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Re: [snort-devel] Chainning pre-processors
From: Hui Cao <hcao () sourcefire com>
Date: Thu, 05 Dec 2013 09:49:49 -0500

Make sure you have frag3 enabled. You can try the application preprocessors (such as http, smtp, ssl, sip etc) to see how it works. If the packets are reassembled, you can see that from snort exit stats for stream etc.

Stream will reassemble the payloads of TCP.

Best,
Hui.

On 12/05/2013 08:26 AM, Emiliano Fausto wrote:
Hello Hui,

you are totally right. I was confused, I didn't have to assemble fragmented IP packets.

So, I configured the Stream5 preprocessor in order it to take into account the port of my application (it's 9090).

I let my AddPreproc() with priority set to PRIORITY_APPLICATION so that it's registered after the Stream5.

Then, I tried to get any packet with (p->flags & FLAG_REBUILT_STREAM), but none of them matched, it seems that the Stream5 preprocessor isn't reassembling my TCP segments.

The SNORT configuration file is set like this:

preprocessor stream5_global: track_tcp yes, \
    track_udp yes, \
    track_icmp no, \
    max_tcp 262144, \
    max_udp 131072, \
    max_active_responses 2, \
    min_response_seconds 5
preprocessor stream5_tcp: policy windows, detect_anomalies, require_3whs 180, \
    overlap_limit 10, small_segments 3 bytes 150, timeout 180, \
    ports client 21 22, \
    ports both 9090
preprocessor stream5_udp: timeout 180

My app works on port 9090.

Maybe, the problem is not related to stream5, maybe the problem is originated because the one in charge of doing the segments reassembly is my preprocessor, at the application level, and not the stream5 preprocessor at the TCP level.

More on this, into wireshark I can see all the packets of the protocol TCP segmented, and if I go to preferences-->protocols-->TCP it has an option that says: "allow dissectors to reassemble TCP". It means that the reassemble isn't being done at the TCP layer, but into the proper application.

Do you know if SNORT has something of this? Or the Stream5 should reassemble the payloads of the TCP packets?

Thanks a lot in advance,
Emiliano.


2013/12/4 Hui Cao <hcao () sourcefire com <mailto:hcao () sourcefire com>>

    Frag3 will deal with IP fragmentation. You might want reassembled
    data from TCP. This means your preprocessor should be after stream
    and you should check
    (SFSnortPacket*)ipacketp)->flags & FLAG_REBUILT_STREAM. Snort will
    only reassemble up to the flush point defined (typically 16K). You
    can't expect snort will give all the data from one reassembled packet.

    Fragmentation from wireshark might mean TCP segmentation. You
    might get full IP packets.

    Best,
    Hui.
    On 12/04/2013 03:17 PM, Emiliano Fausto wrote:
    Hui,

    yes, I understood what you told me about filtering just the
    de-fragmented packets, that's why I added these 3 lines:

    SFSnortPacket *p = (SFSnortPacket *)pkt;
    int fla = (p->flags & FLAG_REBUILT_FRAG);
    _dpd.logMsg("Flags set? %d",fla);

    But I generate fragmented packets (which for instance Wireshark
    detects and show as fragmented and the possibility of reassembly
    them) and I'm always getting with each one the output: "Flags
    set? 0".

    I receive all the fragmented packets, but not the last one
    reassembled.

    Regards,
    Emiliano.


    2013/12/4 Hui Cao <hcao () sourcefire com <mailto:hcao () sourcefire com>>

        It will still get all packets including the raw packets. You
        need to
        use packet flags to filter them.

        Best,
        Hui.

