[Snort-devel] Trivial question

Reinoud Koornstra sockstat at ...445...
Thu Sep 12 16:48:17 EDT 2013

Ok, so i noticed even when i do active ftp and add port 20 to the stream5 config, no reassembly takes place.

However, when i put the following rule in action:


alert tcp any any -> any 20 (msg:"Attempting FTP streaming enabling"; stream_reassemble:enable,both; sid:61245; rev:1;)


Then it does reassembly and i see packet sizes of


1824, 1904 , 2360 , 2496 , 2776 , 2816 


when i use active ftp.

Date: Thu, 12 Sep 2013 16:22:09 -0400
Subject: Re: [Snort-devel] Trivial question
From: rcombs at ...402...
To: sockstat at ...445...
CC: snort-devel at lists.sourceforge.net

Yes.  What matters is whether you are in IPS mode or not.  That can be done with a pcap too.

On Thu, Sep 12, 2013 at 4:19 PM, Reinoud Koornstra <sockstat at ...445...> wrote:

Ok, thanks.
Does this also hold for snort reading a pcap containing an ftp session?


Date: Thu, 12 Sep 2013 15:50:21 -0400
Subject: Re: [Snort-devel] Trivial question
From: rcombs at ...402...
To: sockstat at ...445...
CC: snort-devel at lists.sourceforge.net

Snort reassembles different protocols differently.  The 17K number is close to the paf_max default of 16K.  PDUs (protocol data units like an HTTP response) larger than paf_max are truncated into paf_max blocks for processing by Snort.  The FTP data channel does not get reassembled in that fashion.  Simplifying things, in inline IPS mode, it will reassemble every 2 data segments.  Otherwise, every 2 or more acknowledged data segments, upon acknowledgement.  Typically this will be around 2*1460 = 2920 bytes.

So, based on the limited info in your question, the answer is yes, that is correct.

On Wed, Sep 11, 2013 at 3:17 PM, Reinoud Koornstra <sockstat at ...445...> wrote:

Dear Everyone,
When i run http traffic through snort, while snort is in inline mode and monitoring the sizeo of the packets, I see that every 4 of more full mtu packets a packet of 17k bytes is being processed by snort. I am seeing this with most kind of traffic, but not with ftp.
Is that correct or not?

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