[Snort-users] FAQ Suggestion: snort & iptables

Phil Wood cpw at ...440...
Fri Dec 20 07:32:03 EST 2002

It has been my experience on linux that promiscuous mode sees all the junk
on the wire.  Iptables can be set to block all input, but libpcap will see
all packets on the wire.  Snort uses libpcap.

On Thu, Dec 19, 2002 at 06:58:40PM -0500, Matt Kettler wrote:
> I agree with Michael. This is also becoming a good candidate for the Snort 
> FAQ. I think I've seen this question at least a dozen times on the 
> snort-users list.
> Snort is NOT directly affected by ipchains/iptables/ipf/etc. I've 
> repeatedly used snort with "deny all" rules on linux 2.2.x, linux 2.4.x, 
> OpenBSD. My main snort box is a OpenBSD box set up this way. It sees 
> whatever comes out of or goes into the network adapter. Period.
> In fact, I'd actually recommend that everyone use snort listening on an 
> stealth interface (ie: no IP) _and_ "deny all" rules applied to the packet 
> filter for that interface whenever possible.
> FAQ Maintainer suggested FAQ addition (comments/improvements/modifications 
> welcome):
> Q:  Does snort see packets filtered by IPTables/IPChains/IPF?
> A: Snort operates using libpcap. In general it sees everything the network 
> adapter driver sees. Linux IPTables, Linux IPChains, BSD IPF and other 
> packet filters do not prevent snort from seeing a packet that is present on 
> the network wire. Even if an inbound packet is denied by the packet filter 
> Snort will still see and analyze the packet if it is listening to that 
> interface.
>         Note however that Snort is affected to the extent that the stream 
> of data on the network wire is affected. Thus Snort will not see outbound 
> packets which were denied while being sent since they will never reach the 
> network adapter.
> At 05:45 AM 12/20/2002 +0800, Michael Boman wrote:
> >I beg to differ:
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Phil Wood, cpw at ...440...

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