Snort mailing list archives
Re: HTTP Reassembly issue PAF enabled
From: Russ Combs <rcombs () sourcefire com>
Date: Mon, 8 Apr 2013 10:36:53 -0400
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 -QMy 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------------------------------------------------------------------------------ Minimize network downtime and maximize team effectiveness. Reduce network management and security costs.Learn how to hire the most talented Cisco Certified professionals. Visit the Employer Resources Portal http://www.cisco.com/web/learning/employer_resources/index.html _______________________________________________ Snort-devel mailing list Snort-devel () lists sourceforge net https://lists.sourceforge.net/lists/listinfo/snort-devel Archive: http://sourceforge.net/mailarchive/forum.php?forum_name=snort-devel Please visit http://blog.snort.org for the latest news about Snort!
------------------------------------------------------------------------------ Minimize network downtime and maximize team effectiveness. Reduce network management and security costs.Learn how to hire the most talented Cisco Certified professionals. Visit the Employer Resources Portal http://www.cisco.com/web/learning/employer_resources/index.html _______________________________________________ Snort-devel mailing list Snort-devel () lists sourceforge net https://lists.sourceforge.net/lists/listinfo/snort-devel Archive: http://sourceforge.net/mailarchive/forum.php?forum_name=snort-devel Please visit http://blog.snort.org for the latest news about Snort!
Current thread:
- HTTP Reassembly issue PAF enabled Parmendra Pratap (Apr 02)
- Re: HTTP Reassembly issue PAF enabled Hui Cao (Apr 03)
- <Possible follow-ups>
- Re: HTTP Reassembly issue PAF enabled Parmendra Pratap (Apr 04)
- Re: HTTP Reassembly issue PAF enabled Hui Cao (Apr 04)
- Re: HTTP Reassembly issue PAF enabled Hui Cao (Apr 04)
- Re: HTTP Reassembly issue PAF enabled Parmendra Pratap (Apr 05)
- Re: HTTP Reassembly issue PAF enabled Russ Combs (Apr 05)
- Re: HTTP Reassembly issue PAF enabled Parmendra Pratap (Apr 05)
- Re: HTTP Reassembly issue PAF enabled Russ Combs (Apr 08)
- Re: HTTP Reassembly issue PAF enabled Parmendra Pratap (Apr 08)
- Re: HTTP Reassembly issue PAF enabled Russ Combs (Apr 08)
- Re: HTTP Reassembly issue PAF enabled Parmendra Pratap (Apr 08)
- Re: HTTP Reassembly issue PAF enabled Russ Combs (Apr 12)
- Re: HTTP Reassembly issue PAF enabled Parmendra Pratap (Apr 15)