Re: Round-robin mail & DNS question

In article <bb068t$1g7k$1@xxxxxxxxxxx>,
Alan Murrell  <swasak@xxxxxxxxxxx> wrote:
>Hello,
>
>We are thinking of implementing round-robin mail servers using DNS, but are
>unsure about what would happen if one of the mail servers were to go down. 
>Let's say I have the following setup in the zone record:
>
>@   IN   MX   10   mail.domain.com.
>@   IN   MX   20   server1.domain.com.
>@   IN   MX   30   server2.domain.com.
>mail   IN   A   10.10.10.1
>mail   IN   A   10.10.10.2
>
>Note: 'server1' and 'server2' correspond to the '10.10.10.1' and
>'10.10.10.2' IP's, respectively.
>
>So, we understand that when a lookup is done on 'mail.domain.com', it will
>round-robin between the two IP addresses.  We also understand that
>normally, when the primary MX server becomes unavailable, it will go to the
>secondary MX host.  This is for mail delivery, anyway.  But what happens on
>the client side?  Let's say:
>
>  - 'server1' (10.10.10.1) goes down
>  - a client checks their mail, and on this instance, DNS grabs '10.10.10.1'
>as the ip for 'mail.domain.com'
>
>Will approximately 50% of the mail check requests fail, or will the weighted
>MX records kick in here?  If so, then I guess we would need to look at
>software that does two-way IP address take-over (i.e., if one server goes
>down, the other takes over it's IP address)

MX records are only used for mail delivery, they have nothing to do with
mail reading.

Whether the mail client tries both addresses is up to the client's
programmer.  The DNS query will return all the addresses.  The application
can either use the first one and give up if it fails, or it can try them
all in succession, and this is totally out of your control.  Some clients
will do it one way, others will do it another.  If you want failsafe
operation, you should look into other solutions than round-robin DNS, such
as Local Director.

-- 
Barry Margolin, barry.margolin@xxxxxxxxxx
Level(3), Woburn, MA
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