OK, try to describe it:
When client is logging in through hotspot, everything works fine, pings are fine, in the begining Firefox or other browser is displaying webpages correctly as well. Lets say as example, that
http://www.google.com is my homepage. I see this page, I see all pages, but suddenly, sometimes after 5 min. sometimes after 10 min., sometimes immediately I can see errorr page wit DNS error message (for many pages, not only
http://www.google.com). Certainly ping and other direct IP connections to internet are still working fine. When I use nslookup at this moment and asking for this address, I am receiving error message "no A record for this addresss....". If I try to ask nslookup within another 5-10 min., it is answering correctly, but in a browser still the same DNS error.
For some users I am using IP binding and bypassing hotspot, for these users everything works fine except users with Apple computers (these have DNS problems).
Another important fact is, that domains, which have records on my primary DNS server are working correctly anytime.