At 8/20/03 12:40 PM, Rick Hodger wrote:
>> Note that I queried ns1.businessmedia.co.uk (which worked) for this
>> example, instead of your example of ns0.businessmedia.co.uk (which
>> doesn't work for some different reason). It doesn't appear that
>> ns0.businessmedia.co.uk is registered as a hostname, or if so, it perhaps
>> isn't used by any .uk domains so it isn't published as glue by the
>> registry.
>
>Both ns0 and ns1 are being used to serve domains, although now that I look
>at it, I don't think ns0 is being used on any .uk domains. I'll switch one
>over tomorrow and see if that makes a difference.
Right, I'm almost sure it will.
The only reason the glue is ever necessary in the top level servers is to
resolve the chicken-and-egg problem where a domain "example.tld" uses
nameservers that end with the same "example.tld".
That is, if "example.co.uk" uses "ns0.example.co.uk" as a nameserver,
there is no way for a client to obtain the IP address of
ns0.example.co.uk from the ns0.example.co.uk nameserver (because it
doesn't know the nameserver IP address). The top level glue overcomes
this problem by making a copy of the IP address of potential
chicken-and-egg nameservers available in an additional place (the
top-level nameservers); that's the sole problem that the glue is solving.
If the domain in question doesn't have nameservers ending in the domain
name, the glue is unnecessary. If example.com uses ns0.example.co.uk as a
nameserver, but example.co.uk uses ns1.example.net and ns2.example.net as
nameservers, the glue for ns0.example.co.uk is not necessary: a client
doing a lookup on example.com can get the IP address of ns0.example.co.uk
from ns1.example.net, so there's no chicken-and-egg problem. This is
probably what you're seeing if ns0.businessmedia.co.uk is used only for
non-.uk domains.
(Note also that this is a somewhat simplified explanation; an extremely
pathological case also exists where you have a loop of several records.
See "Gluelessness" at http://cr.yp.to/djbdns/notes.html for more info and
other interesting tidbits.)
Most registries will simply check to see whether a nameserver is used for
any other domain in that TLD when deciding whether to include glue; for
example, Verisign will include glue for any .net nameserver if it's used
for any .com or .net domain. It's theoretically possible that some
registries might omit the glue unless the nameserver is used for that
actual domain name -- if the .uk registry does that (unlikely), you would
have to add ns0.businessmedia.co.uk as a nameserver for
businessmedia.co.uk to get the glue included, rather than just adding it
as a nameserver for some random other .uk domain.
>In my fevered speculation though, it appeared to me as though queries for
>businessmedia.co.uk would attempt to resolve ns0 and ns1 first, and if both
>those servers were down, it seemed logical at the time that no query to
>those nameservers would be able to resolve again.
Nah, it would be okay -- a failure to resolve ns0 at any point in the
lookup process would be retried on the next lookup (well, some resolvers
cache failures for a short time, but it's usually only a few minutes).
>I thank you for your time in explaining it to me.
Sure thing; glad it helped.
-- Robert L Mathews, Tiger Technologies"In storing and handling the Compact Disc, you should apply the same care as with conventional records. If you follow these suggestions, the Compact Disc will provide a lifetime of pure listening enjoyment."
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