DNS Stuff (only for the techs!)
 

Hi guys.
This is not a request for help but rather a post made out of genuine interest.

As far as I knew, you could not have more than one A record for a DNS entry but a web developer I know recommended setting up a DNS entry with 2 A records to create something called a "round robin".

As far as I understand it, this would make every other request go to the alternate server.
Is this right?

From what I've heard this does help in load balancing situations but would be pointless if the content is not identical on both servers, correct?

Re: DNS Stuff (only for the techs!)
 

Correct - it was a common load balancing method.

It doesn't work very well these days as most dns clients cache the first entry they get - they don't keep going back to the dns server.

(you can have as many A records for the same domain as you want - it's not limited to one)

Re: DNS Stuff (only for the techs!)
 

Aah I see yeah.
Windows does cache DNS, if you do a CMD box and type "ipconfig /flushdns" then that would be the equivalent of deleting your temp files. :)

Re: DNS Stuff (only for the techs!)
 

But the DNS servers that you use whilst browsing (type ipconfig /all to see your current ones) dont get flushed when you do this, so they still have the first record cached, meaning you will get the first record the next time you request that site as thats what your dns server knows.....

Hope that makes sense...

As stingray said... round robin was a cheap and easy way of doing load balancing, but it wasnt 100% reliable... the other way some people did it, was to set different a records on each of their nameservers.. but it wasnt a "realistic" way of doing it still.......

Re: DNS Stuff (only for the techs!)
 

Cool, that's interesting.
Apparently Google use some twisted method of abusing TTL to perform load balancing but I'm not sure of the specifics.

At least I know that using "round robin" is essentially crap now though. :D

Re: DNS Stuff (only for the techs!)
 

Google has a very low TTL, so they can change the IP their domain(s) resolve to and it will have an effect within 10 minutes. Crude and error prone.

Re: DNS Stuff (only for the techs!)
 

Even more error prone if you're using the Active Directory DC as your DNS server too, because that will cache stuff very persistently AFAIK.