Selective Forwarding Behaviour

Kevin Darcy kcd at daimlerchrysler.com
Thu Apr 25 21:07:24 UTC 2002


Julian Hayward wrote:

> My company has a large extranet served by 3 DNS servers - 1 sited in the UK,
> 1 in the US & 1 in Asia. These DNS servers are authoratative for the domain
> name (changed because only accessible on our extranet) zone1.zone2.com
>
> For our customer name resolution we have recommended that they selectively
> forward name resolution requests for zone1.zone2.com to our DNS servers,
> with the one nearest geographically to be the first in the selective
> forwarding list (The customer would require BIND 8.2.3 or later to do this).
>
> An example of our recommended customer config:-
>
> zone "zone1.zone2.com" {
>         type forward;
>         forwarders      {UK; US; Asia;};
> };
>
> Question One:-
> Under what circumstances would the client DNS server query the 2nd or 3rd
> server in the selective forwarding list?
>
> A client has suggested that the default behaviour with selective forwarding
> is round robin, and that the three DNS servers would be queried in turn for
> subsequent requests.
>
> DNS & BIND 4th Edition doesn't specifically reference this with regard to
> selective forwarding (forward zones) but does have this to say about
> Forwarder Selection pg 270 - "...These nameservers don't necessarily query
> the forwarders in the order listed; they interpret the name servers in the
> list as "candidate" forwarders and choose which one to query first based on
> roundtrip time, the time it took to respond to previous queries."

This was true in later versions of BIND 8, but it was not true earlier than
that, and last I knew, it wasn't true in BIND 9 either. Those versions try the
forwarders in the order listed.

But, if the forwarder list is sorted by "closeness" anyway, and all of them
stay up, what difference does it make to you?

> Question Two
> Does the above excerpt from DNS & Bind relate to selective forwarding
> forwarders?

I believe the forwarder-selection algorithm is the same, regardless of whether
the forwarding is "global" or "selective".

> Question Three
> Would this behaviour change with different (ie later) versions of BIND?, eg
> BIND 9.x.x?

Yes. See above. I understand that BIND 9 regressed to the earlier (sequential)
algorithm for forwarder selection. This may have changed in the later releases
of BIND 9 -- I don't track changes in forwarding behavior very closely, since
we don't use forwarding here...


- Kevin




More information about the bind-users mailing list