Re: [Snort-Devel] SNORT Detection-Plugin just call once

[John Eure <john.eure () gmail com>]

I've only done this a few times before, but it sounds like you might be
setting something up incorrectly?  My static detection plugin setup code
looks something like this (please pardon the incredibly simplified
pseudocode):

// called once, during setup
MyDetectionPlugin_Setup()
{
RegisterRuleOption("my_option_name", MyDetectionPlugin_MyRuleOption_Init());
}

// called once per rule that uses "my_option_name", during setup
MyDetectionPlugin_MyRuleOption_Init()
{
OptFpList * fpl = AddOptFuncToList(MyDetectionPlugin_MyRuleOption_Detect());
}

// called a lot, at most once per packet per rule, during detection
MyDetectionPlugin_MyRuleOption_Detect()
{
return DETECTION_OPTION_MATCH;
}

If you're not seeing multiple calls to your equivalent of
"MyDetectionPlugin_MyRuleOption_Init()", then that's probably the place to
start investigating.

If that's not the cause, then I'm guessing you're doing something wrong
when assigning values to the state associated with the rules.  You need to
be setting up the OptTreeNode and OptFuncList correctly, including setting
otn->ds_list[plugin_type] to something associated with your rule.  But
that's way more complicated than I feel comfortable explaining, so my
suggestion would be to pick a few simple detection plugins, and follow
their logic?

Good luck,
John



On Wed, Feb 12, 2014 at 12:26 PM, Emiliano Fausto <emiliano.fausto () gmail com
> wrote:

> Hello there,
>
> I'm debugging with gdb a detection plugin I've developed, and there's
> something I am not fully understanding.
>
>  I have this function (which should be called every time a packet matches
> one of my rules which invokes a keyword named "MyDP"):
>
> ...
> typedef struct _MyDPData
> {
>     u_int16_t   num;
> }
> MyDPData;
> ...
>
> int MyDPCheck(void *context, Packet *p)
> {
>
>   LogMessage("My processing function was called\n");
>
>   MyDPData* mydata;
>
>   if (!p->tcph)
>       return DETECTION_OPTION_NOT_MATCH;
>
>   mydata = (MyDPData *) context;
>
>   if (mydata->num == 90) {
>       LogMessage("it matched\n");
>       return DETECTION_OPTION_MATCH;
>   }
>
>   return DETECTION_OPTION_NOT_MATCH;
> }
>
> I'm facing 2 problems here:
>
> 1) When I'm debugging with GDB it seems that this function is being called
> just once. The moment it's called, is when the SNORT is parsing all the
> snort.conf file, and detects there's a keyword into a rule which matches:
> "MyDP". Then it doesn't matter how much packets validate the rest of the
> rule, it doesn't seems to be called (because the breakpoint over the
> function MyDPCheck isn't triggered any more).
>
> Note: It's strange, because my /var/log/snort/alert file is full of alerts
> referring to this alert being triggered, although I don't see the "it
> matched" message.
>
> My question is, shouldn't it be logging the message: "It matched?" each
> time it matches a packet calling this rule with the num==90?
>
> Snort.Conf rule -> alert tcp any any -> any any (MyDP: 90; msg: "My DP was
> called"; sid: 5016666; rev:1;)
>
> 2) I realized that it doesn't matter the number I put into the rule (in
> the previous example I just put 90), the Detection Plugin always returns
> "True" --> DETECTION_OPTION_MATCH.
> So, it doesn't make any sense to me, as it's a really simply comparison
> between the number stored option value and the hard-coded value 90.
>
> In this case, I'm almost sure that the alert should NOT be triggered if I
> put numbers different than 90 in the keyword value, right?
>
> Thanks in advance to anyone which could give me some light on this matter!
> Emiliano.
>
>
>
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