Re: Sensor placement with presence of web proxies

["Jefferson, Shawn" <Shawn.Jefferson () bcferries com>]

Hi Martin,

I have the exact configuration* you are describing, and I have wondered the same thing.  I do get alerts for 
proxy-bound HTTP traffic, so I am fairly comfortable that detection is still working correctly.  I think I've even 
asked the same question on this list with no answers.  I'm interested in verifying this as well.

*We actually have two scenarios:

1. Explicit proxy configuration.
2. WCCP redirect of 80/443 traffic.

Both seem to generate alerts from Snort still.


-----Original Message-----
From: Martin Holste [mailto:mcholste () gmail com] 
Sent: Thursday, January 26, 2012 1:54 PM
To: <snort-users () lists sourceforge net>
Subject: [Snort-users] Sensor placement with presence of web proxies

Our org is looking at using web proxies without changing settings on the client.  This can involve using Cisco's WCCP 
or policy-based routing to marshal traffic that would normally go to the Internet to a proxy.  As I understand it, the 
proxy makes the request, returns the response to the router, and the router returns the response to the client.  My 
question is if anyone has run into problems with a tap or span on the side of the router closest to the client.  That 
is, does the proxy change the traffic enough to interfere?  It seems nonsensical to put the sensor at the edge of the 
network since the requests will have the source IP of the proxy, not the actual client, but that means that the traffic 
the IDS inspects will be inauthentic versus what the remote host on the Internet actually sent.
Theoretically, it should be the same traffic, but I'm wondering if anyone can confirm that.  I'm especially concerned 
with appliances that reorder or normalize HTTP headers, etc.

Thanks,

Martin

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