--- src/VBox/NetworkServices/NAT/Makefile.kmk.orig	2020-07-10 02:57:19.000000000 +1000
+++ src/VBox/NetworkServices/NAT/Makefile.kmk	2020-08-22 02:25:57.918257000 +1000
@@ -114,6 +114,7 @@
  endif
 
  VBoxNetLwipNAT_SOURCES.darwin  += rtmon_bsd.c
+ VBoxNetLwipNAT_SOURCES.bsdsunix += rtmon_bsd.c
  VBoxNetLwipNAT_SOURCES.freebsd += rtmon_bsd.c
  VBoxNetLwipNAT_SOURCES.linux   += rtmon_linux.c
  VBoxNetLwipNAT_SOURCES.solaris += rtmon_bsd.c