This file is indexed.

/etc/network/if-up.d/000resolvconf is in resolvconf 1.63ubuntu11.

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
#
# ifup hook script for resolvconf
#
# This file is part of the resolvconf package.
#

[ -x /sbin/resolvconf ] || exit 0

case "$ADDRFAM" in
  inet|inet6) : ;;
  *) exit 0 ;;
esac

R=""
if [ "$IF_DNS_DOMAIN" ] ; then
	R="${R}domain $IF_DNS_DOMAIN
"
fi
if [ "$IF_DNS_SEARCH" ] ; then
	R="${R}search $IF_DNS_SEARCH
"
fi
if [ "$IF_DNS_SORTLIST" ] ; then
	R="${R}sortlist $IF_DNS_SORTLIST
"
fi
for NS in $IF_DNS_NAMESERVERS ; do
	R="${R}nameserver $NS
"
done

echo -n "$R" | /sbin/resolvconf -a "${IFACE}.${ADDRFAM}"