local process packet puzzled the ppp idle time

Hi guys,

I am implementing a nat box with linux, and the box is requested ping
one public ip every 1 minutes.

but when the wan is pppoe with a specified idle time.
since the packet from local process(ping) will be send wit 1 minite
interval,
the ppp connection will never idled.

I found kernel update the ppp->last_xmit in ppp_generic.c::
ppp_send_frame().
so I just need let ppp ignore all outgoing packet from local process.

but how can I judge if the skb is from local process or recieved from
lan interface?

someone tell me getsocketop() or parse /proc/net/ip_conntrack maybe a
possible way,
is that so?

Can I get the original source address by NATed skb from OUTPUT table?

Any suggestions is sincerely appreciated!!

Sailor