Backup MX - DNS "Fallback"
 

Hello everyone

Today I tried setting up a MX backup server running ISPConfig.

I just created a regular "ISPConfig mailserver" and configured the mail transport according to posts found in the forums here, as well as relay access.

However I don't get the point on how to configure the DNS.

I have MX 1 with Prio. 10 and MX 2 with Prio. 20 as well as a CNAME mail.domain.tld pointing to MX 1.

Sending mails through MX 1 FQDN or CNAME works well.
Sending through domain.tld (without mail CNAME) doesn't work (no SMTP server found).

So MX 2 doesn't work at all.

Is their something I have to do with the DNS? Normally, it should work sending mails "to" domain.tld as it's suposed to lookup the MX records, or do I miss understand this?

Thanks for your help!
MaddinXx

You must not use CNAMEs for MX records. MX records must always point to A records.

Hi falko

Well, at the very end they do point to A records. What I have is:

mail.domain.tld -> CNAME to -> mail.alpha.otherdomain.tld
mail.alpha.otherdomain.tld -> MX -> Prio. 10
mail.beta.otherdomain.tld -> MX -> Prio. 20

mail.alpha.otherdomain.tld -> A -> to mailserver IP
mail.beta.otherdomain.tld -> A -> to 2nd mailserver IP

Both mailservers are running on other servers than the webserver.

Why do you use an MX record for mail.domain.tld? it should be domain.tld unless you use email addresses like user@mail.domain.tld.

Hi falko

The CNAME is for the tld itself. I've attached the two DNS zones:

http://www.img-teufel.de/thumbs/Bild...c8244aejpg.jpg

and

http://www.img-teufel.de/thumbs/Bild...013ba1cjpg.jpg

The DNS records look ok.

For my understanding: sending mails or receiving mails - where is the problem?

Like this, sending mails is only possible via the 1st mailserver and there is no fallback to the 2nd one, if the 1st fails.

Why?
- Because it only works with the CNAME (which points to one server (1st))

So the email client doesn't find out that there are two servers he can try.

But, a question aside, because probably I misunderstood the functionality of a backup MX.

Shouldn't it be possible with this setup to:

- If someone sends an email to info[ @ ]rackster, his mailserver checks if my 1st server is up. If yes -> send to it, if no -> send to the 2nd. As soon as the 1st is up again, 2nd -> sending to 1st -> to the email address finally.

- If I want to send an email, check if 1st server is up -> if not, send via the 2nd.

Thank you falko :)

The backup MX is for receiving mails only. Your mail client will use the configured mail server for sending mails, it doesn't check if there's a fallback if the mail server is down.

Ah and again we can see how wrong "knowledge" can lead to confusion.
I'll retry than I guess :)

Thank you falko!