how does dns choose an ns record in a lookup?

[Sasha Borodin]

If the dns server perfororms a lookup for www.somedomain.com, at some
point in time it gets a list of ns servers for somedomain.com, right?
I'm trying to find some documentation on the behaviour/configuration
of the process w2k dns server uses to choose among those multiple ns
records.

The reason i'm digging into this is because about once a month or so,
there's some domain for which my server fails to resolve lookups.
I've traced this down to the fact that out of, say 4 ns servers for
that domain, one is down, and that's the one that my server keeps
asking...

So, I've tried to clear cache and restart the DNS service, hoping that
next time the lookup request is made, it'll consult a different dns
server for that problem domain, but no luck, it's still trying the
same one. :-(

How does it choose? Can it be configured?

Thanks so much for your help,

-Sasha

 

[Sasha Borodin]

Ace, I've read that book. And I read this explanation from Mr. DNS:

http://www.acmebw.com/askmrdns/archive.php?question=3

However, this is not what seems to be happening in my case. My server
insists on querying the SAME ns server out of a list of say four that
it gets returned (in a recursive query); if that ONE happens to be
down, I would think that my DNS server would move on down the list.
But it doesn't.

So sometimes, I can't resolve names in a particular domain for days,
cause one of their ns server is taken down, and my DNS won't move on
to the others still available...

Any help would be greatly appreciated.

-Sasha

 

[Ace Fekay [MVP]]

In

If the dns server perfororms a lookup for www.somedomain.com, at some
point in time it gets a list of ns servers for somedomain.com, right?
I'm trying to find some documentation on the behaviour/configuration
of the process w2k dns server uses to choose among those multiple ns
records.

The reason i'm digging into this is because about once a month or so,
there's some domain for which my server fails to resolve lookups.
I've traced this down to the fact that out of, say 4 ns servers for
that domain, one is down, and that's the one that my server keeps
asking...

So, I've tried to clear cache and restart the DNS service, hoping that
next time the lookup request is made, it'll consult a different dns
server for that problem domain, but no luck, it's still trying the
same one. :-(

How does it choose? Can it be configured?

Thanks so much for your help,

-Sasha

Normally any server that can provide the answer, either the authorative or a
non-authorative (say thru a recursion from forwarding or Root hints), it
will go with the answer.

O'Reilly's DNS and BIND explains all of this. Good book.


--
Regards,
Ace

Please direct all replies to the newsgroup so all can benefit.
This posting is provided "AS IS" with no warranties.

Ace Fekay, MCSE 2000, MCSE+I, MCSA, MCT, MVP
Microsoft Windows MVP - Active Directory

 

[Jonathan de Boyne Pollard]

SB> If the dns server perfororms a lookup for www.somedomain.com,
SB> at some point in time it gets a list of ns servers for
SB> somedomain.com, right?

If "somedomain.com." is a delegation point.

SB> The reason i'm digging into this is because about once a month
SB> or so, there's some domain for which my server fails to resolve
SB> lookups. I've traced this down to the fact that out of, say 4
SB> ns servers for that domain, one is down, and that's the one that
SB> my server keeps asking...

I've experienced this sort of thing where the fault lies in the published DNS
data, not in one's resolving proxy DNS server. A combination of gluelessness,
lame delegations, and mismatched delegation information causes secure
resolving proxy DNS servers to become locked in on a single lame server for a
period.

What's the domain name that you are looking up ?

 

[Ace Fekay [MVP]]

In

Ace, I've read that book. And I read this explanation from Mr. DNS:

http://www.acmebw.com/askmrdns/archive.php?question=3

However, this is not what seems to be happening in my case. My server
insists on querying the SAME ns server out of a list of say four that
it gets returned (in a recursive query); if that ONE happens to be
down, I would think that my DNS server would move on down the list.
But it doesn't.

That doesn't make sense, it should use the next available, according to the
article and the default behavior.

So sometimes, I can't resolve names in a particular domain for days,
cause one of their ns server is taken down, and my DNS won't move on
to the others still available...

Can you give an example of the domain name or are you speaking of an
internal name? What else is in the Nameservers tab list in those machines?

Are you only pointing to your internal DNS on the client side (DC or user
machines)?