Re: snort seems to stop working after first hit of drop rule

["Russ Combs \(rucombs\) via Snort-sigs" <snort-sigs () lists snort org>]

Hey Stefan,

When you say all traffic on UDP blocked, are you changing the source or destination addresses or ports between 
attempts?  I ask because both of your alerts show the same 4-tuple.  Typically the source port would be ephemeral and 
change each time.  What happens if you wait 60 seconds and send more of the same traffic?

Snort should be blocking specific 4-tuples, not everything.  And the block should time out after 30 seconds (default 
config) and allow the 4-tuple to pass again.

Also, that’s an ancient version of Snort.  For best results, download the source from snort.org and build that.

Russ

From: Snort-sigs <snort-sigs-bounces () lists snort org> on behalf of Stefan Mayer <stefan.mayer () usaneers de>
Date: Saturday, February 22, 2020 at 8:07 AM
To: "snort-sigs () lists snort org" <snort-sigs () lists snort org>
Subject: [Snort-sigs] snort seems to stop working after first hit of drop rule

Hi everyone.

I am using ubuntu 18.04 lts, and also the latest snort version from apt-get, Version 2.9.7.0 GRE (Build 149). It is 
running inline, calling
/usr/sbin/snort -A console -Q -c /etc/snort/snort.conf -i eno1:enp3s0 -N

I set up the snort.conf, setting  $HOME_NET to 10.10.10.0/25 and disabling all rules except local.rules, with the 
following content:
alert  udp any any -> $HOME_NET 30501 (msg:"packet detected"; sid:10000003; rev:1; content:"|45670123|"; depth:4;)

The result is:
02/21-18:11:48.115016  [**] [1:10000003:1] packet detected [**] [Priority: 0] {UDP} 10.10.10.99:30400 -> 
10.10.10.16:30501

At the receiving end, the packets still arrive as they are supposed to. So far, so good.

After changing the rule to
drop  udp any any -> $HOME_NET 30501 (msg:"packet detected"; sid:10000003; rev:1; content:"|45670123|"; depth:4;)

The result is:
02/21-18:12:42.978438  [Drop] [**] [1:10000003:1] packet detected [**] [Priority: 0] {UDP} 10.10.10.99:30400 -> 
10.10.10.16:30501

Once. For the first packet that matches. After that, the traffic on udp stops arriving at the target, the only thing 
still passing the bridge is a ping. All udp traffic, either matching the rule or missing it, is lost, until I restart 
snort.
Changing the rule to sdrop does not help, either.

How can I resolve this issue? Thanks.

Stefan

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