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Date:      Fri, 3 Jul 98 14:36:11 +0200
From:      THIERRY.HERBELOT@telspace.alcatel.fr
To:        kpielorz@tdx.co.uk, freebsd-questions@FreeBSD.ORG
Subject:   =?ISO-8859-1?Q?R=E9p_:_Re:_tcpdump_on_a_dial-in_PPP_connection_??=
Message-ID:  <H000057c016b6bbc@MHS>
In-Reply-To: <359CC63B.1EBE5AF5@tdx.co.uk>
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     Hi
     
     I **have** bpf configured in my kernel and the bpf? devices also 
     exist.
     
     What i'm looking for is a way to start tcpdump when ppp opens the link 
     (ppp itself being started by mgetty upon recognition of a PPP 
     negociation). I can't have tcpdump listening on tun0 all the time, 
     because tun0 only exists when ppp is called by mgetty :
     
     If I start tcpdump without waiting for PPP to be on :
     
     # tcpdump -i tun0
     tcpdump: tun0: Network is down 

     I've got the following in /etc/ppp/ppp.linkup :
     
     MYADDR:
      delete 0
      add 0 0 HISADDR
      !bg tcpdump -l -i tun0 >> /tmp/tun0
      
     but /tmp/tun0 stays empty
     
     I have also modified the permissions of tcpdump and the bpf devices to 
     allow execution by anyone (big security hole - I know ...).
     
     I'm stuck !!!
     
     
     
     
     ____________________________ Séparateur Réponse 
     ________________________________
Objet : Re: tcpdump on a dial-in PPP connection ?
Auteur :  kpielorz@tdx.co.uk
Date :    03/07/98 13:53


THIERRY.HERBELOT@telspace.alcatel.fr wrote: 
> 
>      Hello,
> 
>      I would like to see what packets are exchanged on a dialin PPP link 
> 
>      I manage my modem with mgetty (as indicated in the handbook)  - on an 
>      incoming call, all goes well : ppp is established, ....
[snip]
> 
>      Any ideas ?
     
Try configuring your kernel to use bpfilters - i.e. add
     
pseudo-device   bpfilter        2
     
To your kernel, recompile it, install it, restart the system...
     
When your connected via PPP you should be able to do something like:
     
tcpdump -i tun0
     
(I think bpf's work with tun interfaces ;-)
     
     
Regards,
     
Karl


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