How to resolve problems with DNS when a DNS server is configured
DNS SERVIDOR NETWORKING
Windows Server not using TCP DNS responses
I have configured unbound as a recursive server, and set the option max-udp-size to 35. 35 is enough for the DNS header and DNS question for stackoverflow.com to fit in the UDP reply (the question is repeated back in the reply), but not enough for an answer. This causes unbound to send back a so-called truncated response.