This file is indexed.

/etc/network/if-up.d/upstart is in ifupdown 0.8.10ubuntu1.

This file is owned by root:root, with mode 0o755.

The actual contents of the file can be viewed below.

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#!/bin/sh -e
MARK_DEV_PREFIX="/run/network/ifup."
MARK_STATIC_NETWORK_EMITTED="/run/network/static-network-up-emitted"

if [ -e /lib/lsb/init-functions ]; then
    . /lib/lsb/init-functions
fi

if ! init_is_upstart; then
    exit 0
fi

# Let's ignore meta entries (ifup -a)
if [ "$ADDRFAM" = "meta" ]; then
    exit 0
fi

# lo emission handled by /etc/init/network-interface.conf
if [ "$IFACE" != lo ]; then
    initctl emit -n net-device-up \
        "IFACE=$IFACE" \
        "LOGICAL=$LOGICAL" \
        "ADDRFAM=$ADDRFAM" \
        "METHOD=$METHOD"
fi

get_auto_interfaces() {
	# write to stdout a list of interfaces configured as 'auto' in interfaces(5)
	local found=""
	# stderr redirected as it outputs things like:
	# Ignoring unknown interface eth0=eth0.
	found=$(ifquery --list --allow auto 2>/dev/null) || return
	set -- ${found}
	echo "$@"
}

all_interfaces_up() {
	# return true if all interfaces listed in /etc/network/interfaces as 'auto'
	# are up.  if no interfaces are found there, then "all [given] were up"
	local prefix="$1" iface=""
	for iface in $(get_auto_interfaces); do
		# if cur interface does is not up, then all have not been brought up
		[ -f "${prefix}${iface}" ] || return 1
	done
	return 0
}

# touch our own "marker" indicating that this interface has been brought up.
: > "${MARK_DEV_PREFIX}$IFACE"

if all_interfaces_up "${MARK_DEV_PREFIX}" &&
	mkdir "${MARK_STATIC_NETWORK_EMITTED}" 2>/dev/null; then
	initctl emit --no-wait static-network-up
fi