I suffered the same issue after upgrading to Debian 9. Attached please
find the patch file.
Steven
--
Steven Shiau <steven _at_ stevenshiau org>
Public Key Server PGP Key ID: 4096R/47CF935C
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--- knockd.service.orig 2017-09-01 11:27:21.080168675 +0800
+++ knockd.service 2017-09-01 11:27:35.432233376 +0800
@@ -11,3 +11,7 @@
SuccessExitStatus=0 2 15
ProtectSystem=full
CapabilityBoundingSet=CAP_NET_RAW CAP_NET_ADMIN
+
+[Install]
+WantedBy=multi-user.target
+Alias=knockd.service
