ipfilter traffic blocking and tcpdump snort etc

[Jez Hancock]

Hi Horio,

Cheers for reply.

On Fri, Dec 05, 2003 at 07:33:49PM +0900, horio shoichi wrote:
> On Fri, 5 Dec 2003 00:24:12 +0000
> Jez Hancock <jez.hancock at munk.nu> wrote:
> > Hi,
> > 
> > I've blocked a dozen or so addresses using ipfilter:
> > 
> > block in quick on fxp0 from 208.186.60.116 to any
> > block in quick on fxp0 from 216.230.149.11 to any
> > 
> > etc
> > 
> > but I still see a lot of traffic those hosts in trafshow, snort and
> > other packet capturing utils.  Why is this?
> 
> You are probably seeing the supposedly blocked packets on the "outside" of
> network. Observe them on "inside", i.e., on the interface not fxp0.
Not sure what you mean here, what command would you issue via tcpdump or
snort to do what you suggest?  

> What you are seeing are packets ipfilter is just about to handle.
Right - it's just I would have thought that ipfilter handled packets
before they reached any traffic dumping utils.  I see what you're
getting at.  Presumably snort for example uses the bpf driver via pcap(?) to
capture network traffic...

actually reading bpf(4) clears things up a little:

     Associated with each open instance of a bpf file is a user-settable
     packet filter.  Whenever a packet is received by an interface, all file
     descriptors listening on that interface apply their filter.  Each
     descriptor that accepts the packet receives its own copy.

> > Is there any alternative method of blocking access from certain hosts
> > so that this traffic is not 'seen' by higher level /userland apps?
> I don't understand your second question. Are you thinking about tcp wrapper,
> reset feature of snort, etc ?
Let me rephrase that one :P  I meant is there a method - for example
such as adding some kind of routing via arp - so that packets are
dropped on the floor even quicker than they would be via the firewall
method?

-- 
Jez Hancock
 - System Administrator / PHP Developer

http://munk.nu/