On Wed, Sep 19, 2007 at 04:52:35AM +1000, Anthony Towns wrote: > Ah, that'll be because the ordering's simply rotating, rather than being > random: so we're assuming from A > B,C,D > E and getting: > ABCDE -> ABCDE > BCDEA -> ABCDE > CDEAB -> ACDBE > DEABC -> ADBCE > EABCD -> ABCDE > which biasses B to being in second place, C in third, and D in fourth, > simply because all but twice, B is ahead of C and D in the input because > it's just being rotated, not shuffled.
In fact you can take this further, to the point where getaddrinfo()
is actively unbalancing the load. Suppose you have addresses:
F1.00.00.02 (a)
F1.00.00.03 (b)
01.00.00.02 (c)
01.00.00.03 (d)
Then your longest matching prefix will be either with a and b or c and d.
In that case, you get:
DNS a/b match c/d match
~~~~ ~~~~~~~~~ ~~~~~~~~~
abcd abcd cdab
bcda bacd cdba
cdab abcd cdab
dabc abdc dcab
Which will give three times the load to a and c compared to b and d.
And note that applies to IPv6 as well as IPv4, presuming the DNS presents
IPs in the simplest round-robin fashion.
Cheers,
aj
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