nixpkgs/system/etc/dhclient-exit-hooks

echo "$reason" >> /tmp/dhcp-exit
echo "$exit_status" >> /tmp/dhcp-exit

if test "$reason" = BOUND -o "$reason" = REBOOT; then
    @glibc@/sbin/nscd --invalidate hosts
    @upstart@/sbin/initctl emit ip-up
fi

if test "$reason" = EXPIRE -o "$reason" = RELEASE; then
    @upstart@/sbin/initctl emit ip-down
fi