/etc/ppp/ip-down.d/0dns-down is in pppconfig 2.3.19ubuntu1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
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# $Id: 0dns-down,v 1.2 2004/07/31 20:49:04 john Exp $
# 0dns-down by John Hasler 1999-2004
# Any possessor of a copy of this program may treat it as if it
# were in the public domain. I waive all rights.
# Rev. Apr 12 2004 to use resolvconf if installed.
# 0dns-down takes down what 0dns-up sets up.
# If pppconfig has been removed we are not supposed to do anything.
test -f /usr/sbin/pppconfig || exit 0
# Strip options.
PROVIDER=`echo "$PPP_IPPARAM" | cut -d' ' -f1`
ETC="/etc"
RUNDIR="/var/cache/pppconfig"
RESOLVCONF="$ETC/resolv.conf"
RESOLVBAK="$RUNDIR/resolv.conf.bak.$PROVIDER"
TEMPRESOLV="$RUNDIR/0dns.$PROVIDER"
PPPRESOLV="$ETC/ppp/resolv"
if [ -x /sbin/resolvconf ]; then
[ "$1" = "0dns-clean" ] && exit 0
test -n "$PPP_IFACE" || exit 1
/sbin/resolvconf -d "${PPP_IFACE}.pppconfig"
fi
umask 022
cd "$RUNDIR" || exit 1
# Are we being called by dns-clean? If so fix up /etc/resolv.conf
# and clean out /var/cache/pppconfig.
if [ "$1" = "0dns-clean" ]
then
# Get the name of the provider active when we went down. Assume there was only one.
PROVIDER=`ls -t resolv.conf.bak.* 2>/dev/null | awk 'BEGIN {FS = "." } /resolv\.conf\.bak/ {print $NF} {exit}'`
# If we don't have a provider we have nothing to do.
if [ -n "$PROVIDER " ]
then
RESOLVBAK="$RUNDIR/resolv.conf.bak.$PROVIDER"
[ -s "$RESOLVBAK" ] && /bin/cp -Lp "$RESOLVBAK" "$RESOLVCONF"
fi
exit 0
fi
# If we don't have a provider we have nothing to do.
[ -z "$PROVIDER" ] && exit 0
# Is PROVIDER something we can use?
test -f "$PPPRESOLV/$PROVIDER" || exit 0
# It is not an error for RESOLVBAK not to exist.
if [ ! -f "$RESOLVBAK" ]
then
rm -f "$TEMPRESOLV"
exit 0
fi
# Make sure that the resolv.conf that 0dns-up installed has not been
# altered. If has give up.
if [ `stat -c %Y "$TEMPRESOLV"` -ne `stat -c %Y "$RESOLVCONF"` ]
then
rm -f "$TEMPRESOLV" "$RESOLVBAK"
exit 0
fi
# Restore resolv.conf. Follow symlinks.
/bin/cp -Lp "$RESOLVBAK" "$RESOLVCONF" || exit 1
rm -f "$RESOLVBAK" "$TEMPRESOLV"
# Tell nscd about what we've done.
# Restart nscd because resolv.conf has changed
[ -x /etc/init.d/nscd ] && { invoke-rc.d nscd restart || true ; }
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