorString() const
 	{
 	switch ( flavor ) {
@@ -646,6 +686,16 @@ RecordType::RecordType(type_decl_list* arg_types) : BroType(TYPE_RECORD)
 	num_fields = types ? types->length() : 0;
 	}
 
+// in this case the clone is actually not so shallow, since
+// it gets modified by everyone.
+RecordType* RecordType::ShallowClone() const
+	{
+	auto pass = new type_decl_list();
+	loop_over_list(*types, i)
+		pass->append(new TypeDecl(*(*types)[i]));
+	return new RecordType(pass);
+	}
+
 RecordType::~RecordType()
 	{
 	if ( types )
@@ -991,18 +1041,26 @@ EnumType::EnumType(const string& name)
 	SetName(name);
 	}
 
-EnumType::EnumType(EnumType* e)
+EnumType::EnumType(const EnumType* e)
 	: BroType(TYPE_ENUM)
 	{
 	counter = e->counter;
 	SetName(e->GetName());
 
-	for ( NameMap::iterator it = e->names.begin(); it != e->names.end(); ++it )
+	for ( auto it = e->names.begin(); it != e->names.end(); ++it )
 		names[it->first] = it->second;
 
 	vals = e->vals;
 	}
 
+EnumType* EnumType::ShallowClone() const
+	{
+	if ( counter == 0 )
+		return new EnumType(GetName());
+
+	return new EnumType(this);
+	}
+
 EnumType::~EnumType()
 	{
 	for ( auto& kv : vals )
@@ -1231,6 +1289,11 @@ VectorType::VectorType(BroType* element_type)
 	{
 	}
 
+VectorType* VectorType::ShallowClone() const
+	{
+	return new VectorType(yield_type);
+	}
+
 VectorType::~VectorType()
 	{
 	Unref(yield_type);
diff --git a/src/Type.h b/src/Type.h
index 4825feeb2f..9af6c9c96f 100644
--- a/src/Type.h
+++ b/src/Type.h
@@ -86,7 +86,15 @@ public:
 	explicit BroType(TypeTag tag, bool base_type = false);
 	~BroType() override { }
 
-	BroType* Clone() const;
+	// Performs a shallow clone operation of the Bro type.
+	// This especially means that especially for tables the types
+	// are not recursively cloned; altering one type will in this case
+	// alter one of them.
+	// The main use for this is alias tracking.
+	// Clone operations will mostly be implemented in the derived classes;
+	// in addition cloning will be limited to classes that can be reached by
+	// the script-level.
+	virtual BroType* ShallowClone() const;
 
 	TypeTag Tag() const		{ return tag; }
 	InternalTypeTag InternalType() const	{ return internal_tag; }
@@ -107,7 +115,7 @@ public:
 	// this type is a table[string] of port, then returns the "port"
 	// type.  Returns nil if this is not an index type.
 	virtual BroType* YieldType();
-	const BroType* YieldType() const
+	virtual const BroType* YieldType() const
 		{ return ((BroType*) this)->YieldType(); }
 
 	// Returns true if this type is a record and contains the
@@ -330,7 +338,7 @@ public:
 	TypeList* Indices() const		{ return indices; }
 	const type_list* IndexTypes() const	{ return indices->Types(); }
 	BroType* YieldType() override;
-	const BroType* YieldType() const;
+	const BroType* YieldType() const override;
 
 	void Describe(ODesc* d) const override;
 	void DescribeReST(ODesc* d, bool roles_only = false) const override;
@@ -356,6 +364,8 @@ class TableType : public IndexType {
 public:
 	TableType(TypeList* ind, BroType* yield);
 
+	TableType* ShallowClone() const override;
+
 	// Returns true if this table type is "unspecified", which is
 	// what one gets using an empty "set()" or "table()" constructor.
 	bool IsUnspecifiedTable() const;
@@ -382,12 +392,13 @@ protected:
 class FuncType : public BroType {
 public:
 	FuncType(RecordType* args, BroType* yield, function_flavor f);
+	FuncType* ShallowClone() const override;
 
 	~FuncType() override;
 
 	RecordType* Args() const	{ return args; }
 	BroType* YieldType() override;
-	const BroType* YieldType() const;
+	const BroType* YieldType() const override;
 	void SetYieldType(BroType* arg_yield)	{ yield = arg_yield; }
 	function_flavor Flavor() const { return flavor; }
 	string FlavorString() const;
@@ -405,7 +416,7 @@ public:
 	void DescribeReST(ODesc* d, bool roles_only = false) const override;
 
 protected:
-	FuncType()	{ args = 0; arg_types = 0; yield = 0; flavor = FUNC_FLAVOR_FUNCTION; }
+	FuncType() : BroType(TYPE_FUNC) { args = 0; arg_types = 0; yield = 0; flavor = FUNC_FLAVOR_FUNCTION; }
 	RecordType* args;
 	TypeList* arg_types;
 	BroType* yield;
@@ -415,6 +426,7 @@ protected:
 class TypeType : public BroType {
 public:
 	explicit TypeType(BroType* t) : BroType(TYPE_TYPE)	{ type = t->Ref(); }
+	TypeType* ShallowClone() const override { return new TypeType(type); }
 	~TypeType() override { Unref(type); }
 
 	BroType* Type()	{ return type; }
@@ -447,6 +459,7 @@ typedef PList(TypeDecl) type_decl_list;
 class RecordType : public BroType {
 public:
 	explicit RecordType(type_decl_list* types);
+	RecordType* ShallowClone() const override;
 
 	~RecordType() override;
 
@@ -495,6 +508,7 @@ public:
 class FileType : public BroType {
 public:
 	explicit FileType(BroType* yield_type);
+	FileType* ShallowClone() const override { return new FileType(yield->Ref()); }
 	~FileType() override;
 
 	BroType* YieldType() override;
@@ -510,6 +524,7 @@ protected:
 class OpaqueType : public BroType {
 public:
 	explicit OpaqueType(const string& name);
+	OpaqueType* ShallowClone() const override { return new OpaqueType(name); }
 	~OpaqueType() override { };
 
 	const string& Name() const { return name; }
@@ -527,8 +542,9 @@ class EnumType : public BroType {
 public:
 	typedef std::list<std::pair<string, bro_int_t> > enum_name_list;
 
-	explicit EnumType(EnumType* e);
+	explicit EnumType(const EnumType* e);
 	explicit EnumType(const string& arg_name);
+	EnumType* ShallowClone() const override;
 	~EnumType() override;
 
 	// The value of this name is next internal counter value, starting
@@ -580,9 +596,10 @@ protected:
 class VectorType : public BroType {
 public:
 	explicit VectorType(BroType* t);
+	VectorType* ShallowClone() const override;
 	~VectorType() override;
 	BroType* YieldType() override;
-	const BroType* YieldType() const;
+	const BroType* YieldType() const override;
 
 	int MatchesIndex(ListExpr*& index) const override;
 
diff --git a/src/Var.cc b/src/Var.cc
index 3c056ae194..f0cae1224c 100644
--- a/src/Var.cc
+++ b/src/Var.cc
@@ -273,7 +273,7 @@ void add_type(ID* id, BroType* t, attr_list* attr)
 		tnew = t;
 	else
 		// Clone the type to preserve type name aliasing.
-		tnew = t->Clone();
+		tnew = t->ShallowClone();
 
 	BroType::AddAlias(new_type_name, tnew);