Stephane,

Thanks for response. Actually there is a resolver
library ARES from MIT. The gethostbyname() call is
synchrous. ARES is asynchrous, i.e., send the DNS
query, using the select(), and then call ares
callback. In this way, this is non-blockable query. I
want to know the case where from the user's point of
view, the DNS query is blocked.

Hope this time the problem is clear.

Thanks,

kaiduan
--- Stephane Bortzmeyer wrote:
> On Sun, Nov 28, 2004 at 08:06:45PM -0500,
> kaiduan xie 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".
>
>


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