MX load balancing using round robin method?

Greetings Everybody!

I was wondering how company's, using the round robin mechanism as a form of load balancing for their mail servers, makes it possible for clients to fetch their e-mail? As far as I know, using the round robin technique, mail is sent randomly among the two, or more, mail servers defined in the zone's MX records? So when a client, like myself, want to fetch their e-mail, we simply define a POP3 server, for incoming mail, in our mail client software. But we are often only able to supply information regarding one POP3 server?

So when I check for new e-mails, I could be asking one of the other mail servers, and this one might not hold any e-mails, instead of the one that was picked randomly to hold the new e-mail?

Take a look at this zone information:


Does these company's use some kind of file-sharing or fetch-mail-from-remote-server solution? Another question would be, wouldn't DNS caching cause some problems whenever One take use of the round robin mechanism?

Hope my question makes just a little bit of sense.
Best regards, bluePrint.

I learned something new researching your answer.

Here's a rewritten zone based on your example:

---------------------------------------------------------
domain.tld. IN MX 10 mail.domain.tld.
domain.tld. IN MX 20 backup.mailserver.tld.

mail.domain.tld. 0 IN A 10.0.0.1
mail.domain.tld. 0 IN A 10.0.0.2
---------------------------------------------------------

Changes I have made and why:

1) Removed CNAMES.
1a) You had 2 CNAME records listed under 1 host. This is forbidden by RFC standards.
1b) In this case using CNAME's is redundant and increases dns traffic load somewhat. I had renamed the mx domains to mail for consistency.

2) removed second MX entry with same preference number. We do not want to use round-robin on the MX records. MX records are to be used to place priority of one server over another when the primary server goes down. With this new setup, only mail.domain.tld will be used unless it is down.

3) doubled up the 2 mail server IP's under the mail.domain.tld domain. This domain is what will be configured to use round-robin. The first time somebody resolves the domain it will resolve to the first IP. The next time somebody resolves the domain, it will be the second IP. The third time somebody resolves the domain, it will be the first IP again. round-robin is NOT random. It is purely cyclic order - evenly distributing load across all IPs.

4) set the cache times for the mail.domain.tld 'A' records to 0. This prevents caching and help assures that the load distribution is properly implemented.

I have to thank you very much for your useful reply.

Although, it seems only to clearify how to properly configure DNS when using round robin along with MX records for incoming mail load balancing.

My question also took starting point at how company's handle the problem that must occure whenever a given client want to fetch his/her e-mail from the POP server specified in their mail client software, as you said, when defining two or more A records for the same domain name, these are visited, not randomly, although I have read this a couple of places, but more in the order of the first client gets the first IP and the second client gets the second IP, and so on.

Imagine this scenario, a company uses both multiple outgoing mail servers(SMTP) and multiple incoming mail servers(POP3) to balance the given load:



The client then uses mail.domain.tld as his/her incoming mail server and smtp.domain.tld as his/her outgoing mail server. These are, of course, defined in their mail client software.

But if mail are sent to one of the available mail servers, destined for a client, and the client then checks his/her e-mail, the client software would then try to fetch his/her mail, but the software might not end up fetching mail from the exact one that the e-mail was sent to in the first place, because of the load balance feature?

I don't take too well with multiple questions made at one time... :-( I have a particularly one-track single-tasking mind. ;-) But here we go, on to the next issue:

First I'd like to point out that I hope those "pop" servers specified in the MX records has smtp server as well. Cause smtp clients use mx records to find SMTP servers - not pop3 servers. No software today that I know of looks for pop3 servers via DNS. It is all configured.

But aside of that, you speak of a very real issue - although common. What do administrators do with keeping multiple webservers who fall under a single domain redundant? There are solutions out there that may automate the duplication of part or an entire harddrive's contents between servers. This is so that when you make changes to the content of one server, it is automatically propagated to the rest of the redundant servers. More specifically, when your smtp server saves mail that it recieves and is sent to or used by a pop3 server... there should be an external program which copies the new mail to the other servers. I already predict a fardling mess when a user tries to delete his email from one server, just to have it copied back again from another server. I don't know the specifics of how this type of software works - or how it allows you to configure it to its needs. I'm just presenting you with one theory.

Bada bing, I just came up with another solution. Possibly more practical. Once again you have an external program managing the mail, except now it is completely centralized. All incoming mail is stored on an internal network server. Every time somebody connects to a POP3 server, it access the mail stored on that server. deletes mail from that server. Whatever is required.

A windows solution wouldn't even need an external program. You just have network shares on the centralized mail server, and all dependant mail servers map a drive to that share. This way multiple servers can share the data as if it was stored locally on their drive. Linux computers could have samba installed so that they can make use of shares. Or perhaps you can use some other protocol and mount the connection in windows shares-like fashion.

Ah well, I'm plumb out of ideas. I've never actually seen or researched a business solution like this put into action. I've just got my imagination to work off of here.

Apparently, I also have a one-track single-tasking mind, because you are of course right regarding SMTP and MX records, my mistake.

Anyway, your answer on how company's may handle the mentioned issue, was exactly what I was looking for. And as you mention yourself, the most optimal solution might/should be something like a central server holding all of the e-mail?

To conclude this/these question(s), thank you very much, yet again!

By the way, I also said something about reading somwhere, that using multiple CNAME records to point to multiple A records, could be somewhat useful? But you corrected me, by letting me know that, _that_ was against RFC standards? I found this example in the Apache URL Rewritting Guide:



But the guide examples also state, that it isn't a good practice to use round robin that way? Again, thank you!

I've brought back a topic to the top that I wrote a bit ago called "Things to remember when defining your zones"

It touches on several points including the multi CNAME issue. It boasts explanations why and references to material to back me up. Not that I think you don't believe me, but just so you make completely educated choices with something besides heresay if ever you need to "correct" someone else. :-)

But yeah, centralized all the way man. I positively hate my first idea.

SilentRage,
regarding the second question,
smtp redundancy is intended for external users, making sure that your servers will always be reached.
now, administrators can use those smtp servers to just relay email from WAN to LAN.
so, email sent to URL might arrive to any of your smtp server and be directly relayed to a third smtp server that is your mailbox repository ( this through SMTP).
The third server being also the POP server.

In case of high traffic, I would actually put 2 SMTP servers and dedicate them to receive and send email (SMTP). Then, emails addressed to my users would be delivered to a third server for repositoy and POP retrieval.

I do not have an extended knowledge of mail systems, but Lotus Domino is a server that can manage that very easily.

Thanks for the DNS clarification..
gud

By all means if you know of programs that makes the first option I gave viable and convenient and such, then use them. Do you have any recommendations on such software so that other people who read this post can be pointed in the right direction?

Thanks for your reply.