512 byte limit

[Josh Kuo]

> 1) If a reply is over 512 bytes, which can't in theory be done via UDP,
> should the queried server reply telling my resolver to ask again using
> TCP?  Assuming, as one normally should, that there are firewalls, the
> queried server can't simply reply TCP, as it would get blocked.

I am not sure about the UDP size question, but I am pretty sure this
is a client behavior, i.e. the server does not send back a reply to
tell the client to use TCP port, but client should try UDP, fails, and
switch to using TCP.