On Fri, Jul 10, 2015 at 2:53 PM, 神明達哉 <jin...@wide.ad.jp> wrote:
> On Tue, Jul 7, 2015 at 2:20 AM, <fujiw...@jprs.co.jp> wrote: > [...] > In Introduction it states: > > While negative (non-existence) information of DNS caching mechanism > has been known as DNS negative cache [RFC2308], it requires exact > matching in most cases. [...] > This was because the NXDOMAIN response just says > there is no such name "a.example.com" and it doesn't tell anything > for "b.example.com". > > While I see what it tries to say and don't disagree with it, I think > this is not very accurate. In fact, NXDOMAIN for "a.example.com" says > there is no such name *or any subdomain of it*. So it would still be > usable to suppress unnecessary external query for, e.g., > foo.a.example.com. > That's indeed the literal meaning of NXDOMAIN, but it turns out most current resolver implementations don't treat it that way. The wording in RFC2308, Section 5 is not entirely precise, but it seems to say that negative answers should be cached only for the exact qname, and not (necessarily) for anything below it. Section 3 of http://tools.ietf.org/html/draft-vixie-dnsext-resimprove-00 ("Stopping Downward Cache Search on NXDOMAIN") proposed to fix this resolver behavior. It would be great if this was standardized and adopted. Regarding Section 5 (possible side effect on root servers), I wonder > about the implication of qname-minimization (which I expect will be > deployed much sooner than this proposal). A resolver that supports > qname-minimization would first send a query to "local." to the root > server upon receiving a "foo.local" query, and cache the result of > NXDOMAIN for "local.". It will suppress subsequent external queries > for any subdomain of it. > Yes, this will certainly be a very beneficial result of qname minimization. Shumon.
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