forwarders question

Mark Andrews marka at
Tue Aug 11 01:13:05 UTC 2009

In message <4A808228.2080100 at>, Doug Barton writes:
> Michael Monnerie wrote:
> > We are having 2 sites at different locations now with a DNS resolver on 
> > each site. Internet speed between those two different ISPs is very fast, 
> > and the hosts to resolve will be about the same because of similar 
> > services.
> > 
> > My idea is to use 
> > forward X; 
> > on site Y and 
> > forward Y;
> > on site X, but, as I couldn't find it in the documents, I believe this 
> > could lead to a resolver loop between X and Y and therefore even slower 
> > resolution. Or is BIND clever enough to only ask the other server once?
> If you're getting a response for a name that neither server is
> authoritative for, you have your answer. tcpdump could give you more
> information if you want to pursue it further.
> > There are 2 reasons for this:
> > 1) performance. Having the caches hot on both sides and with a high 
> > chance one caches knows entries the other can use, it should be quick.
> Unless you are turning off your name servers when everyone goes home
> at night I would like to suggest that you're not really gaining
> anything by doing this. There are two possible scenarios:
> 1. Usage patterns are different at your 2 sites.
> 	In that case you gain nothing by doing what you're doing.
> 2. Usage patterns are similar at your 2 sites.
> 	In that case IF the link between your 2 sites is dramatically
> 	faster than the link between your name servers and the outside
> 	world then you will gain a small amount of performance after
> 	the name servers are first booted. After a few hours of normal
> 	use (in other words, the cache is built up on both sides) it
> 	is likely that you are not gaining anything.
> In the even that the link between sites suffers some sort of
> performance problem you are definitely going to be pessimizing your
> DNS with this configuration.
> In short, there are a lot of scenarios when you are going to be doing
> worse, and a very few scenarios when you are doing better, and then
> only for a short period of time. I would therefore suggest that the
> configuration you are suggesting is a lot of added complexity for no
> measurable benefit.
> > 2) reliability. Asking only internal servers which I can control is more 
> > secure than using any ISPs DNS. They start to do the DNS mangling here 
> > in Austria also (instead NXDOMAIN they deliver their web sites A record 
> > to point to their search engine).
> While I agree that local resolvers are a good idea, this has nothing
> to do with your forwarder configuration.
> hope this helps,
> Doug
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	The forwarding concept was developed when 48k external links
	were *FAST* links and having everyone on a campus use one
	or two machine as a super cache provided some real benefit.

	It still provides some benefit if you are dialing up over
	the PSTN.  However if you are using Cable/DSL or similar
	technologies there is little benefit and huge negative
	consequences in the case of the forwarder being down.

	Cross connecting caches is not part of the design strategy
	and will not work well.  It would take code changes to make
	it work well.

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at

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