[Oisf-users] false alerts?

Wed Mar 27 12:34:55 UTC 2013

On Wed, Mar 27, 2013 at 1:03 PM, Jose Paulo <paulo at sistemasolar.com.br>wrote:

> Hello all.
>
> I'm studying Suricata and I got this result:
>
> 11/16/2011-15:00:00.198278  [**] [1:9000005:0] HEX offset 503 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:00:00.198278  [**] [1:9000004:0] HEX offset 510 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:00:09.374228  [**] [1:9000005:0] HEX offset 503 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:00:09.374228  [**] [1:9000004:0] HEX offset 510 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:00:09.374228  [**] [1:9000001:0] HEX no offset  [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:31.769957  [**] [1:9000005:0] HEX offset 503 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:38.380502  [**] [1:9000005:0] HEX offset 503 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:38.380502  [**] [1:9000001:0] HEX no offset  [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:44.609767  [**] [1:9000005:0] HEX offset 503 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:44.609767  [**] [1:9000004:0] HEX offset 510 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:44.609767  [**] [1:9000002:0] HEX offset 514 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:48.726883  [**] [1:9000005:0] HEX offset 503 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:48.726883  [**] [1:9000004:0] HEX offset 510 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:48.726883  [**] [1:9000002:0] HEX offset 514 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:48.726883  [**] [1:9000001:0] HEX no offset  [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
>
> against this rules set:
>
> alert tcp any any <> any 23 (msg:"HEX no offset "; content: "|F8 F8 F8
> F8 40 C3 81 89 A7 81|"; sid:9000001;)
> alert tcp any any <> any 23 (msg:"HEX offset 514"; content: "|F8 F8 F8
> F8 40 C3 81 89 A7 81|"; offset:514; sid:9000002;)
> alert tcp any any <> any 23 (msg:"HEX offset 516"; content: "|F8 F8 F8
> F8 40 C3 81 89 A7 81|"; offset:516; sid:9000003;)
> alert tcp any any <> any 23 (msg:"HEX offset 510"; content: "|F8 F8 F8
> F8 40 C3 81 89 A7 81|"; offset:510; sid:9000004;)
> alert tcp any any <> any 23 (msg:"HEX offset 503"; content: "|F8 F8 F8
> F8 40 C3 81 89 A7 81|"; offset:503; sid:9000005;)
>
> My doubts are:
>
> 1) Why I'm getting alerts for sid's 9000004,5 for the same packet if the
> offset is shifted?
>
> 11/16/2011-15:01:48.726883  [**] [1:9000005:0] HEX offset 503 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:48.726883  [**] [1:9000004:0] HEX offset 510 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:48.726883  [**] [1:9000002:0] HEX offset 514 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:48.726883  [**] [1:9000001:0] HEX no offset  [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
>
> 2) Why I'm not getting alerts for sid 9000001 if I got for the others?
>
> 11/16/2011-15:01:44.609767  [**] [1:9000005:0] HEX offset 503 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:44.609767  [**] [1:9000004:0] HEX offset 510 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:44.609767  [**] [1:9000002:0] HEX offset 514 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
>
> The expected result is only this:
> 11/16/2011-15:00:09.374228  [**] [1:9000004:0] HEX offset 510 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:00:09.374228  [**] [1:9000001:0] HEX no offset  [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:38.380502  [**] [1:9000005:0] HEX offset 503 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:38.380502  [**] [1:9000001:0] HEX no offset  [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:48.726883  [**] [1:9000002:0] HEX offset 514 [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
> 11/16/2011-15:01:48.726883  [**] [1:9000001:0] HEX no offset  [**]
> [Classification: (null)] [Priority: 3] {TCP} 10.31.15.32:23 ->
> 10.85.185.2:43569
>
> I don't understand why the others occurs.
> Any enlightenment will be welcome.
>
> Best regards!
>
> José Paulo
>
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>

Hi,
A couple of questions:
1) 11/16/2011- is that really the time in the current packets?
2) Can you share a pcap , if that is ok?

Thank you

-- 
Regards,
Peter Manev
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