Selective Forwarding Behaviour

[Cricket Liu]

> 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?

With BIND 8 from 8.2.3 on, if it were the fastest to respond.

> 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.

Nope.

> 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."

The same is true for forward zones in these versions of BIND, I believe.

> Question Two
> Does the above excerpt from DNS & Bind relate to selective forwarding
> forwarders?
>
> Lab tests with a DNS running BIND 8.2.3 on a Windows NT Server platform,
> configured to selectively forward zone1.zone1.com requests to the above
> three DNS servers have produced results that do not indicate round robin.
>
> Question Three
> Would this behaviour change with different (ie later) versions of BIND?,
eg
> BIND 9.x.x?

Yes.  BIND 9 tries forwarders in the order in which they're listed.

cricket

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