On Thu, 22 Oct 1998, Alessandro Ambrosini wrote:
> I had supposed to use "active-filter" and the filter expression syntax
> is equal to tcpdump. Well, using tcpdump for experiment and tuning, I
> cannot 'see' the various LCP packet. Also using a typical "tcpdump -e
> -i ppp0" , I see only supported protocol (TCP, UDP, ...). In tcpdump
> man page there was not reference to LCP and/or PPP link layer protocol.
> So, if I cannot create the "filter-expression" for "active-filter"
> using tcpdump, how can I filter the LCP packets ?? :-) My knowledge in
> PPP is not enough to create "on the paper" the right filter-expression.
> I need "try and buy" the right expression :-)
>
> Any idea ?
I'm still using 2.3.3, so my man page may be out of date regarding the use
of 'active-filter'. Is this option available for Linux in 2.3.5? The
2.3.3 man page also says that PPP_FILTER must be defined for the kernel
and pppd at compile time, as well.
I believe that the PPP interface filters out the LCP echoes before tcpdump
sees the packets. You might look in the pppd source and see what a LCP
echo looks like in the code, then create the filter accordingly. It might
also show up in kdebug packet dumps, but I haven't tried this.
Geof
DISCLAIMER: The comments above are my own and may not represent the views
of my employer.
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: Geoffrey P. Goodrum : US Department of Commerce :
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