[Bro] Sensor placement with presence of web proxies

Martin Holste mcholste at gmail.com
Thu Jan 26 18:57:27 PST 2012

Right, but then you lose the power of reporting on srcip, as well as
any chance at correlation.  A very frequent task my analysts do is to
follow-up on any newly-identified hostile dstip's by doing a query
which shows all unique srcip's to hit it.  My httpry_logger.pl script
sets the srcip via X-Forwarded-For if available, but a lot of other
tools don't.

All of that aside, I shouldn't need to worry about XFF if I sniff the
client-side of the connection, right?  If that's true, then my main
concern is whether server response headers are getting altered by the

On Thu, Jan 26, 2012 at 8:01 PM, Russell Fulton <r.fulton at auckland.ac.nz> wrote:
> one thing I do with some of the snort stuff is to pull the packet contents and look for a 'X-FORWRDED-FOR' header.
> Several of my automated scripts do this so this enables me to trace connections back through squid proxies with out problems.
> If the proxies add such headers than you may be able to get a bro script to automatically pull the real IP and report that.
> R
> On 27/01/2012, at 10:54 AM, Martin Holste wrote:
>> 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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