[Snort-users] Getting alerts for every file Snort detects and File Services preprocessor

Pablo Cantos Polaino pcantos at ...16842...
Mon Mar 30 05:53:43 EDT 2015


Hi Victor,

First of all, thanks for your reply!

Regarding your proposal below:

The "file_identify" preprocessor is designed to work more as a "reputation"
> based system. I think you will get better millage by using the new
> "file_type" rule keyword in a plain-old snort rule.
> Something like this:
> alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"GIF file downloaded";
> flow:to_client,established; file_type:GIF; sid:1000000;)
> alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"GIF file downloaded";
> flow:to_client,established; file_type:PNG; sid:1000001;)


We already tried that and we got a lot of alerts. The sizer the file is the
more alerts are fired, and this is not the behavior we expected since we
would overpopulate the disk with repeated events. For instance, after
downloading a BZ file with size of 7,5MB, we've got 855 alerts from one
just rule:

alert tcp any any -> any any (msg: "BZ file"; flow:to_client,established;
file_type:BZ; sid:3000033;)

And we get just one alert if we use the rule below and the following
configuration:

Conf (just type_id, neither signature nor capture)

preprocessor file_inspect: type_id

Rule:

alert (msg: "BZ file"; gid:146; sid:32;)

This is why we discarded some weeks ago the way of writing the rule as you
proposed now. If it were a way to get just one alert per file it would be
great. Meanwhile, we await the bug can be fixed.

Best Regards,

Pablo Cantos
redborder.org / pcantos at ...16842...

2015-03-27 22:46 GMT+01:00 Jaime Nebrera <jnebrera at ...16842...>:

