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Editing File: /lib/udev/ifupdown-hotplug

#!/bin/sh -e
#
# run /sbin/{ifup,ifdown} with the --allow=hotplug option.
#

PATH='/sbin:/bin:/usr/sbin:/usr/bin'

if [ -x /usr/bin/logger ]; then
	LOGGER=/usr/bin/logger
elif [ -x /bin/logger ]; then
	LOGGER=/bin/logger
else
	unset LOGGER
fi

# for diagnostics
if [ -t 1 -a -z "$LOGGER" ] || [ ! -e '/dev/log' ]; then
	mesg() {
		echo "$@" >&2
	}
elif [ -t 1 ]; then
	mesg() {
		echo "$@"
		$LOGGER -t "${0##*/}[$$]" "$@"
	}
else
	mesg() {
		$LOGGER -t "${0##*/}[$$]" "$@"
	}
fi

if [ -z "$INTERFACE" ]; then
    mesg "Bad ifupdown udev helper invocation: \$INTERFACE is not set"
    exit 1
fi

check_program() {
    [ -x $1 ] && return 0

mesg "ERROR: $1 not found. You need to install the ifupdown package."
    mesg "ifupdown udev helper $ACTION event for $INTERFACE not handled."
    exit 1
}

wait_for_interface() {
    local interface=$1
    local state

while :; do
	read state /sys/class/net/$interface/operstate 2>/dev/null || true
	if [ "$state" != down ]; then
		return 0
	fi
	sleep 1
    done
}

net_ifup() {
    check_program /sbin/ifup

# exit if the interface is not configured as allow-hotplug
    TARGET_IFACE=$(/sbin/ifquery --allow hotplug -l "$INTERFACE" || true)
    if [ -z "$TARGET_IFACE" ]; then
        exit 0
    fi

if [ -d /run/systemd/system ]; then
        exec systemctl --no-block start $(systemd-escape --template ifup@.service $TARGET_IFACE)
    fi

wait_for_interface lo

exec ifup --allow=hotplug $INTERFACE
}

net_ifdown() {
    check_program /sbin/ifdown

# systemd will automatically ifdown the interface on device
    # removal by binding the instanced service to the network device
    if [ -d /run/systemd/system ]; then
        exit 0
    fi

exec ifdown --allow=hotplug $INTERFACE
}

do_everything() {

net_ifup
    ;;

remove)
    # the pppd persist option may have been used, so it should not be killed
    case $INTERFACE in
	ppp*)
	exit 0 ;;
    esac

net_ifdown
    ;;

*)
    mesg "NET $ACTION event not supported"
    exit 1
    ;;
esac

}

# under systemd we don't do synchronous operations, so we can run in the
# foreground; we also need to, as forked children get killed right away under
# systemd
if [ -d /run/systemd/system ]; then
    do_everything
else
    # under sysvinit we need to fork as we start the long-running
    # "ifup". but there, forked processes won't get killed.
    # When udev_log="debug" stdout and stderr are pipes connected to udevd.
    # They need to be closed or udevd will wait for this process which will
    # deadlock with udevsettle until the timeout.
    exec > /dev/null 2> /dev/null
    do_everything &
fi

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