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nbar behavior


I'm not sure this post belongs in this forum, but it seems a good place to start.

We are running nbar on a router here to help identify nimda and code red virus traffic. The router is setup according to instructions provided on Cisco's website for accomplishing nimda and Code Red control.

I setup a test environment to isolate traffic and behavior and I'm seeing thing I don't understand. Nbar is configured to identify, mark and drop packets "inbound" based on certain content in the URL. It does this fine. If I send a URL with this certain content through the router from the "inside", the packet with the URL gets passed through the router fine, just like I'd expect. However, I consistently see 2 packets in the response from the target webserver get dropped, one is a plain TCP ACK, the other in an HTTP Server Error packet. The documentation for nbar indicates URL matching looks at the HTTP GET request to identify specified requests. Neither of these blocked packets contains a GET field, nor do they contain the specified content, as far as I can tell with my sniffers. What's extra weird is an identical HTTP Server Error packet makes it through upon the second GET request submission.

Anybody have any ideas about what's going on? This odd behavior doesn't appear to be stopping anyone from doing what they want to do, at least, they are telling me if it is. That's OK too.




Re: nbar behavior

Since there has been no response to your post, it appears to be either too complex or too rare an issue for other forum members to assist you. If you don't get a suitable response to your post, you may wish to review our resources at the online Technical Assistance Center ( or speak with a TAC engineer. You can open a TAC case online at

If anyone else in the forum has some advice, please reply to this thread.

Thank you for posting.

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