On 24 Jun 2005 at 8:30, Rodrigo Severo wrote:

I don't usually comment, but since this is continuing, I guess I will. First I'll state the obvious. If you have something specific in mind you want the software to do to meet some particular personal itch, then feel free to modify your implementation so it does it. With that said, I can't begin to imagine why, as the administrator of a system *sending* email, it's my responsibility to provide load balancing services to the recipients. DNS is *not* a load balancing service though it will provide some crude round-robin or cyclic distribution of the load. If the administrators of the systems receiving email are concerned about load balancing, then they should purchase a true load balancer and have it manage the distribution of the load. (There are different sort of load balancers, but there would then only be one primary MX record.)

Anyway, I want my MTA to implement the appropriate email standards and handle message routing. I do *not* want it to also be a load balancer or any other completely different type of software or device. If I also want something in a different category, I'll set it up or acquire it. Sam's approach certainly strikes me as the correct one and the one I desire. The MTA should ask the resolver for MX records. In the absence of MX records, use any A records. Use the first record the resolver returns. Everything else is the concern of a different type of software.

Now, wearing my DNS admin hat, if I wanted some sort of primitive round robin load balancing for some set of mail servers I ran, but did not want to invest in a true load balancer, I probably wouldn't use multiple MX records of the same priority. I would probably use one MX record at the higher priority and then associate multiple IPs with the canonical name. But it's not something I've ever spent any time working on or worrying about.

I'm pretty sure Sam has already given you the only answer you'll get from him. DNS and load balancing are not within the scope of courier. If you want something different in either of those departments, look for DNS software or load balancers that meet your needs. At least, I hope that remains his answer. The requirement in the standard is that all MX records of the lowest priority be tried before giving up (and preferably all MX records are tried, though the standard doesn't actually mandate that). It is not a requirement that the MTA attempt to 'randomize' MX records if presented with multiple MX records of the same priority. And I would categorize that as undesirable extraneous behavior in my MTA.

Scott