I have been trying to set up an active filter to prevent xntpd keeping my
dialup link open with the following pppd config:


# /etc/ppp/options
/dev/tty00   
115200 crtscts modem
lock 
noauth
defaultroute 
idle
asyncmap 0
mtu 1500 mru 1500
active-filter-in 'port ntp'
active-filter-out 'port ntp'
# /etc/ppp/options ends

#/etc/ppp/peers/demon-red
connect 'chat -vf /etc/ppp/chat/demon-red'
user black-star.demon.co.uk
#/etc/ppp/peers/demon-red

The tcpdump output is as follows
04:45:02.663043 black-star.demon.co.uk.65196 > nnrp-02.news.demon.net.nntp: F 663:663(0) ack 1791 win 17520
04:45:02.681389 nnrp-02.news.demon.net.nntp > black-star.demon.co.uk.65196: F 1791:1791(0) ack 663 win 8760 (DF)
04:45:02.682080 black-star.demon.co.uk.65196 > nnrp-02.news.demon.net.nntp: . ack 1792 win 17520
04:45:02.758688 nnrp-02.news.demon.net.nntp > black-star.demon.co.uk.65196: . ack 664 win 8760 (DF)

<<<<<< Last none ntp packet here >>>>>>
04:45:12.022866 black-star.demon.co.uk.ntp > tele-auth-0-de0.ns.demon.net.ntp:  v3 client strat 3 poll 7 prec -14
04:45:12.161243 tele-auth-0-de0.ns.demon.net.ntp > black-star.demon.co.uk.ntp:  v3 server strat 3 poll 7 prec -17
04:45:33.023430 black-star.demon.co.uk.ntp > ntp.demon.co.uk.ntp:  v3 client strat 3 poll 7 prec -14
04:45:33.160350 ntp.demon.co.uk.ntp > black-star.demon.co.uk.ntp:  v3 server strat 2 poll 7 prec -18
04:46:16.024792 black-star.demon.co.uk.ntp > tele-auth-0-de0.ns.demon.net.ntp:  v3 client strat 3 poll 7 prec -14
04:46:16.157906 tele-auth-0-de0.ns.demon.net.ntp > black-star.demon.co.uk.ntp:  v3 server strat 2 poll 7 prec -17
04:46:37.026457 black-star.demon.co.uk.ntp > ntp.demon.co.uk.ntp:  v3 client strat 3 poll 7 prec -14
04:46:37.157036 ntp.demon.co.uk.ntp > black-star.demon.co.uk.ntp:  v3 server strat 2 poll 7 prec -18

<<<<<<< Expect link termination at 04:47:12 >>>>>>>
04:48:24.028957 black-star.demon.co.uk.ntp > tele-auth-0-de0.ns.demon.net.ntp:  v3 client strat 3 poll 7 prec -14
04:48:24.161243 tele-auth-0-de0.ns.demon.net.ntp > black-star.demon.co.uk.ntp:  v3 server strat 2 poll 7 prec -17


I also tried negating the active filter rule but this made no difference.

Kernel config includes PFIL_HOOKS and PPP_FILTER and pppd was built with 
PPP_FILTER defined.

This is all with current supped 19/11/1999.