Security Basics mailing list archives
Re: Proxy Port detection
From: Alexander Klimov <alserkli () inbox ru>
Date: Thu, 23 Dec 2004 17:53:44 +0200 (IST)
On Wed, 22 Dec 2004, John Madden wrote:
In our enterprise we have URL filtering capabilities and we restrict the usual sites (Porn, Sports, Gambling etc..) We do not use a proxy, so everyone goes directly to the internet.
Curiously, how do you filter URLs without a proxy?
I believe that some users put in their proxy settings an anonymous proxy using port 80 (which is obviously allowed) and in that manner avoid the restriction of the URL filtering. First thoughts: - Blocking all the anonymous proxy is imposible and would be a full time job - The use of a proxy is not an option right now
If it is not an option only because you do not want to force everybody to change their web browsers setups you can use transparent proxy.
Is there any way to detect this type of traffic (HTTP-Proxy) ?
If the following request GET /url HTTP/1.1 Host: some.host.name was sent to IP a.b.c.d which does not belong to some.host.name it is likely that a.b.c.d hosts a proxy. But since DNS can report differnt IPs for the same domain it's not that easy to check if an IP serves some particular domain. Note that if you allow encrypted connections (https) you can't inspect their contents. Also if your user has a webserver A outside they can setup it to make a request on behalve of the user with a translation of urls, e.g., http://B/url is encoded as http://A/B,port/url, A sends request to B and sends the reply to your user (and change all embeded urls). A can also encode all documents (with an embeded JavaScript to auto-decode) to avoid content filtering. So, you should consider how far you want to go into these troubles with rather marginal ROI into enforcing the policy. -- Regards, ASK
Current thread:
- Proxy Port detection John Madden (Dec 22)
- Re: Proxy Port detection Alexander Klimov (Dec 23)
- <Possible follow-ups>
- Re: Proxy Port detection Steve Crapo (Dec 23)
- Re: Proxy Port detection Ivan Coric (Dec 23)
- RE: Proxy Port detection Andrew Shore (Dec 23)