blocked DNS query

[Stephane Bortzmeyer]

On Sun, Nov 28, 2004 at 08:06:45PM -0500,
 kaiduan xie <kaiduanx at yahoo.ca> wrote 
 a message of 13 lines which said:

> Can any one explain the cases where DNS query got blocked

Blocked by who? Why? I must confess the question is unclear.

> and the cases where DNS query is carried by TCP instead of UDP?

It's at the client's discretion (although a good client should start
with UDP and switch to TCP only if the reply was truncated). See RFC
1035, 4.2.

> In other words, asynchrous DNS query is preferred over synchrous DNS
> query.

In the DNS protocol, there is no such thing as an "asynchronous DNS
query". Asynchronicity is a function of the client (libraries exist to
perform asynchronous queries). If you talk about UDP vs. TCP, I've
never seen these protocols referenced as "asynchronous" or
"synchronous".