> Hi Víctor,
>
> I'm not that technical ;) Pablo will coment on your ideas, I just tried to
> give you a deeper context of what Pablo (as part of my team) was doing but
> for sure, they are welcomed. I let them continue
> El 27/03/2015 21:25, "Victor Roemer" <viroemer at ...589...> escribió:
>
>> Jamie,
>>
>> That's neat, I'll keep an eye on it. :)
>>
>> Reviewing over how the "file_identify" works currently, when doing
>> capture or signature
>> its not possible to also get the file type event via the preprocessor.
>> I'll open an bug for this.
>>
>> In order to get the file type events with these further settings, more
>> sophisticated tracking
>> needs to be in-place in the file_identify preproc; the tricky bits being
>> that the file service code
>> is generating the events in question; however once a verdict is passed to
>> file service- an
>> action will be taken at that time.
>>
>> To clarify the verdicts -
>> As I understand it, passing FILE_VERDICT_UNKNOWN is how processing
>> continues further.
>> The FILE_VERDICT_PENDING when passed to file service api, should cause
>> the packet to be
>> dropped, in-order to give the interfacing code time to retrieve a true
>> verdict when the re-transmitted
>> packet is seen.
>>
>> On 03/27/15 16:04, Jaime Nebrera wrote:
>>
>>> Hi Víctor,
>>>
>>> Pablo's work is geared towards a more ambitious goal:
>>>
>>> 1) Intercept the file and send it to a S3 based storage platform for
>>> further analysis or whatever
>>>
>>> 2) Provide as much context about such interception as possible, like from
>>> were to whom, URL, email, etc
>>>
>>> This we hope to open source and make it public as soon as it is usable in
>>> our repository www.github.com/redborder
>>> El 27/03/2015 20:50, "Victor Roemer" <viroemer at ...589...> escribió:
>>>
>>>    Pablo,
>>>>
>>>> Sorry for the delay. The "file_identify" preprocessor is designed to
>>>> work
>>>> more as a "reputation" based system. I think you will get better
>>>> millage by
>>>> using the new "file_type" rule keyword in a plain-old snort rule.
>>>>
>>>> Something like this:
>>>>
>>>> alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"GIF file downloaded";
>>>> flow:to_client,established; file_type:GIF; sid:1000000;)
>>>> alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"GIF file downloaded";
>>>> flow:to_client,established; file_type:PNG; sid:1000001;)
>>>>
>>>>
>>>> You'll still need to have "file_magic.conf" included in your Snort
>>>> configuration, but you will not need the file_identify preprocessor.
>>>>
>>>> ~Victor
>>>>
>>>> On 03/17/15 4:57, Pablo Cantos Polaino wrote:
>>>>
>>>> Hello all,
>>>>
>>>> I already sent this issue to snort-devel with the same subject since I
>>>> am
>>>> not sure if either I am configuring Snort in the right way or there is
>>>> an
>>>> internal malfunction to fix.
>>>>
>>>> I have been testing the new experimental preprocessor called File
>>>> Services
>>>> in order to get an event every time a file go through our network. To
>>>> carry
>>>> on these tests I have used two pcap files. The first one is a 1GB-size
>>>> pcap
>>>> with a great number of files and the second one is a short pcap
>>>> generated
>>>> on my computer when I downloaded a GIF file.
>>>>
>>>> My snort.conf file is configured like this at the end:
>>>>
>>>> include file_magic.conf
>>>>
>>>>   preprocessor file_inspect: type_id, signature, \
>>>>               capture_queue_size 5000, \
>>>>               capture_disk /home/file_capture/tmp/
>>>>
>>>>
>>>>
>>>> In both cases files are captured by the preprocessor, as you can see
>>>> below
>>>> (1GB pcap output):
>>>>
>>>> ============================================================
>>>> ===================
>>>>
>>>>   Action Stats:
>>>>       Alerts:            0 (  0.000%)
>>>>       Logged:            0 (  0.000%)
>>>>       Passed:            0 (  0.000%)
>>>> Limits:
>>>>        Match:            0
>>>>        Queue:            0
>>>>          Log:            0
>>>>        Event:            0
>>>>        Alert:            0
>>>> Verdicts:
>>>>        Allow:      8418451 ( 97.482%)
>>>>        Block:            0 (  0.000%)
>>>>      Replace:            0 (  0.000%)
>>>>    Whitelist:       217492 (  2.518%)
>>>>    Blacklist:            0 (  0.000%)
>>>>       Ignore:            0 (  0.000%)
>>>>
>>>> ============================================================
>>>> ===================
>>>>
>>>>
>>>>
>>>> ============================================================
>>>> ===================
>>>>
>>>>   File Preprocessor Statistics
>>>>    Total file type callbacks:            576
>>>>    Total file signature callbacks:       578
>>>>    Total files would saved to disk:      574
>>>>    Total files saved to disk:            320
>>>>    Total file data saved to disk:        483039    bytes
>>>>    Total files duplicated:               254
>>>>    Total files reserving failed:         2
>>>>    Total file capture min:               0
>>>>    Total file capture max:               2
>>>>    Total file capture memcap:            0
>>>>    Total files reading failed:           0
>>>>    Total file agent memcap failures:     0
>>>>    Total files sent:                     0
>>>>    Total file data sent:                 0
