[Snort-devel] HTTP Reassembly issue PAF enabled

Parmendra Pratap parmendra.pratap at ...398...
Mon Apr 8 12:51:01 EDT 2013

Instead of --daq dump --daq-var load-mode=read-file , I used --daq dump --daq-var load-mode=passive, as load_mode=read_file was not starting and looking for a file path.
I can see its functionally working for sure as far as TCP headers and HTTP PDU reassembly is concerned.
There are couple of issues though - it creates a dump file, namely inline-pcap.out file which keeps growing until I run out of disk space.
Isn't there a way to turn tcp normalization ON on without needing to dump pcap to file system.
Performance seems to be OK but would defeinitely be desirable from my end to be able to do TCP normalization without needing to dump to the filesystem.
Is there a reason behind deactivating TCP Normalization in IDS mode?


 From: Russ Combs <rcombs at ...402...>
To: Parmendra Pratap <parmendra.pratap at ...398...> 
Cc: "snort-devel at lists.sourceforge.net" <snort-devel at lists.sourceforge.net> 
Sent: Monday, 8 April 2013 3:36 PM
Subject: Re: [Snort-devel] HTTP Reassembly issue PAF enabled
On Fri, Apr 5, 2013 at 7:14 PM, Parmendra Pratap
<parmendra.pratap at ...398...> wrote:
> Brilliant - Thanks Russ normalization on tcp sorts this out.
> Just a a small note - I used load_mode=passive.

Not clear what you did but glad it works for you.

> So if I go it right with normalization on , reverse Ack is not awaited for
> in S5 prior to flushing , rest is same and PAF still works as expected to
> for supported protocols/preprocessors ?

> Also , is there at all a flip side to running snort in "forced" inline mode
> on top pcap like this, performance wise may be?

--daq dump --daq-var load-mode=read-file will be slower than straight
readback because it also writes to file all packets that weren't
blocked as well as any injected packets.  Hopefully performance isn't
the major concern in readback mode.

> Thanks
> Parmendra
> ________________________________
> From: Russ Combs <rcombs at ...402...>
> To: Parmendra Pratap <parmendra.pratap at ...398...>
> Cc: "snort-devel at lists.sourceforge.net" <snort-devel at ...2764...t>
> Sent: Friday, 5 April 2013 5:15 PM
> Subject: Re: [Snort-devel] HTTP Reassembly issue PAF enabled
> On Fri, Apr 5, 2013 at 11:56 AM, Parmendra Pratap
> <parmendra.pratap at ...398...> wrote:
>> Hi Hui
>> Thanks again for a quick response.
>> Have tried -Q flag , but since I am using Pcap DAQ it fails to start with
>> -Q
>> set.
>> However it does start wiith --test-inline mode , which I assume works no
>> different from -Q except that drops are not enabled.
> Actually, normalizations are not enabled in that case.  You should see
> something like this in your startup output:
> WARNING: tcp normalizations disabled because not inline.
> Try running with:
> --daq dump --daq-var load-mode=read-file -Q
>> My snort.conf does have preprocessor normalize_tcp: ips have set.
>> But even with the above set up I can still replicate the same issue
>> related
>> to incorrect tcp flags.
>> As I said before , I think it seems like a matter of setting the tcp
>> header
>> of the last packet that completes the PDU inside Packet->tcph->th_flags in
>> the s5 preprocessor when doing a PAF based flushing.
>> I can see that the direction i.e. sourceIP/port and destIP/port being
>> reversed for the same reason in the Packet struct when doing a reassembly
>> based flushing from s5.
>> Thanks
>> Parmendra
>> __________________________________________________________________________
>> Hi Parmendra,
>> To be clear, you must use IPS mode to get what you want, so you need to
>> 1) use -Q  when you run snort
>> 2) Enable Normalization for TCP:
>> preprocessor normalize_tcp: ips
>> Best,
>> Hui.
>> On 04/04/2013 08:25 AM, Parmendra Pratap wrote:
>>> Hi Hui
>>> Thanks for a quick reply.
>>> I tried the use case with Snort
>>> Does not make a difference.
>>> Issue is still replicable with the steps outlined in my root email.
>>> This is what I think is going on , based on few tests and source
>>> lookups <excuse my newbieness if it reflects anywhere below :) >-
>>> Stream5 reassembly does not tag a packet as complete PDU until it
>>> recieves subsequent ack gainst the packet no matter wheter or not the
>>> packet actually holds complete PDU  (in this case HTTP) or not.
>>> With PAF enabled this prevents the URI Bufs from being created and
>>> inspected in HTTP inspect module until the next packet arrives (ie ack
>>> against the original packet that contained the HTTP req).
>>> When the HTTP inspect URI Bufs based match fires, with PAF ON,  its
>>> always(mostly?) when the ack on reverse direction is received.
>>> The spo_alert* modules simply uses the header data provided in the
>>> Packet->tcph which holds the header from the current packet ie ack
>>> packet from the server ... and hence the incorrect TCP header display
>>> with PAF on.
>>> There is no way curently to get the correct TCP headers unless Stream
>>> 5 is queried to give the original raw packet <spo_log_tcp_dump.c does
>>> that>.
>>> Realworld issue arising from the above is incorrect TCP header data in
>>> alerts. TCP dumps are OK though for reason mentioned above.
>>> There seems multiple ways to get around this:
>>> Generate TCP dump on all alerts and assume the alerts will have
>>> incorrect TCP headers
>>> Write an alert output plugin that inspects the raw packet for correct
>>> TCP headers etc
>>> Add more metadata to Packet struct which can provide the correct TCP
>>> headers at least for the last packet that completed the PDU in the
>>> alert output plugins.
>>> Last option looks the most organic and least sub optimal one to me.
>>> Given my little experience with snort so far , I wont be surprised if
>>> any of the above stated flow is incorrect.
>>> I will more than appreciate if someone can correct me above and
>>> enlighten me more about the internals :) .
>>> Thanks
>>> Parmendra
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