Selective Forwarding Behaviour

[Julian Hayward]

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."

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?

I would like to get a definitive answer to the manner in which the selective 
forwarding forwarders are queried and so would be grateful for input from 
this list.

Regards,

Julian Hayward



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