I have a 3900 running 1.0.6 firmware. It appears that "out of the box" it acts as an open dns relay on it's wan port. I am not running dns on my lan but the 3900 itself acts as a dns server for the lan caching and onwardly querying my ISP's dns servers. It is good that it does this for the lan but it should not do it for the wan and certainly not by default. I didn't find any way to turn this off but a workaround is to forward UDP port 53 from the 3900's address to a non-existent address on the lan. You can confirm before and after behaviour from the internet using dig or nslookup
if the public address of your 3900 is 1.1.1.1
dig 1.1.1.1 ibm.com any (linux)
nslookup ibm.com 1.1.1.1 (windows)
if you find that you get a response to the above then you have the same problem as me and need the workaround described above. I had the same problem with the 3900 in a data centre where the wan is configured as static as in the office where it is PPPoE so it appears fairly general.

I also had this issue before mine died completely today. When I get working again I will apply your workaround until draytek fix!

ask for new beta firmware, I was told this was fixed with the 1.0.6 firmware but maybe not, I know the betas do though.

Voodle wrote: ask for new beta firmware, I was told this was fixed with the 1.0.6 firmware but maybe not, I know the betas do though.

Well I am currently in possession of the 1.0.6.1RC3a build 2021 and 1.0.7 build 2005 as a result of my current issues. When they are sorted I will let you know if they do indeed fix it!

It is indeed fixed with the 1.0.6.1 code