/etc/ppp/ip-down.d/000resolvconf is in resolvconf 1.78ubuntu2.
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 12 13 14 15 16 17 18 19 20 | #!/bin/sh
#
# PPP down hook script for resolvconf
#
# Reconfigures resolver to take into account
# the disappearance of the ppp interface.
#
# This file is part of the resolvconf package.
#
[ -x /sbin/resolvconf ] || exit 0
case "$6" in
nm-pptp-service-*|nm-l2tp-service-*|/org/freedesktop/NetworkManager/PPP/*)
# NetworkManager handles it
exit 0
;;
esac
/sbin/resolvconf -d "${PPP_IFACE}.pppd"
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