[Snort-users] Capturing incoming packets?

Erek Adams erek at ...950...
Sat Jun 14 14:43:06 EDT 2003

On Fri, 13 Jun 2003 guano at ...9464... wrote:

> Unfortunately, the option you provided will not work:
> > 	snort -l <logdir> -b 'not net <local_lan>'
> This will filter out things that I want to capture, such as someone
> initiating a port scan against my network.
> In addition, this command does not take sessions or initiation
> direction into account.

My bad.  I forgot one part:

	'src not net <local>'

After reading the below, I realized that you're going to have some
issues, so that may not work as you want.

> What I have is a WAN connection (e.g., cable modem or DSL) and
> a firewall with NAT protecting the LAN.  The firewall logs only
> the basics (when, what) but not the details (packet content, fractional
> packets, anything TCP beyond SYN).
> My Linux IDS is tapped into the area between the WAN connection and
> the firewall (e.g., DMZ).  So it is in a position to see all traffic
> leaving the firewall, as well as everything that comes toward the
> firewall.  In particular, it is in a position to see everything
> that does not make it though the firewall.
> What I want to capture are only the packets that are:
> (1) Heading toward the firewall from the WAN, *and*

Easy enough.  Since you're using NAT, all you need is the "front" IP of
the firewall.  Then just do something like:

	snort -l <dir> -b 'not src <IP>'

> (2) Not in reply to anything sent out from the firewall/NAT.

Easily done.  See above.

> Thus, only unrequested packets (sniffs, attacks, "something unexpected")
> will be captured.

Well, for what you want, you might be better off using tcpdump.  You don't
seem to care about the rules, you just want the traffic.  If that's the
case just use tcpdump instead.

If you do care about the rules, just write some that ignore what you want
and alert on everything else.

	alert tcp $EXTERNAL_NET any -> $HOME_NET 22 (msg: "Incoming port
22 connection.';)

There's been some good threads on 'anomaly detection' in the archives.
I'd suggest you read over that and make use of some of those ideas/rules.


Erek Adams

   "When things get weird, the weird turn pro."   H.S. Thompson

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