Best way to share multiple links to the net?

Re: Best way to share multiple links to the net?

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At 4:28 PM -0500 3/13/03, Robert Woodhead wrote:
>At 8:00 PM +0000 3/13/03, QuickDNS Talk wrote:
>>Let me try to clarify the issues involved.
>>
>>You're asking about web service, not so much about DNS service,
>>right? You want to use DNS to solve the problem for web service, as I
>>see it.
>
>Yep.
>
>>Solution: Yes, set your web/mail/other-non-DNS servers to have one IP
>>on each subnet. Put one DNS server on each subnet. (You could use
>>more than one for each subnet, but that might be overkill.)
>>
>>Make each DNS server a master server for your zones. Don't edit them
>>together with QuickDNS 3.x or above - edit data on each server
>>separately. On each server, for the web service, list only the same
>>web server IP address as that DNS server is on. For DNS and mail
>>services, list both. So, more concretely:
>>
>>You have link A and link B. You have a web server with IP address A
>>and IP address B. You have two DNS servers, DNS A and DNS B. On DNS
>>A, the record for web service points to IP address A. On DNS B, the
>>record for web service points to IP address B. If link A goes down,
>>the DNS servers of the world will only find DNS B, thus only finding
>>IP address B for web service.
>>
>>Have I answered the question clearly? I hope so. This is an easy
>>solution once you grok it.
>
>Yes, I understand what you're saying, and it is cute. But questions:
>
>1) Wouldn't it require a short refresh time?

Yes. Actually, short TTL's on the A records in question. 5 minutes
(300 seconds) is a good figure.

>Consider the case of someone who happens to have accessed DNS and
>gotten the IP that comes in over link A. Then link A goes down.
>Until the DNS info expires, how are they going to find out about B?

Actually, unfortunately, they won't, not with any DNS-based fault
tolerance solution. Client software (i.e. web browsers and such)
ignore TTL's.

>2) This means that all the DNS servers have to be inside my area
>(behind links A and B), right?

Yes.

>3) Are DNS lookups smart enough that, assuming there are 2 master
>servers listed, they will pick one at random.

Yes. (It has been reported that Microsoft DNS isn't smart enough, but
this has been disputed by knowledgeable sources.)

>Otherwise everyone is going to come down pipe A until it goes down
>and the DNS info expires.
>
>If this IS the case, then a partial solution might be to shuffle the
>order of the DNS servers listed for each domain I control (so a.com
>lists the pipe A dns server first, b,com lists the pipe B dns server
>first, etc).

Yes, if MS DNS works as has been described (i.e. stupidly), this may
be a useful trick. However, you don't need to shuffle the order in
your zones so much as in the zone delegation records.

>More details on exactly how to tweak this kind of config would be
>much appreciated.

You've pretty much figured it out. I'm not the expert, merely the
parrot - while I've never used this myself, I know several of our
list members have described making this work, and work well.
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