load balance with multiple A record

[Len Conrad]

>if i have 2 A record for a single host,
>
>server.foo.com        IN    A        10.1.1.1        (from ISP A)
>server.foo.com        IN    A        20.1.1.1        (from ISP B)
>
>when isp B goes down, does the DNS continue to resolve the server
>address to 20.1.1.1

DNS doesn´t know anything is down, so it continues to answer with the 
entire RRset.

It´s the querying machines that will detect no response.  Whether they will 
try the other A record depends on the application.


>,hence half of my connection request is actually droped.

50% of queriers will go to the dead ip.

>will i be able to do someting like the exchange priority on an A
>record, where the second entry via
>ISP B have a lower priority than A?

no, with with DNS and A records.

MX and SRV records provide params to do that, but the queriers need to know 
how to play along.

Len



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