[Pdns-users] CNAME extra balast for DNS server ?

Lorens Kockum lorens-pdns-3987 at tagged.lorens.org
Wed Aug 25 16:20:44 UTC 2004

On Wed, Aug 25, 2004 at 10:22:14AM -0400, Ben Kennedy wrote:
> On 25 8 2004 at 4:14 am -0400, Roeland Buisman wrote:
> >domeinnaam.nl MX mail.axess.nl
> I thought that strictly speaking, MX records had to point to CNAMEs (in
> your example mail.axess.nl is an A record).

No! You've got it backwards; an MX may *not* point to a
CNAME. It should point to an A.

webmail.axess.nl        A
webmail.axess.nl        A
pop.axess.nl            A
pop.axess.nl            A
mail.axess.nl           A
mail.axess.nl           A

mail.domeinnaam.nl      CNAME pop.axess.nl
domeinnaam.nl           MX    mail.axess.nl

is quite OK. I suppose mail.domeinnaam.nl is used to configure
the POP clients.

It would be more "classic", though, to say

mail1.axess.nl          A
mail2.axess.nl          A
domeinnaam.nl           MX 10 mail1.axess.nl
domeinnaam.nl           MX 10 mail2.axess.nl

That way the situation is more clearly defined in the case where
one of your mail.axess.nl IPs goes down -- I am not quite sure
whether an MTA, given an MX with two IPs, will immediately try
the second if the first does not respond. It might even be that
some implementations might always choose the same one . . .
never tested that.

> >My question here is: Can this give any problems with the load on the DNS
> >server ( 7000 domains), or any other problems? Or is there a better way
> >? ( we donÃ
½t want our customers to have to change all there settings. )

Assuming the same TTLs for both configurations, it will increase
the traffic by the amount of lookups on "mail.domeinnaam.nl",
because when the result is a CNAME, the CNAME will have to be
resolved.  That shouldn't be a problem, though. The alternative
is to keep

mail.domeinnaam.nl      A
mail.domeinnaam.nl      A

and update that for all clients; if I understand correctly
this is what you wish to avoid. Another alternative is a load

HOWEVER! Bear in mind that a cname is an alias...
for everything! That means that mail adressed to
user at mail.domeinnaam.nl may be rewritten to user at pop.axess.nl.
Postfix does it.  This is usually *not* what people want, and a
good reason for totally avoiding cnames.

#include <std_disclaim.h>                          Lorens Kockum

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