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Firewalls can be a huge headache
when you want computers to work together. Hardware firewalls
are less of a problem because it's usually very clear
whether or not there is a hardware firewall between
your computers. Software firewalls ("personal firewalls",
software that runs on the same computer that you're
trying to protect) are more difficult because sometimes
they cause problems even when you think that they're
turned off.
In some cases (described below), turning on a
firewall solved the problem! (What happened was that
the firewall had been disabled incorrectly.)
Firewalls can prevent one computer from connecting to
another. When you're using Extramon what happens is
that you press the button to activate the other computer
and a minute later a message box is displayed with an
error such as:
www.kazoom.biz
could not be contacted. Failed to connect to (nil) IP
number 1:10061
This means that there is firewall software preventing
the software from accessing the network.
One customer reported that Norton Internet Security
did this even after it was disabled. He uninstalled
the firewall software entirely and then the software
worked fine.
The Windows firewall can prevent connections
from one computer to the other. When using Windows XP
SP2, see our Windows XP SP2
page. For Windows XP original and SP1, the Windows firewall
can be configured by selecting Start - Control Panel
- Network Connections - Local Area Connection - Properties
- Advanced. Either turn the firewall off or add an entry
for Extramon (TCP port 5250 on the computer 'localhost').
One customer reported that Zone Alarm continued
to block network connections even if he did not start
it (e.g., he deleted the icon from the Start Up menu).
But if he started Zone Alarm and the stopped it, then
Extramon worked fine. The reason for this behaviour
is that Zone Alarm also has a service running in the
background (the True Vector engine) which starts as
soon as the computer starts, even if the Zone Alarm
icon is deleted.
A customer reported that Computer Associates E-trust
software behaved in a similar way; it blocked network
connections even though it was disabled in msconfig.
He re-enabled the firewall software and the firewall
then permitted the network connection to succeed.
The
Telnet Test
It is sometimes helpful to use the telnet utility that
comes with Windows to test whether or not a firewall
is preventing a connection to your computer.
The telnet test is helpful because it helps confirm
that a software firewall is the problem, even though
you might think that the firewall has been disabled.
Step 1. Go to the computer that you are
trying to connect to.
Step 2. Start a command prompt by selecting
Run from the Start menu and typing "cmd" into
the box, then press OK.
Step 3. At the command prompt, type "telnet
localhost NNNN", where NNNN is the port number
that the application uses, usually 5250.
You should see a message such as "Extramon is running
and the port is open". If not, the software is
not running.
Step 4. On the same computer, type "telnet
x.x.x.x NNNN", where x.x.x.x is the computer's
IP address, and NNNN is the application's port number.
You should see the same message as above. (Note: Different
firewalls behave differently. The ZoneAlarm firewall
will prevent the message from being displayed, but the
Windows firewall will not. So just because you see the
message as expected, a firewall might still be causing
a problem.)
Step 5. Go to the computer that you're
trying to connect from. Open a command prompt as above
and type "telnet x.x.x.x NNNN", as described
above. You should see the same message as above. If
not, it means that you have a software firewall on this
computer that is preventing telnet from accessing the
other computer.
If telnet is able to connect from one computer to the
other, then Extramon should be able to connect too.
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