ipfilter traffic blocking and tcpdump snort etc

horio shoichi bugsgrief at bugsgrief.net
Fri Dec 5 18:37:00 PST 2003


On Fri, 5 Dec 2003 10:58:39 +0000
Jez Hancock <jez.hancock at munk.nu> wrote:
> 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?  

Um, that's my bad assumption !
I thought your box is a filtering router, and has at least two interfaces.

> 
> > 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.
> 
The "log" keyword on blocking rules would have helped...

> > > 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?

In my observation, packet filters are the quickest since blocked packets die
in ip_input(), below which is where ethernet interrupt handlers are laid out.


horio shoichi

> 
> -- 
> Jez Hancock
>  - System Administrator / PHP Developer
> 
> http://munk.nu/
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