[BlueOnyx:01252] Re: SMTP Relay servers

[Chris Gebhardt - VIRTBIZ Internet]

Colin Jack wrote:
> Just looking for a bit of guidance.
> 
> We want to run two or three SMTP relay servers for our clients but with the same host name (e.g. relay.domain.com) - so they round-robin or if one is down the others are still available. Is this just a matter of having three A records with the same host name but different IPs and metrics or is there a better way of doing this?

Realistically, the idea is a non-starter.  The problem you will run into 
is that the end-user's ISP will cache a lookup.  Then of course the 
client machine will do a lookup and resolve the hostname to the IP. 
Then it will cache.

Then if one of your SMTP boxes dies for some reason, anybody who had 
already been using that would be down, but the other customers that 
achieved the lookup to another box would still ostensibly go through.

Sure, you could monkey with TTL's and expires, but then you're creating 
more overhead on your end with all the lookups, and there's the added 
issue that in order to reduce network load many of the larger ISPs 
completely ignore your TTL and expire times and cache for whatever time 
period they see fit.   This appears to be an increasing trend.

The better idea is just to have your SMTP not "go down", and if you're 
really concerned, or you have a history of flaky SMTP boxes, have a 
warm-spare standing by.

Alternatively, you could set up some fancy NAT-based load balancing at 
your server-side.

But DNS is not an ideal way to handle this.

-- 
Chris Gebhardt
VIRTBIZ Internet Services
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