Index: Mk/bsd.port.mk
===================================================================
--- Mk/bsd.port.mk
+++ Mk/bsd.port.mk
@@ -1144,6 +1144,15 @@
 .endif
 _EXPORTED_VARS+=	OSVERSION
 
+.if ${OPSYS} == FreeBSD # && (${OSVERSION} < 1003000 || (${OSVERSION} >= 1100000 && ${OSVERSION} < 1100122))
+_UNSUPPORTED_SYSTEM_MESSAGE=	${OPSYS} version you are running is not supported by the Ports Collection, no port is guaranteed to build. Please upgrade to a supported release or, as a last resort, use a ports tree snapshot dated within your system support lifetime.
+. if defined(ALLOW_UNSUPPORTED_SYSTEM)
+WARNING+=						"${_UNSUPPORTED_SYSTEM_MESSAGE}"
+. else
+UNSUPPORTED_SYSTEM_ERROR=		${_UNSUPPORTED_SYSTEM_MESSAGE} You may define ALLOW_UNSUPPORTED_SYSTEM to proceed, but you have been warned.
+. endif
+.endif
+
 # Convert OSVERSION to major release number
 _OSVERSION_MAJOR=	${OSVERSION:C/([0-9]?[0-9])([0-9][0-9])[0-9]{3}/\1/}
 # Sanity checks for chroot/jail building.
@@ -5154,6 +5163,13 @@
 .endif
 .endif #DEVELOPER
 
+.if defined(UNSUPPORTED_SYSTEM_ERROR)
+show-unsupported-system-error:
+	@${ECHO_MSG} "/!\\ ERROR: /!\\"
+	@${ECHO_MSG} "${UNSUPPORTED_SYSTEM_ERROR}" | ${FMT} 75 79
+	@${FALSE}
+.endif
+
 # Please note that the order of the following targets is important, and
 # should not be modified.
 
@@ -5168,7 +5184,8 @@
 # If you change the pre-foo and post-foo values here, go and keep them in sync
 # in _OPTIONS_TARGETS in bsd.options.mk
 
-_SANITY_SEQ=	050:post-chroot 100:pre-everything 150:check-makefile \
+_SANITY_SEQ=	050:post-chroot 100:pre-everything \
+				125:show-unsupported-system-error 150:check-makefile \
 				200:show-warnings 210:show-dev-warnings 220:show-dev-errors \
 				250:check-categories 300:check-makevars \
 				350:check-desktop-entries 400:check-depends \