OT: Load Balancing & redundancy using Multiple A records

Steve Freegard steve.freegard at fsl.com
Fri Nov 23 11:13:11 GMT 2007

ram wrote:
> We use multiple MX records for our MailServers for all our clients. 
> We have already added one data center and had to inform all customers to
> add more MX records 
> Can I avoid changing of MX records by adding the new ips as multiple A
> records and using DNS round robin

First of all - read this: 

However, I personally still believe this to be a worthwhile approach and 
tend to recommend this to service providers.  IMO it is best to add the 
records like so:

mx1.domain.com. 60 IN A
                 60 IN A
                 60 IN A
                 60 IN A

The '60' is the TTL value, as the above article states, this is probably 
the minimum value you will get away with anyway and it will increase the 
load on your DNS servers.

As you should always have at least 2 MX records and the fact that some 
DNS resolvers and implementation are very broken, I specify the same 
list again in reverse order, just to be on the safe side.

mx2.domain.com. 60 IN A
                 60 IN A
                 60 IN A
                 60 IN A

Then specify the actual MX records

domain.com.      IN MX  10   mx1.domain.com.
                  IN MX  10   mx2.domain.com.

> What are the problems associated with this kind of setup 

It works, but don't expect it to load balance well.  For that you really 
do need a proper load-balancer that can examine metrics of each of the 
back-end systems and direct connections to them accordingly.

Kind regards,

Steve Freegard
Fort Systems Ltd.
Tel: +1 202 595 7760 x801

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