Need help on delegation to subdomain/external servers

Merton Campbell Crockett m.c.crockett at
Thu Sep 24 04:47:01 UTC 2009

The re-design of the DNS network architecture was one of the few  
internal projects where a credible "Concept of Operations" document  
was produced.  It had detailed graphics showing the flow of network  
traffic between local and regional name servers.  There were detailed  
discussions and graphics explaining how local name servers would "fail  
over" to another regional name server and which regional name server  
would be used under certain failure conditions.

The "unfortunate" aspect is, simply, that IT colleagues (and  
management) believe that the document is "right".

On 21 Sep 2009, at 14:31:39, Kevin Darcy wrote:

> What is "unfortunate" about BIND picking a forwarder based on real,  
> up-to-date information about the thing that the end-user ultimately  
> cares about -- how quickly the queries get answered?
> Surely this is better than hardcoding a bunch of assumptions into  
> your forwarding configs, assumptions that can be trivially  
> invalidated by such factors as nameserver failures, nameserver  
> performance bottlenecks, network congestion, route flapping and  
> other topographical anomalies, query usage patterns, etc.
> If you want more fine-grained control of your query traffic, for  
> reasons other than pure query performance (e.g. you might pay per- 
> byte on link A but a flat rate on link B), then you might have to  
> resort to devices and/or mechanisms outside of named in order to  
> accomplish it, e.g. traffic shapers, transparent DNS proxies and the  
> like.
>                                                                                                                                         - Kevin
> Merton Campbell Crockett wrote:
>> 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 <mailto:kcd at 
>>> >> wrote:
>>>    RUOFF LARS wrote:
>>>            [mailto:bind-users-bounces at
>>>            <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 <mailto:m.c.crockett at>
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Merton Campbell Crockett
m.c.crockett at

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