Why the response is slow?

[koancui]

Hi,
I have 2 machines, i configured one of them, such as A, as the DNS,
run named, set B's NS point to A.
When i run telnet B on A, there is obvious delay after displaying
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Trying xxx.xxx.xxx.xxx...
Connected to B.
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I had to wait a while here, then got
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Escape character is '^]'

OS
login:
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if i set B's NS point a normal DNS, everything is ok.
Can you explain it?
Thanx.

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Koan