/usr/share/doc/ppp/examples/filters is in ppp 2.4.5-5ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 | #
# These filter rules should prevent unwanted internet services to
# keep your connections up by ignoring their connection requests
# and your 'go way' responses.
#
# Activate them by activating the line 'file /etc/ppp/filters' in
# /etc/ppp/options.
#
# Note: This has nothing to do with firewall rules. It only affects
# the idle time calculation of the kernel/pppd.
#
active-filter 'outbound and not icmp[0] == 3 and not tcp[13] & 4 != 0'
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