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DNS for MX mail exchangers with multiple servers / IP addresses

Posted: Fri Feb 27, 2015 2:53 pm
by Nanook
From time to time customers report that mail from some site is undeliverable.

Almost always this is because of broken DNS on the sending site. For mail to be accepted for delivery here, we require proper inverse -> forward -> inverse DNS mapping.

Some sites do not have their DNS properly mapped, and often when we notify them, they site the need for multiple IPs, frequently because of load distributors or failover devices.

It is possible to map a single hostname such as mxall.shitheads.net to multiple IP addresses and still maintain proper inverse -> forward -> inverse mapping.

To do this, first assign separate hostnames for each IP address in the group, for example, let's say you want mxall.shitheads.net to map to multiple IP addresses. You can do this without breaking inverse -> forward -> inverse DNS relationships by first assigning a hostname to each IP address.

For example, let's say you have three IPs you want to assign to this host, and they are 204.122.31.2, 204.122.31.3, and 204.122.31.4. You would create individual hostnames for each such as:

204.122.31.2 -> mxall02.shitheads.net
204.122.31.3 -> mxall03.shitheads.net
204.122.31.4 -> mxall04.shitheads.net

You would then create A records mapping each hostname to it's corrosponding IP address as well as PTR records mapping each IP address back to it's corrosponding hostname.

Then in addition, you would create A records with mxall.shitheads.net pointing to all of the IPs (just make three A records, same host, but different IPs).

This will allow you to connect to multiple IPs with one hostname but still provide the necessary inverse -> forward -> inverse DNS relationship needed to make mail servers accept your e-mail.