>>>>    Total file transfer failures:         0
>>>>
>>>> ============================================================
>>>> ===================
>>>> File type stats:
>>>>           Type              Download   (Bytes)      Upload     (Bytes)
>>>>            GZ( 33)          2          5580056      0          0
>>>>           SWF( 52)          1          65991        0          0
>>>>           GIF( 62)          7          16516        0          0
>>>>           GIF( 63)          275        151718       0          0
>>>>           PNG( 69)          266        256724       0          0
>>>>          JPEG( 70)          2          35566        0          0
>>>>           BMP(148)          2          4204         0          0
>>>>           ICO(149)          21         187894       0          0
>>>>              Total          576        6298669      0          0
>>>> File signature stats:
>>>>           Type              Download   Upload
>>>>            GZ( 33)          2          0
>>>>           SWF( 52)          1          0
>>>>           GIF( 62)          7          0
>>>>           GIF( 63)          275        0
>>>>           PNG( 69)          266        0
>>>>          JPEG( 70)          2          0
>>>>           BMP(148)          2          0
>>>>           ICO(149)          21         0
>>>>              Total          576        0
>>>> File type verdicts:
>>>>          UNKNOWN:           576
>>>>              LOG:           0
>>>>             STOP:           0
>>>>            BLOCK:           0
>>>>           REJECT:           0
>>>>          PENDING:           0
>>>>     STOP CAPTURE:           0
>>>>            Total:           576
>>>> File signature verdicts:
>>>>          UNKNOWN:           578
>>>>              LOG:           0
>>>>             STOP:           0
>>>>            BLOCK:           0
>>>>           REJECT:           0
>>>>          PENDING:           0
>>>>     STOP CAPTURE:           0
>>>>            Total:           578
>>>> Total files processed:             68985
>>>> Total files data processed:        97156439  bytes
>>>> Total files buffered:              576
>>>> Total files released:              574
>>>> Total files freed:                 2
>>>> Total files captured:              574
>>>> Total files within one packet:     561
>>>> Total buffers allocated:           641
>>>> Total buffers freed:               64
>>>> Total buffers released:            577
>>>> Maximum file buffers used:         64
>>>> Total buffers free errors:         0
>>>> Total buffers release errors:      0
>>>> Total memcap failures:             0
>>>> Total memcap failures at reserve:  0
>>>> Total reserve failures:            0
>>>> Total file capture size min:       0
>>>> Total file capture size max:       0
>>>> Total capture max before reserve:  2
>>>> Total file signature max:          0
>>>> Maximum buffers can allocate:      3196
>>>> Number of buffers in use:          0
>>>> Number of buffers in free list:    2619
>>>> Number of buffers in release list: 577
>>>>
>>>> ============================================================
>>>> ===================
>>>>
>>>>
>>>>
>>>> Following the instructions given as examples inside the file
>>>> README.file, I
>>>> have included the following rules to get an alert every time Snort
>>>> detects
>>>> a file:
>>>>
>>>> alert (msg: "GIF file"; gid:146; sid:63; rev:1; metadata: rule-type
>>>>
>>>>   preproc;)
>>>>
>>>>
>>>>
>>>> After that, no alert showed up.
>>>>
>>>> I went deep inside the code to find out what the reason is and found the
>>>> following piece of code that confused me:
>>>>
>>>> snort/src/dynamic-preprocessors/file/file_agent.c:601-614
>>>>
>>>> /*
>>>>
>>>>    * File type callback when file type is identified
>>>>   *
>>>>   * For file capture or file signature, FILE_VERDICT_PENDING must be
>>>> returned
>>>>   */
>>>> static File_Verdict file_agent_type_callback(void* p, void* ssnptr,
>>>>          uint32_t file_type_id, bool upload, uint32_t file_id)
>>>> {
>>>>      file_inspect_stats.file_types_total++;
>>>>      if (file_signature_enabled || file_capture_enabled)
>>>>          return FILE_VERDICT_UNKNOWN;
>>>>      else
>>>>          return FILE_VERDICT_LOG;
>>>> }
>>>>
>>>>
>>>>
>>>> You can read on the description that FILE_VERDICT_PENDING must be
>>>> returned
>>>> when file capture OR file signature is enabled, but what really the code
>>>> does is to return FILE_VERDICT_UNKNOWN when capture or signature are
>>>> enabled.
>>>>
>>>> After see that, I have modified the snort.conf by carrying on the
>>>> following
>>>> changes:
>>>>
>>>> Replace this:
>>>>
>>>> preprocessor file_inspect: type_id, signature, \
>>>>
>>>>                capture_queue_size 5000, \
>>>>               capture_disk /home/file_capture/tmp/
>>>>
>>>>
>>>>
>>>> By:
>>>>
>>>> preprocessor file_inspect: type_id
>>>>
>>>>
>>>> This way I forced to go through the ELSE and return a FILE_VERDICT_LOG.
>>>> After this change, and using the same two alert rules, we run snort,
>>>> getting alerts like these below:
>>>>
>>>> 03/16-12:50:22.350000  [**] [146:63:1] GIF [**] [Priority: 0] {TCP}
>>>>
>>>>   192.168.202.78:80 <http://192.168.202.78/> <http://192.168.202.78/>
>>>> -> 192.168.203.61:38976
>>>> 03/16-12:50:22.350000  [**] [146:63:1] GIF [**] [Priority: 0] {TCP}