        On Wed, Dec 4, 2013 at 3:00 PM, Emiliano Fausto
        <emiliano.fausto () gmail com
        <mailto:emiliano.fausto () gmail com>> wrote:
        > Yes,
        >
        > that's what I thought, but for some reason the TCP packets
        keep on coming
        > fragmented to my preprocessor.
        >
        > No I took off all the preprocessors and just let the frag3
        and mines, I'll
        > try to figure out if they are called in the correct order,
        but they are not
        > being chained.
        >
        > Thanks,
        > Emiliano.
        >
        >
        > 2013/12/4 Hui Cao <hcao () sourcefire com
        <mailto:hcao () sourcefire com>>
        >>
        >> It looks good to me.
        >>
        >> Best,
        >> Hui.
        >>
        >> On Wed, Dec 4, 2013 at 2:44 PM, Emiliano Fausto
        >> <emiliano.fausto () gmail com
        <mailto:emiliano.fausto () gmail com>> wrote:
        >> > Hi Hui,
        >> >
        >> > I've seen that I was using PRIORITY_TRANSPORT, which is
        lower than the
        >> > PRIORITY_NETWORK that uses frag3.
        >> >
        >> > Anyway, I put the priority: PRIORITY_LAST to my own
        preprocessor, but
        >> > when
        >> > the TCP packets keep arriving fragmented to my preprocessor.
        >> >
        >> > Is there anything else I should take into account?
        >> >
        >> > Thanks in advance,
        >> > Emiliano.
        >> >
        >> >
        >> > 2013/12/4 Hui Cao <hcao () sourcefire com
        <mailto:hcao () sourcefire com>>
        >> >>
        >> >> In sr/preprocids.h
        >> >>
        >> >> Best,
        >> >> Hui.
        >> >>
        >> >> On 12/04/2013 02:36 PM, Emiliano Fausto wrote:
        >> >>
        >> >> Great,
        >> >>
        >> >> so, the pre-processors are "chained" by default, and
        the order that
        >> >> SNORT
        >> >> follows to call them is set by the PRIORITY variable.
        >> >>
        >> >> Do you know where's defined this PRIORITY variable?
        Because I saw that
        >> >> the
        >> >> frag3 is being registered with PRIORITY_NETWORK, so I'd
        like to set the
        >> >> priority of my own preprocessor as (PRIORITY_NETWORK -1).
        >> >>
        >> >> Thanks in advance,
        >> >> Emiliano
        >> >>
        >> >>
        >> >> 2013/12/4 Hui Cao <hcao () sourcefire com
        <mailto:hcao () sourcefire com>>
        >> >>>
        >> >>> sc means snort configuration. We use PRIORITY to sort
        the processing.
        >> >>> All
        >> >>> processors enabled will be called and processed based
        on priority. You
        >> >>> have
        >> >>> to rely on the code to figure out what exactly snort does.
        >> >>>
        >> >>> The checking is correct. You will only process rebuilt
        packets.
        >> >>>
        >> >>> Best,
        >> >>> Hui.
        >> >>> On 12/04/2013 02:19 PM, Emiliano Fausto wrote:
        >> >>>
        >> >>> Hello Hui,
        >> >>>
        >> >>> thanks a lot for your answer.
        >> >>>
        >> >>> Right now I have registered my preprocessor (let's call it
        >> >>> examplePreprocess as you said, because right now I'm
        using the one
        >> >>> provided
        >> >>> with the DPX) with this line:
        >> >>>
        >> >>> _dpd.addPreproc(ExampleProcess, PRIORITY_TRANSPORT, 10000,
        >> >>> PROTO_BIT__TCP);
        >> >>>
        >> >>> So, the only change is to add previous to the parameter
        >> >>> ExampleProcess,
        >> >>> the "sc". What does it mean? Do you know if there's
        any documentation
        >> >>> about
        >> >>> this chaining preprocesses?
        >> >>>
        >> >>> So, checking the flags, should be:
        >> >>>
        >> >>> (SFSnortPacket*)tcppacket)->flags & FLAG_REBUILT_FRAG
        >> >>>
        >> >>> right?
        >> >>>
        >> >>> Thanks again!
        >> >>> Emiliano.
        >> >>>
        >> >>>
        >> >>>
        >> >>> Then, I'll have to register my own preprocessor where?
        >> >>>
        >> >>>
        >> >>> 2013/12/4 Hui Cao <hcao () sourcefire com
        <mailto:hcao () sourcefire com>>
        >> >>>>
        >> >>>> Yes, it is possible. You can register you
        preprocessor like this:
        >> >>>>
        >> >>>> _dpd.addPreproc( sc, ExampleProcess,
        PRIORITY_TRANSPORT, You_PP_ID,
        >> >>>> PROTO_BIT__IP );
        >> >>>>
        >> >>>> Remember check the following flag in your ExampleProcess:
        >> >>>>
        >> >>>> (SFSnortPacket*)ipacketp)->flags & FLAG_REBUILT_FRAG
        >> >>>>
        >> >>>> Best,
        >> >>>> Hui.
        >> >>>>
        >> >>>>
        >> >>>> On 12/04/2013 12:52 PM, Emiliano Fausto wrote:
        >> >>>>
        >> >>>> Hi everybody,
        >> >>>>
        >> >>>> I'm creating a new preprocessor which needs to have
        the whole content
        >> >>>> in
        >> >>>> a packet which was fragmented.
        >> >>>>
        >> >>>> So I thought of using the frag3 preprocessor to
        re-assembly the
        >> >>>> packets,
        >> >>>> and then, when this reassembly is done, sent it to my own
        >> >>>> preprocessor.
        >> >>>>
        >> >>>> Do you know if this is possible? May I have the
        output of frag3 being
        >> >>>> the input of my own preprocessor?
        >> >>>>
        >> >>>> Regards,
        >> >>>> Emiliano.
        >> >>>>
        >> >>>>
        >> >>>>
        >> >>>>
        >> >>>>
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