Multiple MX or A Records?

[Max Clark]

Hi all,

I have seen and used both multiple MX records in DNS for a zone, as
well as one MX record pointing to multiple A records. For our
customers it is much easier to tell them to point to mx.foo.com vs.
mx1.foo.com and mx2.foo.com.

I am curious - what is the impact / benefits of these different
approaches. Will multiple A records tied to one MX entry give me
sufficient failover?

TIA,
Max

--
Max Clark
http://www.clarksys.com

[Wietse Venema]

Max Clark:

> Hi all,
>
> I have seen and used both multiple MX records in DNS for a zone, as
> well as one MX record pointing to multiple A records. For our
> customers it is much easier to tell them to point to mx.foo.com vs.
> mx1.foo.com and mx2.foo.com.

Does the client perform MX lookups?

> I am curious - what is the impact / benefits of these different
> approaches. Will multiple A records tied to one MX entry give me
> sufficient failover?

Does the client perform MX lookups?

Wietse

[Victor Duchovni]

On Thu, Apr 06, 2006 at 10:37:40AM -0700, Max Clark wrote:

> Hi all,
>
> I have seen and used both multiple MX records in DNS for a zone, as
> well as one MX record pointing to multiple A records. For our
> customers it is much easier to tell them to point to mx.foo.com vs.
> mx1.foo.com and mx2.foo.com.
>
> I am curious - what is the impact / benefits of these different
> approaches. Will multiple A records tied to one MX entry give me
> sufficient failover?

Postfix will not care, but Sendmail will make at most one connection
per-host per delivery, so if the first A record returns 4XX, Sendmail
will skip all further A records for that host.

The other place were you may see a difference is with TLS peername
verification. A single multi-homed host makes secure channels
easier.

--
Viktor.

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[Max Clark]

> Postfix will not care, but Sendmail will make at most one connection
> per-host per delivery, so if the first A record returns 4XX, Sendmail
> will skip all further A records for that host.

Interesting - do you know what the behavior for yahoo, aol, hotmail,
gmail is? This leads me to want to have at least two mx records listed
for the domain.

Thanks,
Max

On 4/6/06, Victor Duchovni <Victor....@morganstanley.com> wrote:
> On Thu, Apr 06, 2006 at 10:37:40AM -0700, Max Clark wrote:
>

> > Hi all,
> >
> > I have seen and used both multiple MX records in DNS for a zone, as
> > well as one MX record pointing to multiple A records. For our
> > customers it is much easier to tell them to point to mx.foo.com vs.
> > mx1.foo.com and mx2.foo.com.
> >
> > I am curious - what is the impact / benefits of these different
> > approaches. Will multiple A records tied to one MX entry give me
> > sufficient failover?
>

> Postfix will not care, but Sendmail will make at most one connection
> per-host per delivery, so if the first A record returns 4XX, Sendmail
> will skip all further A records for that host.
>
> The other place were you may see a difference is with TLS peername
> verification. A single multi-homed host makes secure channels
> easier.
>
> --
> Viktor.
>
> Disclaimer: off-list followups get on-list replies or get ignored.
> Please do not ignore the "Reply-To" header.
>
> To unsubscribe from the postfix-users list, visit
> http://www.postfix.org/lists.html or click the link below:
> <mailto:majo...@postfix.org?body=unsubscribe%20postfix-users>
>
> If my response solves your problem, the best way to thank me is to not
> send an "it worked, thanks" follow-up. If you must respond, please put
> "It worked, thanks" in the "Subject" so I can delete these quickly.
>

[Victor Duchovni]

On Thu, Apr 06, 2006 at 10:52:31AM -0700, Max Clark wrote:

> > Postfix will not care, but Sendmail will make at most one connection
> > per-host per delivery, so if the first A record returns 4XX, Sendmail
> > will skip all further A records for that host.
>
> Interesting - do you know what the behavior for yahoo, aol, hotmail,
> gmail is? This leads me to want to have at least two mx records listed
> for the domain.

Perhaps, but how often will you be issuing 4XX replies for mail that
is immediately deliverable on another MX host? I only know about the
behaviour of Postfix and Sendmail in this space.