>>>> 192.168.202.78:80 <http://192.168.202.78/> <http://192.168.202.78/> ->
>>>> 192.168.203.61:38976
>>>> 03/16-12:50:22.350000  [**] [146:63:1] GIF [**] [Priority: 0] {TCP}
>>>> 192.168.202.78:80 <http://192.168.202.78/> <http://192.168.202.78/> ->
>>>> 192.168.203.61:38977
>>>>
>>>>
>>>> ...
>>>>
>>>>   and getting the following output at the end:
>>>>
>>>> ============================================================
>>>> ===================
>>>>
>>>>   Action Stats:
>>>>       Alerts:          275 (  0.003%)
>>>>       Logged:          275 (  0.003%)
>>>>       Passed:            0 (  0.000%)
>>>> Limits:
>>>>        Match:            0
>>>>        Queue:            0
>>>>          Log:            0
>>>>        Event:            0
>>>>        Alert:            0
>>>> Verdicts:
>>>>        Allow:      8418514 ( 97.482%)
>>>>        Block:            0 (  0.000%)
>>>>      Replace:            0 (  0.000%)
>>>>    Whitelist:       217429 (  2.518%)
>>>>    Blacklist:            0 (  0.000%)
>>>>       Ignore:            0 (  0.000%)
>>>>
>>>> ============================================================
>>>> ===================
>>>>
>>>>
>>>>
>>>>
>>>> ============================================================
>>>> ===================
>>>> File Preprocessor Statistics
>>>>    Total file type callbacks:            576
>>>>    Total file signature callbacks:       0
>>>>    Total files would saved to disk:      0
>>>>    Total files saved to disk:            0
>>>>    Total file data saved to disk:        0         bytes
>>>>    Total files duplicated:               0
>>>>    Total files reserving failed:         0
>>>>    Total file capture min:               0
>>>>    Total file capture max:               0
>>>>    Total file capture memcap:            0
>>>>    Total files reading failed:           0
>>>>    Total file agent memcap failures:     0
>>>>    Total files sent:                     0
>>>>    Total file data sent:                 0
>>>>    Total file transfer failures:         0
>>>>
>>>> ============================================================
>>>> ===================
>>>> File type stats:
>>>>           Type              Download   (Bytes)      Upload     (Bytes)
>>>>            GZ( 33)          2          0            0          0
>>>>           SWF( 52)          1          0            0          0
>>>>           GIF( 62)          7          0            0          0
>>>>           GIF( 63)          275        0            0          0
>>>>           PNG( 69)          266        0            0          0
>>>>          JPEG( 70)          2          0            0          0
>>>>           BMP(148)          2          0            0          0
>>>>           ICO(149)          21         0            0          0
>>>>              Total          576        0            0          0
>>>> File signature stats:
>>>>           Type              Download   Upload
>>>>              Total          0          0
>>>> File type verdicts:
>>>>          UNKNOWN:           0
>>>>              LOG:           576
>>>>             STOP:           0
>>>>            BLOCK:           0
>>>>           REJECT:           0
>>>>          PENDING:           0
>>>>     STOP CAPTURE:           0
>>>>            Total:           576
>>>> File signature verdicts:
>>>>          UNKNOWN:           0
>>>>              LOG:           0
>>>>             STOP:           0
>>>>            BLOCK:           0
>>>>           REJECT:           0
>>>>          PENDING:           0
>>>>     STOP CAPTURE:           0
>>>>            Total:           0
>>>> Total files processed:             68987
>>>> Total files data processed:        42751396  bytes
>>>> Total files buffered:              0
>>>> Total files released:              0
>>>> Total files freed:                 0
>>>> Total files captured:              0
>>>> Total files within one packet:     0
>>>> Total buffers allocated:           0
>>>> Total buffers freed:               0
>>>> Total buffers released:            0
>>>> Maximum file buffers used:         0
>>>> Total buffers free errors:         0
>>>> Total buffers release errors:      0
>>>> Total memcap failures:             0
>>>> Total memcap failures at reserve:  0
>>>> Total reserve failures:            0
>>>> Total file capture size min:       0
>>>> Total file capture size max:       0
>>>> Total capture max before reserve:  0
>>>> Total file signature max:          0
>>>>
>>>> ============================================================
>>>> ===================
>>>>
>>>>
>>>>
>>>> As you can see, in the "File type verdicts" section I got all the files
>>>> with verdict LOG. Also, I got 275 alerts that match the 275 GIF files
>>>> detected by Snort.
>>>>
>>>> I am not sure if this is the expected behavior of this feature or maybe
>>>> I
>>>> am not configuring Snort properly.
>>>>
>>>> Am I doing something wrong or configuring the preprocessor in a wrong
>>>> way?
>>>>
>>>> Thanks for your help and best Regards,
>>>>
>>>> Pablo Cantosredborder.org / pcantos at ...16842...
>>>>
>>>>
>>>>
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>>>>
>>>>
>>>>
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>>>>
>>>>
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