syntax = "proto2"; import 'alltypes.proto'; extend AllTypes { optional int32 AllTypes_extensionfield1 = 255 [default = 5]; } message ExtensionMessage { extend AllTypes { optional ExtensionMessage AllTypes_extensionfield2 = 254; // required ExtensionMessage AllTypes_extensionfield3 = 253; // No longer allowed by protobuf 3 repeated ExtensionMessage AllTypes_extensionfield4 = 252; } required string test1 = 1; required int32 test2 = 2; }