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Re: Company Firewall's IP Address
From: Bill Hamel <billh () bugs hamel net>
Date: Wed, 13 Nov 2002 19:49:28 -0500 (EST)

Hi Ed,

I believe in the original post, the poster was concerned that the IP
address of their firewall showed up in a pop-up using a browser, and how
to keep that from happening. This would suggest outbound port 80 activity,
maybe SSL. I am assuming this is the external address of the firewall.

All I am saying is that the external ip address of a firewall will
always be known by whomever or whatever you are connecting to. if the machine
this user is conecting to, out on the internet,
is showing their internal RFC1597 IP address IMHO that is impossible since
private IP space doesn't route.

As far as port 25 goes, inbound connections on 25 TCP typically go to an
SMTP server, hopefully on a DMZ. I think what you are saying is that
outbound SMTP traffic will show in the email header the internal IP
address of the original sending client. Which is true with stateful
inspection or less firewalls. but most application proxy level firewalls
will remove this information (if configured correctly).

Some mail (SMTP) servers need to be on the trusted network for technical
reasons (Yuk!), in which case a firewall that utilizes it's own mail
servers is the best bet because not one bit from the internet ever
directly gets to the SMTP server.

Jeesh, I hope I didn't cloud the issue :)


On Wed, 13 Nov 2002, Edward N Schofield wrote:

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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