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Re: HTTP Reassembly issue PAF enabled
From: Parmendra Pratap <parmendra.pratap () yahoo com>
Date: Mon, 8 Apr 2013 15:49:41 -0700 (PDT)

Exact command I am using is:
snort --daq dump --daq-var load-mode=read-file -Q -i eth0 -c etc/snort.conf 
Fails to start with following error
Can't initialize DAQ dump (-1) - eth0: No such file or directory

No typo issue as such, but its perhaps just that I am trying to run it from a device instead of a pcap file.

Following works for me as desired:

snort --daq dump --daq-var load-mode=passive -Q -i eth0 -c etc/snort.conf


Thanks file=/dev/null takes care of the dump file and disk saturation.

I do get why normalization in general makes more sense for inline mode, but putting reassembly "first" seems to be 
something just not exclusive to inline mode.
Would be great if there was a config hook to enable/disable this in NIDS mode.
I guess its more of a matter of opinion :)

Thanks for the help. 
The promptness here is impressive and motivates me more to go the snort route for my current IDS and n/w traffic 
analysis needs.

Thanks
Parmendra


________________________________
 From: Russ Combs <rcombs () sourcefire com>
To: Parmendra Pratap <parmendra.pratap () yahoo com> 
Cc: "snort-devel () lists sourceforge net" <snort-devel () lists sourceforge net> 
Sent: Monday, 8 April 2013 6:33 PM
Subject: Re: [Snort-devel] HTTP Reassembly issue PAF enabled
 
On Mon, Apr 8, 2013 at 12:51 PM, Parmendra Pratap
<parmendra.pratap () yahoo com> wrote:
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.

Maybe just a typo, but that should be "load-mode=read-file" ( '-' not '_' ).

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?

Yes - normalizations require inline operation.  No point changing a
packet if isn't being forwarded.  Also, stream5 tries to reassemble
the way the receiving host does.  This won't be the case if not
inline.

You can try something like --daq-var file=/dev/null if you don't want
to write the file.

Regards
Parmendra


From: Russ Combs <rcombs () sourcefire com>
To: Parmendra Pratap <parmendra.pratap () yahoo com>
Cc: "snort-devel () lists sourceforge net" <snort-devel () 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 () yahoo com> 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 ?

Yes

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 () sourcefire com>
To: Parmendra Pratap <parmendra.pratap () yahoo com>
Cc: "snort-devel () lists sourceforge net"
<snort-devel () lists sourceforge net>
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 () yahoo com> 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 2.9.4.5.
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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Reduce network management and security costs.Learn how to hire 
the most talented Cisco Certified professionals. Visit the 
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_______________________________________________
Snort-devel mailing list
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