we are investigating the behavior of Network Address Translation (NAT)
with regard to NAT traversal. Today, most households use a Network
Address Translator (NAT) integrated into their DSL-Router to connect to
the internet. The NAT translates between the public and the private
realm and creates a mapping for each outgoing connection. This works
well as long as connections are established from the inside because the
NAT is aware of all incoming packets and can forward them to the
appropriate private host. But as soon as a user wants to provide a
service behind a NAT, the NAT is not able to map inbound packets to this
host because no state exists for these packets. This problem is called
the NAT-Traversal Problem. There are already a number of approaches to
solve this problem, but these approaches depend on the NAT behavior.

The implementation of a NAT is not standardized. The behavior of NATs
not only varies from vendor to vendor, but also from type to type. To
evaluate and test as many NATs as possible, we created a small
application that determines the properties of a NAT and makes some
connectivity tests with our test server.

Therefore, we kindly ask you to download the NAT-Tester and run it on
your home computer. The test takes 1-2 minutes and is done
automatically. After the test, you will be directed to an internet-page
where you can provide some information about the tested NAT (vendor and
model). The test client is available for Windows and Linux. The
Source-Code can be found within the Linux package. If you have any
questions, please contact us.

Visit http://gex.cs.uni-tuebingen.de at your convenience to participate
and look at all the results.

Kind Regards
Andreas Klenk and Andreas Müller