Pre-emptively define IPPROTO_SPACER to 32767, the same value as PROTO_SPACER

to document that this value is globally assigned for a special purpose and
may not be reused within the IPPROTO number space.

sys/netinet/in.h

@@ -243,6 +243,12 @@ __END_DECLS
 /* Only used internally, so can be outside the range of valid IP protocols. */
 #define	IPPROTO_DIVERT		258		/* divert pseudo-protocol */
 
+/*
+ * Defined to avoid confusion.  The master value is defined by
+ * PROTO_SPACER in sys/protosw.h.
+ */
+#define	IPPROTO_SPACER		32767		/* spacer for loadable protos */
+
 /*
  * Local port number conventions:
  *