Re: ipchains and bridging

From: Ulrich Keil (ulrich@der-keiler.de)
Date: 07/08/02


Date: Mon, 8 Jul 2002 23:28:37 +0200
From: Ulrich Keil <ulrich@der-keiler.de>
To: "Security-Basics (E-mail)" <security-basics@securityfocus.com>


On Mon, Jul 08, 2002 at 10:11:44AM -0400, Chris Santerre wrote:
> I have a firewall I have been working on. 3 NICs. I have real IP addresses
> for the outside NIC, DMZ NIC, and servers in the DMZ. I used bridging to get
> packets from the internet to the servers in the DMZ. Here is the problem.
> Bridging seems to be at a lower level then packet filtering. I can't filter
> anything coming IN to the DMZ, only out. It works, and stops everything, but
> it is NOT the best setup at all!!! I am well aware of ways to attempt to
> comprimise the servers in the DMZ. A DOS or ping of death could work easily.
> Any thoughts on how to go about fixing this, or have I doomed myself using
> bridging?

Linux BRIDGE-STP-HOWTO:

http://www.tldp.org/HOWTO/BRIDGE-STP-HOWTO/rules-on-bridging.html

Section 4:

"-A bridge knows nothing about higher protocols than ARP"

Means: It is normally not possible to filter packets on a bridge ... but ...

There is a patch available to make ipchains/iptables work on a bridge

http://www.tldp.org/HOWTO/BRIDGE-STP-HOWTO/advanced-bridge.html#IPCHAINS
http://bridge.sourceforge.net/download.html

> Should I have virtually hosted the WEB and EMAIL server on the outside NIC
> of the firewall, and ipportfwd them to DMZ machines on a 192.x.x.x network?

I prefer this option, because you normally don't have any advantages
using a bridge against using NAT.

Ulrich

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