diff --git a/usr.sbin/bhyve/config.h b/usr.sbin/bhyve/config.h
--- a/usr.sbin/bhyve/config.h
+++ b/usr.sbin/bhyve/config.h
@@ -99,12 +99,24 @@
 void	set_config_value_node(nvlist_t *parent, const char *name,
     const char *value);
 
+/*
+ * Similar to set_config_value_node but only sets value if it's unset yet.
+ */
+void 	set_config_value_node_if_unset(nvlist_t *const parent,
+    const char *const name, const char *const value);
+
 /*
  * Similar to set_config_value_node but expects a full path to the
  * leaf node.
  */
 void	set_config_value(const char *path, const char *value);
 
+/*
+ * Similar to set_config_value but only sets the value if it's unset yet.
+ */
+void 	set_config_value_if_unset(const char *const path,
+    const char *const value);
+
 /* Convenience wrappers for boolean variables. */
 bool	get_config_bool(const char *path);
 bool	get_config_bool_node(const nvlist_t *parent, const char *name);
diff --git a/usr.sbin/bhyve/config.c b/usr.sbin/bhyve/config.c
--- a/usr.sbin/bhyve/config.c
+++ b/usr.sbin/bhyve/config.c
@@ -135,6 +135,17 @@
 	nvlist_add_string(parent, name, value);
 }
 
+void
+set_config_value_node_if_unset(nvlist_t *const parent, const char *const name,
+    const char *const value)
+{
+	if (get_config_value_node(parent, name) != NULL) {
+		return;
+	}
+
+	set_config_value_node(parent, name, value);
+}
+
 void
 set_config_value(const char *path, const char *value)
 {
@@ -167,6 +178,16 @@
 	set_config_value_node(nvl, name, value);
 }
 
+void
+set_config_value_if_unset(const char *const path, const char *const value)
+{
+	if (get_config_value(path) != NULL) {
+		return;
+	}
+
+	set_config_value(path, value);
+}
+
 static const char *
 get_raw_config_value(const char *path)
 {