DNS load balancing

Blake Hudson blake at ispn.net
Wed May 26 11:06:47 EDT 2010

Andy Bennett wrote:
> In failure cases existing clients will wobble for a bit until their 
> cache expires and then the connections will have to be reestablished. 

I wish it were that straightforward. After performing several
switchovers where DNS A records were repointed, many clients (days
later) continue trying to access the old servers. TTL on the DNS records
are set appropriately short, this is simply a case of  mail clients
inappropriately caching DNS results. The fix for this is to restart the
client, sometimes requiring a reboot of the machine to accomplish.

Additionally, several clients perform a capability check when they
initially connect to a server, but don't re-verify this until restarted.
If the two servers are different, it can cause problems. This typically
involves supported/preferred authentication mechanisms between systems -
not typically a problem in a cluster, but could be a problem in a fail
over scenario to a different system.


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