/etc/network/if-down.d/resolvconf is in openresolv 3.5.2-1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 | #!/bin/sh
# ifdown hook script for resolvconf
# Written by Roy Marples <roy@marples.name> under the BSD-2 license
[ -x /sbin/resolvconf ] || exit 0
case "$ADDRFAM" in
inet|inet6) : ;;
*) exit 0;;
esac
[ "$METHOD" = dhcp ] && /sbin/resolvconf -f -d "$IFACE"
/sbin/resolvconf -f -d "$IFACE.$ADDRFAM"
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