Index: etc/defaults/periodic.conf
===================================================================
--- etc/defaults/periodic.conf
+++ etc/defaults/periodic.conf
@@ -22,6 +22,8 @@
 # periodic script dirs
 local_periodic="/usr/local/etc/periodic"
 
+# Max time to sleep to avoid causing congestion on download servers
+anticongestion_sleeptime=3600
 
 # Daily options
 
@@ -136,8 +138,6 @@
 
 # 480.leapfile-ntpd
 daily_ntpd_leapfile_enable="YES"			# Fetch NTP leapfile
-daily_ntpd_avoid_congestion="YES"			# Avoid congesting
-							# leapfile sources
 
 # 480.status-ntpd
 daily_status_ntpd_enable="NO"				# Check NTP status
@@ -307,6 +307,18 @@
 if [ -z "${source_periodic_confs_defined}" ]; then
         source_periodic_confs_defined=yes
 
+	# Sleep for a random amount of time in order to mitigate the thundering
+	# herd problem of multiple hosts running periodic simultaneously.
+	# Will not sleep when used interactively.
+	# Will sleep at most once per invocation of periodic
+	anticongestion() {
+		[ -n "$PERIODIC_IS_INTERACTIVE" ] && return
+		if [ -f "$PERIODIC_ANTICONGESTION_FILE" ]; then
+			rm -f $PERIODIC_ANTICONGESTION_FILE
+			sleep `jot -r 1 0 ${anticongestion_sleeptime}`
+		fi
+	}
+
 	# Compatibility with old daily variable names.
 	# They can be removed in stable/11.
 	security_daily_compat_var() {
Index: etc/periodic/daily/480.leapfile-ntpd
===================================================================
--- etc/periodic/daily/480.leapfile-ntpd
+++ etc/periodic/daily/480.leapfile-ntpd
@@ -13,16 +13,9 @@
 
 case "$daily_ntpd_leapfile_enable" in
     [Yy][Ee][Ss])
-	case "$daily_ntpd_avoid_congestion" in
-    	[Yy][Ee][Ss])
-	    # Avoid dogpiling
-	    (sleep $(jot -r 1 0 3600); service ntpd onefetch) &
-	    ;;
-	*)
-	    service ntpd onefetch
-	    ;;
-	esac
-	;;
+        anticongestion
+        service ntpd onefetch
+        ;;
 esac
 
 exit $rc
Index: usr.sbin/periodic/periodic.sh
===================================================================
--- usr.sbin/periodic/periodic.sh
+++ usr.sbin/periodic/periodic.sh
@@ -76,6 +76,12 @@
 shift
 arg=$1
 
+if [ -z "$PERIODIC_ANTICONGESTION_FILE" ] ; then
+	export PERIODIC_ANTICONGESTION_FILE=`mktemp ${TMPDIR:-/tmp}/periodic.anticongestion.XXXXXXXXXX`
+fi
+if tty > /dev/null 2>&1; then
+	export PERIODIC_IS_INTERACTIVE=1
+fi
 tmp_output=`mktemp ${TMPDIR:-/tmp}/periodic.XXXXXXXXXX`
 context="$PERIODIC"
 export PERIODIC="$arg${PERIODIC:+ }${PERIODIC}"
@@ -141,3 +147,4 @@
 } | output_pipe $arg "$context"
 
 rm -f $tmp_output
+rm -f $PERIODIC_ANTICONGESTION_FILE