Linux as a Backup E-mail Server
One Friday morning several months ago, the Microsoft Exchange e-mail server I'm in charge of crashed. At the time, I was a fairly new Windows NT administrator. The server, which we call tuccster, was not running properly again until late the following Sunday evening.
Unfortunately, at this point my problems had just begun. Our departmental e-mail server had been down for nearly three days and megabytes of important e-mail were spooled on remote servers all over the Internet. There was no way to predict when the mail would arrive and how much e-mail might never show up. Since different e-mail servers try to re-send e-mail at different intervals, what did show up would arrive out of chronological order. While I had learned a lot about Windows NT disaster recovery, the whole event was a major inconvenience for the users in my department and a horrible embarrassment for me.
While I'm not convinced I could have prevented the failure of Microsoft Exchange, I could have set up a fall-back e-mail server to spool all incoming mail while tuccster was down. Shortly after the mishap, I was able to find an old Gateway 486/66 with an Ethernet card that was being replaced with a faster Pentium system. Using Debian Linux and sendmail, I set up a fall-back e-mail server that receives and spools any incoming e-mail whenever tuccster, the primary Windows-based server, is down. The addition of a web server and a simple CGI script written in Perl provided a simple user interface into the system. I configured the web server so that a web browser could be used from certain trusted hosts to check if anything is waiting in the sendmail queue. Once our primary server is ready to begin receiving e-mail, the sendmail queue can be flushed by clicking a link on the same web page.
A “fall-back e-mail server” is an old idea on the Internet—the functionality to set one up is actually built into the Domain Name Server (DNS) protocol. The intent was that every important e-mail server would have a backup in place. A Domain Name Server contains many types of records. The most common of these types are SOA records which indicate authority for a domain's data; NS records which list name servers for a domain; A records which map a name to an address; PTR records which perform the reverse, mapping an address to a name; and MX records which describe “mail exchangers”. MX records allow one to define the actual host responsible for receiving mail directed at any particular host. The host actually responsible for receiving e-mail need not be the host to which the mail appears to be addressed.
To illustrate why this would be useful, imagine a set of workstations called larry, curly and moe. To reduce the load on curly and moe, we would like all incoming e-mail to be directed to larry, regardless of the host to which the mail was actually addressed. MX records provide a way to achieve this goal. Suppose we program our DNS server with the following:
larry.tucc.uab.edu. IN MX 1 larry.tucc.uab.edu. curly.tucc.uab.edu. IN MX 1 larry.tucc.uab.edu. moe.tucc.uab.edu. IN MX 1 larry.tucc.uab.edu.
If somebody tries to send e-mail to firstname.lastname@example.org, the mail transport agent (MTA) should look up the DNS record and see that larry is responsible for all e-mail directed to moe. Not all MTAs properly implement MX redirection. The mail will then be delivered to larry as if it were addressed to email@example.com.
While this is useful, it is not all that can be accomplished with MX records. The number appearing in the example between “MX” and “larry.tucc.uab.edu” is a preference value. Suppose I was worried that student projects running on larry might cause the system to crash periodically, or that larry was running a less-than-robust e-mail server. I could set up curly as a fall-back server by using the following DNS entries:
larry.tucc.uab.edu. IN MX 1 larry.tucc.uab.edu. larry.tucc.uab.edu. IN MX 2 curly.tucc.uab.edu. curly.tucc.uab.edu. IN MX 1 larry.tucc.uab.edu. curly.tucc.uab.edu. IN MX 2 curly.tucc.uab.edu. moe.tucc.uab.edu. IN MX 1 larry.tucc.uab.edu. moe.tucc.uab.edu. IN MX 2 curly.tucc.uab.edu.
Now suppose that larry is down for some reason. A remote host attempting to send e-mail to larry would discover that larry is unavailable. It would then learn from DNS that curly is the next preferred e-mail server for larry. The remote host will send the message to curly. The mail transport agent (such as sendmail) on curly will then realize that larry is preferred over curly as a mail exchange. It then spools the message locally, periodically attempting to pass the message on until it succeeds.