This file is indexed.

/etc/network/if-up.d/ethtool is in ethtool 1:3.16-1.

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

ETHTOOL=/sbin/ethtool

test -x $ETHTOOL || exit 0

[ "$IFACE" != "lo" ] || exit 0

# Find settings with a given prefix and print them as they appeared in
# /etc/network/interfaces, only with the prefix removed.
# This actually prints each name and value on a separate line, but that
# doesn't matter to the shell.
gather_settings () {
    set | sed -n "
/^IF_$1[A-Za-z0-9_]*=/ {
    h;                             # hold line
    s/^IF_$1//; s/=.*//; s/_/-/g;  # get name without prefix
    y/ABCDEFGHIJKLMNOPQRSTUVWXYZ/abcdefghijklmnopqrstuvwxyz/;  # lower-case
    p;
    g;                             # restore line
    s/^[^=]*=//; s/^'\(.*\)'/\1/;  # get value
    p;
}"
}

# Gather together the mixed bag of settings applied with -s/--change
SETTINGS="\
${IF_LINK_SPEED:+ speed $IF_LINK_SPEED}\
${IF_LINK_DUPLEX:+ duplex $IF_LINK_DUPLEX}\
"

# WOL has an optional pass-key
set -- $IF_ETHERNET_WOL
SETTINGS="$SETTINGS${1:+ wol $1}${2:+ sopass $2}"

# Autonegotiation can be on|off or an advertising mask
case "$IF_ETHERNET_AUTONEG" in
'')	;;
on|off)	SETTINGS="$SETTINGS autoneg $IF_ETHERNET_AUTONEG" ;;
*)	SETTINGS="$SETTINGS autoneg on advertise $IF_ETHERNET_AUTONEG" ;;
esac

[ -z "$SETTINGS" ] || $ETHTOOL --change "$IFACE" $SETTINGS

SETTINGS="$(gather_settings ETHERNET_PAUSE_)"
[ -z "$SETTINGS" ] || $ETHTOOL --pause "$IFACE" $SETTINGS

SETTINGS="$(gather_settings HARDWARE_IRQ_COALESCE_)"
[ -z "$SETTINGS" ] || $ETHTOOL --coalesce "$IFACE" $SETTINGS

SETTINGS="$(gather_settings HARDWARE_DMA_RING_)"
[ -z "$SETTINGS" ] || $ETHTOOL --set-ring "$IFACE" $SETTINGS

SETTINGS="$(gather_settings OFFLOAD_)"
[ -z "$SETTINGS" ] || $ETHTOOL --offload "$IFACE" $SETTINGS