Figure 2: Morningstar PPP filter configuration file

$ cat Filter
#
#/etc/ppp/Filter - PPP configuration file binding packet type
#                  to actions.
#

internet-gateway
bringup !ntp !3/icmp !5/icmp !11/icmp !who !route !nntp
pass    nntp/137.39.1.2 !nntp
!telnet/syn/recv
!ftp/syn/recv !login/syn/recv !shell/syn/recv !who
!sunrpc !chargen !tftp !supdup/syn/recv !exec !syslog
!route !6000/tcp/syn/send
keepup  !send !ntp !3/icmp !5/icmp !11/icmp !who !route
log     rejected

default bringup !ntp !3/icmp !5/icmp !11/icmp !who !route
keepup  !send !ntp !3/icmp !5/icmp !11/icmp !who !route
$