Yes, it basically means if Rule ID 500 is triggered, look at this rule also.
So in the case of the rule you posted:
1. If Rule ID 500 is triggered,
2. and if this is a First Time Seen event,
3. and the string "Agent started" is found in the event,
4. trigger Rule ID 501 at level 3.

On Thu, Sep 16, 2010 at 10:36 AM, Aamir Niazi <[email protected]> wrote:
> Can someone please explain what does if_sid mean exactly. Does it mean
> if rule matches ID=500 do the following?
>
> Just a bit confused how rules are triggered.
>
>
> <rule id="501" level="3">
>    <if_sid>500</if_sid>
>    <if_fts />
>    <options>alert_by_email</options>
>    <match>Agent started</match>
>    <description>New ossec agent connected.</description>
>  </rule>
>
>
>
>
> --
> Best Regards,
>
> Aamir Niazi
> Senior Security Analyst
>

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