round robin

[Kevin Darcy]

"Cyclic" ordering, the default ordering scheme for BIND 8, has not been the same as true "round robin" for some number of years. You will notice, however, that while the first address in the response is chosen at random, the following addresses always observe "rotational" order. If you have addresses A, B and C, in other words, listed in that order in the zone file, then "cyclic" ordering will hand out, randomly, the sequences ABC, BCA and CAB. It will not, however, ever hand out a ACB, BAC or CBA sequence, since those are out of rotational sequence.

Cyclic ordering sucks pretty badly when one of the nodes fails -- the *next* node in the rotational sequence then gets approximately twice as many hits as usual. Note that "true round robin" fares badly in this scenario also. Random ordering is better for the failure scenario; unfortunately, random ordering can lead to occasional "spikes" of activity on one node, even when everything is operating normally. Ultimately, the "fairest" ordering scheme would be "permuted" ordering, where the nameserver steps through every permutation of answer for each RRset. Unfortunately, that would require
keeping track of a significant amount of per-RRset state (read: increased memory footprint), not to mention all of the extra code required. Don't hold your breath waiting for this feature to arrive in BIND...

                                                                                                                                                - Kevin

Armin Safarians wrote:

> I have a host with three ip's setup to do round robin and according to
> the bind book it should work.
> 1,2,3 - 2,3,1 - 3,2,1..  I'm sure you know how Round Robin works.
>
> Unfortunately my server acts nothing like this. It is as random as it
> can be. I have not been able to get a pattern.
>
> Is there anything I'm missing.
>
> Thank you for you help.
> AMS :-)
>
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