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Silly22 01-01-2007 10:06 AM

iptables: Using shorewall and the priority (dominance) of rules
I'm editing the /etc/shorewall/rules file in order to block all internet access for a local (internal) client with the IP address This internal computer is a liability if it were able to become infected from the Internet, so I have to restrict its access to the WWW.

I wrote a line in the /etc/shorewall/rules file:
Headings included for clarification...

#ACTION  SOURCE            DEST        PROTO        DEST    SOURCE        ORIGINAL#
#                                                PORT    PORT(S) DEST    #
REJECT  loc:  net        all  #this is the line I added

This partly worked; I couldn't SSH to a host on port 22, and my pings were unable to reach Internet hosts. However, I could still surf http:// websites using Netscape.

And then I noticed the line in the rules file just below the recently added rule.

REDIRECT loc                    3128            tcp        www
This is when I first realized just how much I relied on the squid proxy server on my linux machine for port 80 website surfing. By default Mandrake 10.1 is set-up this way.

Why is it that when I commented the second line out (rules in question reproduced below), the one about redirecting local port 80 traffic to a squid server on port 3128, I was finally able to accomplish my goal of blocking all internet access for the internal machine?

REJECT  loc:  net        all  #this is the line I added
#REDIRECT loc                    3128            tcp        www

I thought that this applied... quote from website

Rules for each pair of zones (source zone, destination zone) are evaluated in the order that they appear in the file the first match determines the disposition of the connection
So that would mean the REDIRECT rule after my REJECT rule shouldn't have caught that internal client's traffic? Am I wrong here? Am I understanding the traffic incorrectly?

Your thoughts please....

jschiwal 01-02-2007 07:03 PM

Does "net" refer to any nonlocal address or your proxy. If you want to block html access, I think you also need to block traffic from to the proxy server IP address.

Silly22 01-13-2007 11:46 PM

"net" refers to any nonlocal address. Well, thanks for framing the idea in a question. Now I understand that the "pair of zones" as described by shorewall documentation is evaluated in order. The first pair in my rules being loc to net, and the second pair being loc to proxy on port 3128.

Now here's another thing I did so that I wouldn't have to nullify the utility of the Squid proxy.
I changed the redirect rule to the following instead of commenting it out.

#ACTION  SOURCE            DEST        PROTO        DEST    SOURCE        ORIGINAL#
#                                                PORT    PORT(S) DEST    #
REDIRECT loc                    3128            tcp        www        -        !

It worked, I can't surf the web from the internal machine, BUT the rule doesn't really make sense. Because the way it is written actually means the following:
Redirect all local machine TCP port 80 (www) traffic to port 3128 on the proxy (my router), unless the traffic was originally intended to go to machine at address
The rule kind of makes sense since, I quote the example in the rules file, "This example shows yet another use for the ORIGINAL DEST column; here, connection requests that were NOT (notice the “!”) originally destined to [] are redirected to local port 3128."

If I think about this in terms of traffic coming from the proxy back to (if the proxy itself is considered in the "loc" group, I don't know), then the rule implies traffic that is originally destined for does not get redirected to 3128. This is confusing...

Perhaps I should write a rule like the following to make it explicit what my goal is...

REJECT  loc:  3128        tcp        www
Any other ideas? Note that by default, I think Mandrake names my router/firewall, where the proxy resides, "fw". So if you can write me a rule that's clear and doesn't hinder internal port 80 traffic from, please let us see.

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