Selective Forwarding Behaviour
Julian Hayward
hayward_julian at hotmail.com
Thu Apr 25 07:31:42 UTC 2002
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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