Index: head/Mk/bsd.port.mk
===================================================================
--- head/Mk/bsd.port.mk
+++ head/Mk/bsd.port.mk
@@ -1144,6 +1144,24 @@
 .endif
 _EXPORTED_VARS+=	OSVERSION
 
+.if (${OPSYS} == FreeBSD && (${OSVERSION} < 1003000 || (${OSVERSION} >= 1100000 && ${OSVERSION} < 1100122))) || \
+    (${OPSYS} == DragonFly && ${DFLYVERSION} < 400400)
+_UNSUPPORTED_SYSTEM_MESSAGE=	Ports Collection support for your ${OPSYS} version has ended, and no ports\
+								are guaranteed to build on this system. Please upgrade to a supported release.
+. if defined(ALLOW_UNSUPPORTED_SYSTEM)
+WARNING+=			"${_UNSUPPORTED_SYSTEM_MESSAGE}"
+. else
+show-unsupported-system-error:
+	@${ECHO_MSG} "/!\\ ERROR: /!\\"
+	@${ECHO_MSG}
+	@${ECHO_MSG} "${_UNSUPPORTED_SYSTEM_MESSAGE}" | ${FMT} 75 79
+	@${ECHO_MSG}
+	@${ECHO_MSG} "No support will be provided if you silence this message by defining ALLOW_UNSUPPORTED_SYSTEM." | ${FMT} 75 79
+	@${ECHO_MSG}
+	@${FALSE}
+. 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.
@@ -5168,7 +5186,8 @@
 # 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 \
Index: head/UPDATING
===================================================================
--- head/UPDATING
+++ head/UPDATING
@@ -5,6 +5,16 @@
 You should get into the habit of checking this file for changes each time
 you update your ports collection, before attempting any port upgrades.
 
+20170117:
+  AFFECTS: users of EoL'ed FreeBSD versions (<10.3, 11 prior to 11.0)
+  AUTHOR: amdmi3@FreeBSD.org
+
+  The ports system will now refuse to build anything if it's run on
+  outdated system version. This protects users from unexpected build
+  failures after code to support EoL'ed systems is removed from the
+  tree. You may define ALLOW_UNSUPPORTED_SYSTEM to override this and
+  allow builds, but no support will be provided in case of failures.
+
 20170116:
   AFFECTS: users of java/wildfly10
   AUTHOR: olgeni@FreeBSD.org