About this test: In this suite of tests we determine basic UDP connectivity available from your computer. In particular, we test whether your machine is allowed to send the following to our server:

• UDP datagrams on arbitrary ports. A number of applications require this connectivity, which may be blocked by default.
• Correct fragmentation of large UDP datagrams. Support for fragmented IP datagrams is important for ensuring robust operation regardless of your network MTU.
• UDP-based DNS requests. The ability to send DNS requests to arbitrary resolver of your choosing is important, since it allows you to choose a different DNS service (such as OpenDNS.com. On the other hand, this freedom also implies that you are at risk of inadvertently using an unwanted DNS resolver or can bypass controls which are part of a commercial network.
• Other services, particularly UDP port 1434, once used by the notorious SQL Slammer worm.

What if this test reports a problem: If you are using firewalling software on your computer, make sure it does not interfere incorrectly. If you are unable to configure arbitrary DNS resolvers but would like to do so, contact your ISP. If we report a path MTU hole, it means that a network component is unable to process IP datagrams that are larger than its local interface MTUs. There is little you can do about this, but see the path MTU help page for more information.

For additional information on these tests, please consult the Netalyzr help site over at the New Scientist.