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Re: Tool to find hidden web proxy server
From: Javier Fernandez-Sanguino <jfernandez () germinus com>
Date: Thu, 02 Sep 2004 15:03:04 +0200

vinay mangal wrote:

Dear all,

Thanks for your suggestions. May be I am not able to define my question
properly.

This problem is strictly with in company internet access firewall and in the
LAN only. In a company, policy for Internet access says it is through IP
only. The others can not browse the internet. This policy is implemented on
firewall. Few smart guys have installed free proxy server running on non
default ports and distributed the internet access to their friends. The
firewall sees the traffic coming from the authorized IP and does not stop
them. We want to know who has installed proxy on there machine.


Since you say that the authorised IPs that can browse the Internet are a known subset of your company, I suggest you could (in an increasing level of complexity):

- port-scan those systems and determine if there are open ports on them that act as a proxy (try using 'nmap -sV'). Of course this will not work if they have added a firewall in the system and are blocking access to that port to everyone save their friends.

- analyse the outgoing HTTP traffic through the firewall from those IP addresses and look for proxy 'give-aways' in the HTTP headers ('X-Forwarded-For:' or 'Via:'). If they have configured the proxy server to not print these headers this might not spot out any culprits. Use ngrep for this.

- analyse the outgoing HTTP traffic through the firewall and analyse it (you can probably use a lot of accounting tools to extract data from tcpdump captures). This will allow you to determine which servers are responsible for most of the outgoing HTTP traffic and that might be an indication of a proxy in use. You can use ntop for this.

- access your switches and analyse the traffic statistics of the ports used by the IP address that can access the Internet. Ports with a high incoming/outgoing byte counts might allow you to distinguish legitimate vs. illegitimate accesses. Most clients (if only used for browsing) will generate a high incoming byte count but a low outgoing byte cout so a high outgoing byte count might be an indication of traffic being proxied to other clients.

- (if your switches permit) use traffic monitoring (port spanning) or netflows to do accounting on communications between the different IP address of your local company, discard known servers and analyse the traffic to detect uncommon client-server relationships that generate an uncommon ammount traffic. In most office environments you should only see traffic going from clients to known servers (or to the Internet) as there is rarely a need for clients to communicate amongst themselves (unless sharing resources). That could allow you to detect both the IP addresses of illegitimate servers and the IP addresses of those using them. You can use ntop for this.

Good luck.

Javier

PS: You basicly need tools to do traffic analysis, but you first have to place yourself in a position in which you know what you want to capture, and how to analyse it to obtain your rogue users so before thinking about tools, think about how to capture that data you need.

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