diff --git a/src/proto/SConscript b/src/proto/SConscript
index ef6bc2aca..665d16493 100644
--- a/src/proto/SConscript
+++ b/src/proto/SConscript
@@ -1,6 +1,6 @@
 # -*- mode:python -*-
 
-# Copyright (c) 2012 ARM Limited
+# Copyright (c) 2012,2017 ARM Limited
 # All rights reserved.
 #
 # The license below extends only to copyright in the software and shall
@@ -45,3 +45,8 @@ if env['HAVE_PROTOBUF']:
     ProtoBuf('packet.proto')
     ProtoBuf('inst.proto')
     Source('protoio.cc')
+
+    # protoc relies on the fact that undefined preprocessor symbols are
+    # explanded to 0 but since we use -Wundef they end up generating
+    # warnings.
+    env.Append(CCFLAGS='-DPROTOBUF_INLINE_NOT_IN_HEADERS=0')
diff --git a/src/proto/inst.proto b/src/proto/inst.proto
index 3e8dcde8b..5ef266455 100644
--- a/src/proto/inst.proto
+++ b/src/proto/inst.proto
@@ -1,4 +1,4 @@
-// Copyright (c) 2014 ARM Limited
+// Copyright (c) 2014,2017 ARM Limited
 // All rights reserved
 //
 // The license below extends only to copyright in the software and shall
@@ -35,6 +35,7 @@
 //
 // Authors: Ali Saidi
 
+syntax = "proto2";
 
 // Put all the generated messages in a namespace
 package ProtoMessage;
diff --git a/src/proto/inst_dep_record.proto b/src/proto/inst_dep_record.proto
index f89e51d81..808837e12 100644
--- a/src/proto/inst_dep_record.proto
+++ b/src/proto/inst_dep_record.proto
@@ -1,4 +1,4 @@
-// Copyright (c) 2013 ARM Limited
+// Copyright (c) 2013,2017 ARM Limited
 // All rights reserved
 //
 // The license below extends only to copyright in the software and shall
@@ -35,6 +35,8 @@
 //
 // Authors: Radhika Jagtap
 
+syntax = "proto2";
+
 // Put all the generated messages in a namespace
 package ProtoMessage;
 
diff --git a/src/proto/packet.proto b/src/proto/packet.proto
index c07206742..53f69e533 100644
--- a/src/proto/packet.proto
+++ b/src/proto/packet.proto
@@ -1,4 +1,4 @@
-// Copyright (c) 2012-2013 ARM Limited
+// Copyright (c) 2012-2013,2017 ARM Limited
 // All rights reserved
 //
 // The license below extends only to copyright in the software and shall
@@ -35,6 +35,8 @@
 //
 // Authors: Andreas Hansson
 
+syntax = "proto2";
+
 // Put all the generated messages in a namespace
 package ProtoMessage;