As some of you noticed, there are some problems with DNS cache service. I use v.2.9.27 and it stops working after few minutes:
http://forum.mikrotik.com/t/dns-problem/15495/1
Perhaps someone is poisoning DNS service when router has public ip.
maybe this problem is fixed in 2.9.28, or later? 
update your software
I have the same problem. Using 2.9.50
Upgrade to 3.2 doesnt help. Dont know what to DO !!!
see http://forum.mikrotik.com/t/dns-doesnt-work-propertly/18752/1
you can use firewall to filter networks which can access your DNS service. Regarding the problem - please write to support with detailed description and a supout.rif file. We will try to repeat and fix it.
That’s what I thought. I’ll use firewall and prevent external access to DNS service. I think there’s no bug in DNS, but there’re some problems when DNS is overloaded with requests.
no, there is serious bug in DNS service in MT. Why is it so difficult to find that bug? noone is able to give me advice and answer. I sent supout file, but i only get answer to use “redirect” instead of dst-nat. But I would like to use DNS cache service like in past time. Is it possible, when I set my router to default and agains set it, it will be working?
thanks for reaction.
I was convinced that there was some bug in DNS cache server too.
Tired of weird random DNS misbehavior, i’ve started to sniff some traffic and voila, i was simply being dumb from the beginning: Every DNS query that was over the 512 byte limit cannot be transfered using normal UDP DNS packets. It MUST use TCP communication with the name server instead, or the name will never resolve. So, the dumb part was that my firewall rules were blocking direct access to TCP port 53 on the router… Now everything seems to be normal again!
TL