Consider the following scenario
You only want to give specified remote Internet users access to
your network (or your single machine, if only one machine is connected
to the Internet).
If the remote users have static IP addresses, there is no problem.
You simply BLOCK all access to specific services on your network
with either IPNetSentry or IPNetRouter, then "punch holes"
in these filters to PASS the specified remote users with static
BUT, what do you do when remote users have dynamic IP addresses?
(which is often the case) In this situation you cannot simply punch
holes in your firewall filters because you never know the exact
IP address of the remote user.
This is the function of a new feature in IPNetSentry (v1.3.8 or
later) when used in conjunction with the IPNetAuthorize client:
providing authorized access to remote users who have dynamic IP
Setting It Up
1. You must have IPNetSentry v1.3.8 or later installed.
2. Turn OFF IPNetSentry with the IPNetSentry Companion.
3. Open the IPNetSentry Config file with SimpleText (or BBEdit,
etc.). Choose a port for which you wish to use for authorization.
Many ISPs intentionally block incoming traffic on ports below 1024,
so you might wish to choose a port above this range. For this example
we will assume we have chosen port 2525.
4. Add the following line to your IPNetSentry Config file (right
below the #set/payload_inspection commands):
The above command will authorize a remote user who comes in on
UDP Port 2525 using the Password "trythis". Access will
be permitted for 600 seconds. When access is granted, the alert
will display "Password Access" as the service.
The general syntax of the #set\password command is:
If no interface is specificed, the default interface will be used
(as setup in the TCP/IP control panel)
If no notification is provided, the default IPNetSentry notification
mechanism will be used.
5. IF you are not running IPNetRouter, then you will need to add
the BLOCK service filters in the IPNetSentry Config file. For example,
let's say you are running a web server on Port 80 and a mail server
on Ports 25 and 110. BUT you only want authorized remote users to
have access to this server. You need to block ALL access to these
servers, which can be done with the following filter commands in
the IPNetSentry Config file:
Another approach is to just block ALL tcp ports below 1024, e.g.:
IF you ARE running IPNetRouter, you can add similar filters directly
through the IPNetRouter Filter window.
6. Save the IPNetSentry Config file and close it. Turn ON IPNetSentry.
7. On a remote machine, download and install the appropriate IPNetAuthorize
client for your remote platform:
Client for Mac OS X
Client for Classic Mac OS
Client for Windows (95, Me, NT, 2000, XP)
8. Launch the IPNetAuthorize client on the remote machine. Enter
the destination (IP address or Domain Name), Port (2525 in our example),
and password ("trythis" in our example). Click the "Authorize"
button. IPNetSentry should recognize the incoming datagram and insert
a PASS filter for this specific IP address. This filter will expire
in 600 seconds (as set in our example #set/password command).
- You cannot use the same password simultaneously from two different
remote IP addresses. Using the same password more than once will
automatically remove any existing PASS filters which were invoked
by that same password. This is a security feature. For example,
say your PPP connection dropped during your session. You then reconnect
to your dial-up ISP, and are assigned a new IP address. You then
re-authorize using the IPNetAuthorize client. Your re-authorization
would automatically drop the existing PASS filter for your previous
- Unauthorization can only be performed by the aged filter timeout
mechanism. While this has some limitations, it also ensures that
unattended access will always timeout.
- There is NO encryption performed by IPNetAuthorize nor IPNetSentry.
Hence this solution is not a general replacement for virtual private
networks (VPNs). In many cases, however, users are not so concerned
about data encryption as they are about unauthorized access to their
server(s) or network.
There are situations when you might also wish to limit access to
services on internal LAN machines. For example, say that you want
to give LAN clients universal access to web and email services.
But only one machine on the LAN at a time should have general access
for all other services (webcasts, streaming audio, etc.). You can
easily do this with IPNetRouter, IPNetSentry, and IPNetAutorize.
First you would add the filters to IPNetRouter which prevent access
to all services below port 1024. You would then punch holes in this
main blocking filter to give access to essential services such as
HTTP, POP, SMTP, DNS, DHCP (if needed).
Then, in the IPNetSentry Config file you would add a #set\password
command to permit access by any one of the client machines to all
#set\password\Ethernet slot J9\2525\trythis\Password Access\\600
Note that we have specified the interface on which this password
command will be set (we are assuming our private LAN is connected
to the Ethernet slot J9 port).
In this manner only one of the client machines at a time will have
access to all services. In addition, when any client machine requests
such access, you will have a log of this event in this IPNetSentry
Please send questions, comments, or suggestions using our general
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