Re: Use of flowbits in Snort to alert upon reception of second identical packet

[Alex McDonnell <amcdonnell () sourcefire com>]

I don't know what setup you've got or what your rule is but if you set the
rule to "drop" instead of "alert" it should kill the connection once it
hits that count.

Alex

On Tue, Oct 8, 2019 at 4:31 PM stephane potier <stephane.potier () yahoo fr>
wrote:

> Hi Alex,
>
> Thanks, sounds interesting.
> I set count to 1 to reach my objective.
> I observe that the alarm is raised only after tcp connection is closed. Is
> that normal ?
> I need this alarm to be raised before the tcp connection is closed.
>
> Regards,
> Stephane
>
> Le mardi 8 octobre 2019 à 18:26:09 UTC+2, Alex McDonnell <
> amcdonnell () sourcefire com> a écrit :
>
>
> I think you could do this more easily by using detection_filter. This will
> allow you to specify after how many identical (according to the rule)
> packets to alert, and over how long a period of time.
> http://manual-snort-org.s3-website-us-east-1.amazonaws.com/node34.html#SECTION00477000000000000000
>
> Alex McDonnell
>
> On Tue, Oct 8, 2019 at 11:49 AM stephane potier via Snort-sigs <
> snort-sigs () lists snort org> wrote:
>
> Hi,
>
> I am trying to write a snort rule that alert only if the same packet is
> received twice (ip, port and content are identical).
> Idea is to raise the alert only in case of reception of the second same
> packet. Several other frames can be received between those 2 same packets.
> I do not find clear indications how using flowbits for this case. I have
> found a rule that seems to do similar job in
> /etc/snort/rules/web-client.rules (see below), but I am not very clear how
> it really works.
>
> alert tcp $EXTERNAL_NET any -> $HTTP_SERVERS 443 (msg:"WEB-MISC SSLv2
> Client_Hello with pad request"; flow:to_server,established;
> flowbits:isnotset,sslv2.client_hello.request;
> flowbits:isnotset,sslv3.client_hello.request;
> flowbits:isnotset,tlsv1.client_hello.request; byte_test:1,<,128,0;
> content:"|01|"; depth:1; offset:3; content:"|00 02|"; depth:2; offset:6;
> flowbits:set,sslv2.client_hello.request; flowbits:noalert;
> classtype:protocol-command-decode; sid:2659; rev:4;)
> Any explanation of the previous rule and idea is welcome. Particularly how
> flowbits isnotset and set can be written in the same rule, and their
> position in the rule.
>
> Thanks.
>
> Herl
>
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