When I was transferred into our corporate IT Networks group, I
inherited a DNS architecture based on forwarding DNS queries to
regional name servers. The regional name servers had access to the
Internet and were able to provide name and address resolution for both
Intranet and Internet queries.
The designers of the DNS architecture carefully configured the
forwarders statement on each name server so that the name server for
the region was listed first. It was followed by the other regional
name servers ordered by distance from the local name server.
Had this been implemented under BIND 8, it would have worked as the
designers had expected. Unfortunately, it was implemented under BIND
9.3. The list of name servers in the forwarders statement was no
longer treated as a sequential list as it had been in BIND 8. Under
BIND 9.3 and later releases, the selection of name server from the
forwarders list is performed the same way as the selection of name
server for a DNS zone: its round-robin with a preference given to the
name server with the smallest RTT.
Another item that the designers didn't anticipate was how RTT is
calculated. It is not based on the RTT to the forwarder but on the
time that it takes for the forwarder to return a result. Given the
physical locations of the regional name servers, the calculated RTT is
roughly identical even at sites where there is a local name server co-
located with the regional name server. In our particular environment,
the "primary" forwarder changes approximately every 20-30 seconds.
Given this behavior, I'm not sure what advantage there is in having
"online" and "offline" name servers. I would opt for having all name
servers "online" and let BIND select the more desirable name server.
On 17 Sep 2009, at 11:15:59, Ben Croswell wrote:
I have done some testing of the RTT forwarding and found that as
long as only one, or the other of the two "nameservers" that you
forward to is active at any given time the switch over is actually
very quick.
The exception being the first query when the currently active
forwarder dies and the second comes up. The reason being that the
first query has to wait for a timeout cycle before trying the second
forwarder and readjusting the RTT values for both.
So theoretically if your forwarders are 10.1.1.1 and 10.2.1.1 as
long as only one will answer queries at a given time with their own
"right" answer it should failover fairly quickly. If both answer
then you will be at the mercy of the RTT as to which answer you will
get.
--
-Ben Croswell
On Thu, Sep 17, 2009 at 12:27 PM, Kevin Darcy <k...@chrysler.com>
wrote:
RUOFF LARS wrote:
[mailto:bind-users-boun...@lists.isc.org] On Behalf Of Kevin Darcy
BTW, at the moment I am experimenting a solution usign a forward zone:
zone "dummy.ts" IN {
type forward;
forward only;
forwarders { 172.25.32.171; 192.168.2.3; };
};
It seems to work.
I guess that the requests are not sent simultaneously though?
Correct, it's similar to the algorithm that a stub resolver uses:
try one forwarder, if it times out, try another, and so on.
In fact, the way I like to think of forwarding is: when you forward,
you're turning named *into* a stub resolver with a cache, at least
for part of the namespace. If you forward "globally" (i.e. in
"options"), and have some authoritative zones and/or stub zones with
"forwarders { }" defined, then those are just selective "overrides"
of your stub-resolver+cache function. And if you have "forward
first" anywhere, then you're just giving named a second chance to
resolve names iteratively, in case the initial stub-resolver+cache
approach fails (because the forwarders aren't available/reachable).
Seems like extreme overkill to use a big heavyweight process like
named, to perform a simple stub-resolver function that can otherwise
be accomplished with a few library routines, doesn't it? Well it
*should* seem like overkill, because it's usually the wrong tool for
the job. Forwarding is generally to be avoided, unless you need to
deal with a limited-connectivity situation (e.g. trying to resolve
Internet names to internal clients through a firewalled environment)
or, in certain select cases, to forward to a richly-populated
central cache, with ample capacity, over fast internal links, in
order to speed up the average name resolution time for a local set
of clients.
What delay do I have to expect when only the second server
(192.168.2.3)
is active?
I'm not sure, I'd have to look through the code. I don't believe
this delay is configurable, by the way.
What search policy is applied by default? (round-robin vs sequential?)
Can I modify it?
Obviously I would prefer a policy where we always forward to the last
active, unless we time out; Then try the alternate.
Will check that out.
I believe that forwarder-selection uses the same algorithm as NS-
selection, i.e. it's based on the historical RTT data. So it might
not switch over as fast as you'd like.
- Kevin
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Merton Campbell Crockett
m.c.crock...@roadrunner.com
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