How to get a user's IDLE time?

[hanxin <hanxin@xxxxxxxxxxxxxxxxxx>]

Hi,
I have a Redhat 7.2 box with kernel 2.4.18. I use iptables for NAT and
there are 20 clients in my firewall.
Now I want to know is there a way to get a client PC's IDLE time(the
time that the PC doesn't send/receive packets through firewall).

Thanks.

--
hanxin