Can we load balance traf[f]ic for CNAME records?

[Manish Rane]

Ok - let me rephrase the question. I guess Charles got it right.

I understand that Mail Delivery load balance can be achieved by usingMX
priorities. My concern is not that, rather I am more worries about users
who will be using A record to configure their mail clients like IMAP or
POP. I am thinking on load balancing their since I want users to access the
both the ISPs to connect. I can have A/CNAME? record configured in my zone
with *lower TTL value say 180* so that if any of the link goes down I can
edit the zone and have the faulty entry removed which eventually would cost
me less downtime. That way I dont need to do any configuration at client
end since the A/CNAME record is gonna be the same.

That is why I was wondering if A or CNAME can be configured for two
different IP addresses which also holds MX Records and thus configure the
load balancing by that way?

Please advise.

On Fri, Dec 14, 2012 at 12:01 PM, Charles Swiger <cswiger at me.com> wrote:

> On Dec 13, 2012, at 9:27 PM, Manish Rane <manishr78 at gmail.com> wrote:
> > I m planning to add cname records to two A records to different IPs
> provided by two ISP. Can I loadbalance traffic then?
>
> Using CNAMES and load-balancing traffic isn't directly related.  You can
> implement simple load-balancing by listing multiple A records for a name
> without using a CNAME at all, for example.
>
> > Like say i need to play with mail services and I have 2 ISPs.say ISP A
> and ISP B.
> >
> > MX 10 mail.example.com  20.20.20.20 (from isp A)
> >
> > Mx 20 mail01.example.com 30.30.30.30 (from isp B)
> >
> > CNAME cas.example.com
> > mail.example.com 20.20.20.20
> > mail01.example.com 30.30.30.30
> >
> > Now when users wil confugure their mail boxes they will use
> cas.example.com.
>
>
> MX records are used to define a destination for SMTP mail delivery; SMTP
> uses MX records or falls back to A records in order to canonify the domain
> name portion; trying to use CNAMES to alter the canonical destination for a
> domain is a Bad Idea (tm).
>
> When you speak of users "configuring their mail boxes", you're probably
> referring to POP or IMAP clients, which could use a CNAME if you wanted.
>  Mail reading (ie, POP/IMAP/etc) and mail delivery (ie, SMTP) are different
> protocols, and large sites almost invariably have separate pools of
> external MX, internal MX, and reader boxes.
>
> > Can I load balance the traffic for cas.example.com?
>
> Sure.  But I suspect your real question involves something more
> complicated beyond the trivial round-robin behavior of multiple DNS
> records, such as load-balancing with session affinity, or liveness testing
> of destinations, or allocating traffic to the least busy destination, or
> geographic traffic management between multiple destinations (ie, data
> centers).
>
> You can get somewhere with something like RFC-2391 LSNAT, for example, but
> there is a reason why folks pay for hardware load-balancing hardware and
> CDNs….
>
> Regards,
> --
> -Chuck
>
>
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