how to filter out teardrop in linux2.4

[zhengchuanbo]

i use linux2.4 as a router. when i tested the device i found that teardrop packets can penetrate the router. i remember that in linux2.2 there is a defragmant option. that can filter out the teardrop packets. but i think in linux2.4 the defragmen is action in default.
i tried the -f option in iptables,but that also could not stop the teardrop packets.
so how to filter out teardrop packets in linux2.4?
please cc. thanks.

regards,
chuanbo zheng
[e-mail]