[NTLUG:Discuss] Use DNS for redundant geographic sites?

[Leroy Tennison]

ntlug at thorshammer.org wrote:
> 	DNS caching will get you. Someone's resolver will cache webA (that it got from dnsA). dnsA goes down, but the resolver still has the record cached. Traffic will go to webA even though dnsA is down because the resolver will hold on to the record. The resolver will have to expire the record for webA and fail over to dnsB before it will hit webB.
>
> 	You can control how long a record will be cached, but 'sain' levels are 5-min to 30-min to 3-hrs to 24-hrs depending on how much traffic you get (higher times for more traffic). I've heard that some ISPs will not recognize low TTLs (like anything under 30 minutes) though I have no idea how true or if true how widespread this is.
>
> 	Check this out for a suggestion, especially the last paragraph:
>
> http://en.wikipedia.org/wiki/Round_robin_DNS
>
> Robert
>
>
> On Sun, Nov 19, 2006 at 10:52:47PM -0600, Neil Aggarwal wrote:
>   
>> Hello:
>>
>> I am trying to figure out a way to create a web site
>> that is multi-homed geographically.
>>
>> I just thought of this idea:
>>
>> I will have two machines, each located in
>> a different place.
>>
>> Machine A will run:
>> 	dnsA.jammconsulting.com (DNS server)
>> 	webA.jammconsulting.com (Web Server)
>>
>> Machine B will run:
>> 	dnsB.jammconsulting.com (DNS server)
>> 	webB.jammconsulting.com (Web Server)
>>
>> Now, lets assume that I register a new domain
>> name.  For discussion, lets use jammconsulting.com
>> as the domain name.
>>
>> In the domain records, I set the DNS servers to
>> be:
>> 	dnsA.jammconsulting.com
>> 	dnsB.jammconsulting.com
>>
>> Now, here is the strange part:
>>
>> dnsA will always resolve www.jammconsultng.com to 
>> the IP address of webA and dnsB will always resolve
>> www.jammconsulting.com to the IP address of webB.
>>
>> This way, if either machine goes down, the machine
>> will not respond to DNS queries.  The DNS will query
>> the other machine and all traffic will go to it.
>>
>> I know this does not protect me against the situation
>> where dnsA is up and webA is down, but as long as
>> the machine is up, I can set up monitoring of the
>> web server.  Usually, Apache httpd runs without a
>> problem.
>>
>> Will DNS clients handle a situation where the 
>> nameservers give different responses?
>>
>> Are there other downsides I have not considered?
>>
>> Thanks,
>> 	Neil
>>
>> --
>> Neil Aggarwal, (214)986-3533, www.JAMMConsulting.com
>> FREE! Eliminate junk email and reclaim your inbox.
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>>
>>
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>>
>>     
>
> _______________________________________________
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>
>   
You've been given some problems to consider and the Wiki discussion of 
DNS-Round-Robin accurately points out problems with that approach.  
Maybe it's time to ask what your ultimate goal is rather than a specific 
way to accomplish it.  Possibly that will open up other avenues of 
approach such as clustering.