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Re: Company Firewall's IP Address
From: Edward N Schofield <shuffle3 () insightbb com>
Date: Wed, 13 Nov 2002 18:16:45 -0600

Unless someone knew nothing about firewall configuration, the trusted interface should only be addressable by the firewall, assuming that Network address translation(NAT) algorithms in the firewall or by an external gateway router are being used. If NAT is being used, even knowing the trusted interface address would not bypass the firewall. It would be difficult to imagine anyone setting up a firewall to directly accept the trusted interface address from the untrusted side of the firewall (or else why have a firewall?) Passing through email messages just means the firewall is being told to not filter messages coming in for email services (TCP port 25 ( a logical port), if my holey memory recalls correctly). A stateful packet inspection firewall such as Checkpoint checks the characteristics of the packet to ensure it only gets the services for email, in this case. The message then goes to the email client, and the reply is returned from the email client's address, not the firewall. Most organizations pass outgoing messages through the firewall without checking the services. It is developing security practice to have the firewall permit only the services you let into your organization's network to exit the network. (i.e. if you permit only HTTP (TCP Port 80) or email (TCP Port25) to enter your network, only permit these services to exit.) This hinders someone using a code exploit to generate FTP services packets. (Port 23), as an example. This is a tough sell, but at least one consultant demonstrated that , given an exploitable code vulnerability, it is possible to generate file transfers of desired files without granting access to these services through the firewall. That went through this list last fall. If you contact me off-list, I can supply the name, but I think it would be contrary to Mike's guidelines to give someone a free plug.

Hope it helps.

Bill Hamel wrote:

Unless I am missing something in the question, no matter what you do,
what/whoever you connect to through a firewall will always know the IP
address of the the trusted interface of the firewall.


On Wed, 13 Nov 2002, Meritt James wrote:

"an" IP Address - not necessarily the originating individual.  There are
a LOT of ways around that.


Leonard.Ong () nokia com wrote:

There is nothing new about finding your IP Address and display it on the web page.
James W. Meritt CISSP, CISA
Booz | Allen | Hamilton
phone: (410) 684-6566

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