diff --git a/src/messenger.cpp b/src/messenger.cpp
index 96da14a5..572bcf98 100644
--- a/src/messenger.cpp
+++ b/src/messenger.cpp
@@ -349,6 +349,15 @@ void osd_messenger_t::check_peer_config(osd_client_t *cl)
                 err = true;
                 printf("Connected to OSD %lu instead of OSD %lu, peer state is outdated, disconnecting peer\n", config["osd_num"].uint64_value(), cl->osd_num);
             }
+            else if (config["protocol_version"].uint64_value() != OSD_PROTOCOL_VERSION)
+            {
+                err = true;
+                printf(
+                    "OSD %lu protocol version is %lu, but only version %u is supported.\n"
+                    " If you need to upgrade from 0.5.x please request it via the issue tracker.\n",
+                    cl->osd_num, config["protocol_version"].uint64_value(), OSD_PROTOCOL_VERSION
+                );
+            }
         }
         if (err)
         {
diff --git a/src/osd_ops.h b/src/osd_ops.h
index ee80d742..76b3ad3a 100644
--- a/src/osd_ops.h
+++ b/src/osd_ops.h
@@ -35,6 +35,7 @@
 #define MEM_ALIGNMENT               512
 #endif
 #define OSD_RW_MAX                  64*1024*1024
+#define OSD_PROTOCOL_VERSION        1
 
 // common request and reply headers
 struct __attribute__((__packed__)) osd_op_header_t
diff --git a/src/osd_secondary.cpp b/src/osd_secondary.cpp
index 5f4fb46f..bb11269a 100644
--- a/src/osd_secondary.cpp
+++ b/src/osd_secondary.cpp
@@ -145,7 +145,9 @@ void osd_t::exec_secondary(osd_op_t *cur_op)
 void osd_t::exec_show_config(osd_op_t *cur_op)
 {
     // FIXME: Send the real config, not its source
-    std::string cfg_str = json11::Json(config).dump();
+    auto cfg_copy = config;
+    cfg_copy["protocol_version"] = std::to_string(OSD_PROTOCOL_VERSION);
+    std::string cfg_str = json11::Json(cfg_copy).dump();
     cur_op->buf = malloc_or_die(cfg_str.size()+1);
     memcpy(cur_op->buf, cfg_str.c_str(), cfg_str.size()+1);
     cur_op->iov.push_back(cur_op->buf, cfg_str.size()+1);