Index: sys/kern/uipc_socket.c
===================================================================
--- sys/kern/uipc_socket.c
+++ sys/kern/uipc_socket.c
@@ -2334,6 +2334,9 @@
 
 	if (!(how == SHUT_RD || how == SHUT_WR || how == SHUT_RDWR))
 		return (EINVAL);
+	if ((so->so_state &
+	    (SS_ISCONNECTED | SS_ISCONNECTING | SS_ISDISCONNECTING)) == 0)
+		return (ENOTCONN);
 
 	CURVNET_SET(so->so_vnet);
 	if (pr->pr_usrreqs->pru_flush != NULL)
Index: sys/kern/uipc_syscalls.c
===================================================================
--- sys/kern/uipc_syscalls.c
+++ sys/kern/uipc_syscalls.c
@@ -1383,6 +1383,15 @@
 	if (error == 0) {
 		so = fp->f_data;
 		error = soshutdown(so, uap->how);
+		/*
+		 * Previous versions did not return ENOTCONN, but 0 in
+		 * case the socket was not connected. Some important
+		 * programs like syslogd up to r279016, 2015-02-19,
+		 * still depend on this behavior.
+		 */
+		if (error == ENOTCONN &&
+		    td->td_proc->p_osrel < P_OSREL_SHUTDOWN_ENOTCONN)
+			error = 0;
 		fdrop(fp, td);
 	}
 	return (error);
Index: sys/sys/param.h
===================================================================
--- sys/sys/param.h
+++ sys/sys/param.h
@@ -77,12 +77,13 @@
 #define __FreeBSD_kernel__
 
 #ifdef _KERNEL
-#define	P_OSREL_SIGWAIT		700000
-#define	P_OSREL_SIGSEGV		700004
-#define	P_OSREL_MAP_ANON	800104
-#define	P_OSREL_MAP_FSTRICT	1100036
+#define	P_OSREL_SIGWAIT			700000
+#define	P_OSREL_SIGSEGV			700004
+#define	P_OSREL_MAP_ANON		800104
+#define	P_OSREL_MAP_FSTRICT		1100036
+#define	P_OSREL_SHUTDOWN_ENOTCONN	1100077
 
-#define	P_OSREL_MAJOR(x)	((x) / 100000)
+#define	P_OSREL_MAJOR(x)		((x) / 100000)
 #endif
 
 #ifndef LOCORE