This article is part of the NAT problem guideline
Checking the nat indicator and torrent smilies
If you never see a green Torrent health icon and only see yellow smilies on your torrents once they're running you probably have a NAT issue. If you see green health icons from time to time it's probably a different issue, have a look at the Torrent health page.
You can also look at the nat status indicator on the bottom... if it's anything but grey or green you either have a nat problem or there are no peers on your torrents at all. This indicator is derived from the NAT status of individual torrents. It will show a red indicator if none of your active torrents have been green in the life of the Azureus session. Red therefore does not indicate a definite NAT problem as you may have an active torrent that has never received an incoming connection due to it having only seeds (for example).
To make sure you really have a nat problem check the next chapter.
UDP (used also for the Distributed DB):
Just like for the TCP port there also is a indicator in the status bar for the UDP port. If it is yellow or red you can check if the DDB is working at all under Tools > Statistics > Distributed DB. If that tab shows you a usercount of several 100k users and you have incoming traffic it's at least operational and the NAT problem just makes it less efficient.
If you have allowed azureus in your firewall, forwarded the UDP and TCP port in your router and have a green TCP nat indicator that might be the case, proceed otherwise.
Note: UDP port settings are not visible for 'Beginner' users.
The built-in nat/firewall tester
If the status bar icon or torrent health icons tell you you might have a nat problem you should use the built-in test under Help -> NAT/Firewall Test (just hit the test button).
Note: There is no explicit test for the UDP port, the DDB will tell you if there might be a problem. If the 'Reachable' status in DDB screen is 'yes', you should be ok.
You can't always rely on the built-in NAT/firewall test; the server that runs this service may be overburdened or unavailable. you can use an external port-scanner like https://grc.com/x/portprobe=52525 <- replace the port 52525 in this example URL with your port number (the "Incoming TCP Listen Port") in Options > Connection.
Note: You must have Azureus running when you run this test. Otherwise there's nothing to respond to the port request.
If the test returns...