[Snort-users] Question about SO Rule 3:21355
jesler at ...1935...
Thu Sep 5 11:42:44 EDT 2013
We'll make sure the information is also put into documentation form for the SID.
On Thu, Sep 5, 2013 at 11:02 AM, Jeremy Hoel <jthoel at ...11827...> wrote:
> Patrick, thank you for the reminder.. we have source! I keep
> forgetting to go there and look for answers. Thanks for the quick
> response and overview. Mental note to self..
> So, to provide more information:
> 1 - Before yesterday, we had never gotten this alert before, ever.
> 2 - As of this morning we have about 120 alerts for this signature
> (1:21355). At first it was just 5 DC's that had been providing DNS at
> some offices (out of 40+). It was limited to just NYT domains. This
> morning we see more events from some other domains and one more site.
> 3 - We (as far as I get from the operation Team) hadn't changed our
> DNS settings. Our DC's send requests up to 4 central DC's and those 4
> do caching and send new requests out to a parent domain that should
> provide the info. What we are seeing is that for the NYT domains, the
> 4 local DC"s gave server failure errors and then the originating DC
> made the DNS request directly to NYTs for the answer. We still are
> trying to figure out how the DC got the IP for the NYT server, since
> it isn't supposed to be doing root lookup or have root hints.
> 4 - Oddly enough, after the DNS request came back, for the next two
> hours, no one sent any traffic to the returned IP address. So if a
> client was asking to go to www.nyt, after it got the answer, it never
> went. I need to see about these new domains today to see if that
> still holds true.
> 5 - If we where under attack, the source of the replies should be
> different from the requested domain right? I mean, in this case, the
> IP of the reply was from the NYT block of IP space, and the new alerts
> this morning are from different IPs and my guess is they will be from
> the requested domains.
> 6 - I asked my ops people to make sure the servers are patched and
> while they think it is, they are going to get back to me.
> On Thu, Sep 5, 2013 at 1:23 PM, Patrick Mullen <pmullen at ...1935...> wrote:
>> Thank you for your query. Before I begin, I would like to remind
>> everyone that the purpose of Shared Object Rules is to provide the
>> ability to write rules in C, not to obfuscate detection. At this
>> point, most SO Rules are open source. The source for this particular
>> rule is in bad-traffic_dns-spoof-mismatched-txid.c and as it happens
>> that file has several paragraphs at the beginning explaining what it
>> does at length.
>> In short, the rule (pair of rules, technically) looks for a DNS reply
>> where the TXID is different than the query(ies) we have on record.
>> This is why you see the alert on the reply, not the query. All of
>> this is explained more fully in the comment at the top of the source
>> as well as a known potential for FPs. But that being said, the rules
>> have a pretty good tolerance for FPs.
>> Your email says nothing about how many alerts you've been getting or
>> if this is a new thing. If you are getting a lot of alerts, are you
>> sure you're not under attack? Did you recently change your internal
>> dns server to not cache? Are your users just suddenly all going to
>> NYTimes.com at the same time due to recent developments around the
>> On Wed, Sep 4, 2013 at 6:46 PM, Jeremy Hoel <jthoel at ...11827...> wrote:
>>> We started seeing this today from some of our DC's when doing lookups
>>> to various nytimes.com sites The MS Bulletin references issues with
>>> Exchange and SMTP and the CVE references the DNS lookup in the
>>> smtpsvc.dll in regards to dns caching poisoning.
>>> We are only seeing these for responses from the NYT DNS servers, which
>>> is also odd, not the original request going outboung which makes me
>>> wonder how/what in the response would trigger this?
>>> And finally.. if the servers are patched with MS10-024, then the could
>>> something else be causing the FP?
>>> Being a SO rule, I don't have much to go on.
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>> Patrick Mullen
>> Response Research Manager
>> Sourcefire VRT
> Learn the latest--Visual Studio 2012, SharePoint 2013, SQL 2012, more!
> Discover the easy way to master current and previous Microsoft technologies
> and advance your career. Get an incredible 1,500+ hours of step-by-step
> tutorial videos with LearnDevNow. Subscribe today and save!
> Snort-users mailing list
> Snort-users at lists.sourceforge.net
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> Snort-users list archive:
> Please visit http://blog.snort.org to stay current on all the latest Snort news!
Senior Research Engineer, VRT
OpenSource Community Manager
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