RE: [fw-wiz] Strange setup
From: Bill Royds (broyds_at_rogers.com)
To: "'Melson, Paul'" <PMelson@sequoianet.com>, "'franco segna'" <email@example.com>, <firstname.lastname@example.org> Date: Thu, 26 Feb 2004 20:02:18 -0500
Actually there is a good reason for the ISA to be configured this way.
The ISA server acts as an HTTP cache/authenticator.
So if you have the inside boxes with default route going to internal address
of ISA server, it can validate the user of that workstation with Windows
authentication and either, forward the request out to the Internet and cache
the reply , or return the cached value.
This adds to security by ensuring that all internal workstation access is by
The straight through branch is used for server traffic like mail that is
coming towards the internal network or is being forwarded by authorized MTA
style servers that use the FW as default route.
ISA may not be a great firewall but it is a good proxy/authenticator for a
Windows network. Its examination it does of HTTP traffic before caching it
is a bonus.
[mailto:email@example.com] On Behalf Of Melson, Paul
Sent: February 26, 2004 2:24 PM
To: franco segna; firstname.lastname@example.org
Subject: RE: [fw-wiz] Strange setup
Without seeing the rule sets on either system, it is impossible to say.
Clearly this design misses the point of a 'DMZ' network. There's no
reason that I can think of for the ISA server to be dual-homed. There's
no reason that outbound proxy traffic, RAS/VPN traffic, or reverse proxy
traffic can't pass through two sets of firewall rules, one between the
outside and DMZ, and another between the DMZ and inside. If you're
tasked with redesigning this network, that should be first on your to-do
That said, a possible explanation is that the ISA server is a reverse
proxy for servers on the internal network, and the firewall only allows
inbound traffic to the DMZ for NAT purposes. In this situation, the ISA
server could provide extra access controls at the application layer in
the form of authentication or restricting access to specific
pages/services through destination lists. Workstation browsing and
other inside->out traffic could pass directly through the firewall
without going through ISA.
That's just a theory, though. Looking at the rules on both the
SonicWall and the ISA Server will give you a better idea of the intended
function of this design.
PS - I can't help but notice that a disproportionately large number of
European, and specifically German IP networks (including my previous
employer's) have been designed using internal addressing schemes that do
not conform to RFC1918. Anybody have an educated guess as to why this
is? It's just a personal curiosity of mine.
> -----Original Message-----
> Hi everybody,
> I'm being confronted with the following existing setup:
> T1 --------------------------------
> (Internet | LAN backbone |
> and VPNs) ------------+---+---+-+-+-+-+---
> | | | | | | | |
> | +-------+ local x.x.x.254/24 | | | | | | +-
> | | Sonic +---------------------+ | | | | +-
> +--+ Wall | | | | |
> | Pro +------+ | | | +- SQL
> +-------+ dmz | | | +-- mail
> (?) | +--------+ | +--- etc.
> | | MS ISA | |
> +--+ 2000 +------+
> | Server | x.x.x.251/24
> The public web server is hosted elsewhere. The LAN comprises
> 30 workstations.
> To complicate the matter, the LAN address family x.x.x. is
> NOT RFC1918-compliant (and is conflicting with existing
> Internet hosts).
> The system is up and running, but I cannot understand the
> bypassing of the ISA server through the direct connection
> firewall/LAN. And the meaning of DMZ seems to be lost.
> Anyone can help me to understand the matter ? Thanks in advance
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