Need help on delegation to subdomain/external servers

Merton Campbell Crockett m.c.crockett at
Sun Sep 20 17:13:13 UTC 2009

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 and 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 <kcd at>  
> wrote:
> RUOFF LARS wrote:
> [mailto:bind-users-bounces at] 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 {;; };
> };
> 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  
> (
> 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.crockett at

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