DNS all records requests

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According to the PIX, for the past few weeks (on every day except
Mondays) I've been getting "DNS all records requests" from a few IPs.
And it looks like these requests always immediately triggered another
all records request from my server to another IP. I don't know any of
the addresses, and they come in frequently (with hour gaps from time to
time) at random-ish times. There are 50 or so from one IP, then change
to another with very little overlap. And chkrootkit doesn't see anything
odd.

Can anyone tell me what might be going on?

- --
Glenn English
ghe@xxxxxxxxxxx

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