Using Sendmail as a Multi-Platform Mail Router
Is e-mail the wave of the future? No way. It's a mission-critical, gotta-have application today. Survey those business cards you've been collecting from associates lately. Odds are most of them are sporting Internet e-mail addresses. In the April 1996 issue of Microsoft Magazine, Bill Gates said, “[E-mail] is probably the most mission-critical application for Microsoft in terms of running the company. If we had to pick one application that would keep running no matter what, E-mail would absolutely be it.” People want fast, reliable, written communications with people in their own company as well as with others across the Net.
My job is to keep e-mail running smoothly for my company. A user recently complained to me because it took a whole 12 minutes for her e-mail message to be delivered. People have ever-growing expectations about what e-mail should do for them.
Providing e-mail in a homogeneous environment is simple. If a shop consists entirely of Linux workstations, e-mail is practically automatic. The same is true for all other major variants of Unix. No special gateways are required to connect to the Internet since the e-mail protocol spoken, SMTP, is the same across the board. SMTP stands for Simple Mail Transport Protocol and has been the de facto standard for Internet mail transmission for years. Exchanging mail is almost as simple with non-SMTP commercial packages, as long as everyone in the company uses the same one. The situation is more complex when several incompatible systems are in use. Getting mail to flow seamlessly between them and additionally the Internet can pose a lot of problems. In this article I will explain how Caliber Technology solved these problems quickly and inexpensively with Linux and sendmail.
Caliber System, through its operating units RPS, Viking Freight, Caliber Logistics, Roberts Express and Caliber Technology, is a value added provider of transportation, logistics, and information services. Caliber Technology provides integrated information services to customers, Caliber companies and other interested users.
The Caliber companies operate in vastly different computing environments: IBM mainframes, AS/400s, HP/UX midrange systems, Novell and NT LANs, Tandem minicomputers and more. The e-mail platforms in use include Microsoft Mail, Lotus cc:Mail, Tandem PS Mail, IBM OV/400, TAO/Emc2 and Microsoft Exchange. Unfortunately these mail systems are generally not compatible with each other. We have plans to migrate all the Caliber companies to compatible e-mail platforms, but the migration effort will take at least two years. The good news is gateways are available for all these systems. The function of a mail gateway is to translate messages from one format to another. Some gateways translate messages from one proprietary format to another: for example, from Lotus cc:Mail to Microsoft Mail and vice versa. Other gateways translate messages between a proprietary format and a standard format. An example here would be cc:Mail to and from SMTP. The two most common standard formats are SMTP and X.400. X.400 is the CCITT/ISO standard and is supported by many commercial networks. Since Caliber's goal was to have all its internal mail systems conversing with each other and with the Internet, SMTP was the obvious choice as the common language.
Here's where it gets complicated. Like many companies, Caliber Technology uses non-obvious login names for its mainframe and LAN systems. In a typical Unix environment, my login name might be tlowery.
But in Caliber's multi-platform environment my login name is, say, xyz123. This was a legacy decision to provide better security. There are too many IDs and they're too ingrained in our systems to let us change them, even if we wanted to. The problem is, most of the gateways we use do not allow the SMTP e-mail name to differ from the local login ID. There's just no way to specify that in the configuration. In other words, my Internet mail address would have to be email@example.com. We found this distasteful for obvious reasons; Actually, it's even worse. The gateways add their network host name to the address for outbound mail. My address would really be firstname.lastname@example.org. So the first problem to solve is to convert, for outbound messages, the private gateway-based addresses to the public Internet addresses, for example from email@example.com to firstname.lastname@example.org.
Solving that problem creates another one. The calibersys.com e-mail domain is made up of several mail systems. If mail is coming inbound from the Internet to email@example.com, how is the decision made to send it to gateway1 (say, Microsoft Mail) rather than gateway2 (TAO/Emc2)? Some system along the way has to look up tlowery and decide to send the message to gateway1. At Caliber, we accept mail for calibersys.com, logistics.calibersys.com, shiprps.com, vikingfreight.com and roberts.com and perform name lookups for all of them.
Like many companies, our internal TCP/IP network is connected to the Internet via a commercial firewall. We purchased the firewall long before deciding to provide Internet mail service to all the Caliber companies. In a way, it can be thought of as a legacy system with some of its own drawbacks. Although our firewall can solve either the first or second problem, it can't solve both at the same time. According to our firewall vendor, we were their first customer who needed name mapping and gateway hiding for multiple domains, all at the same time. Commercial X.500-based directory systems can be purchased to tie everything together elegantly; the problem is cost. It doesn't make much sense to plunk down $200,000 for a solution that, we hope, won't be needed two years from now. With no quick fix from the firewall vendor in sight and hoards of users beating down my door for inter-company and Internet mail, I sat down and studied sendmail to see if it offered any answers. Luckily it does.
The solution we have in place today can be seen in Figure 1. Any mail message, including those to or from the Internet, that travels from one system to another travels through a central hub. For added reliability, there are actually two hubs. One serves as the primary; the other is a backup in case the primary fails. The sole purpose of these hubs is to hide the e-mail addressing details from users. Let's look at an example.
First, let's say I at firstname.lastname@example.org want to send a message to Jane at email@example.com. Both the sender and recipient addresses exist inside the Caliber firewall, so the message will not be sent to the Internet. My mailbox name on Microsoft Mail is xyz123 and the SMTP gateway for Microsoft Mail is called gateway1. When the message leaves my mail system, it looks like this:
From: firstname.lastname@example.org To: email@example.com
The message is routed to mhub, our primary address mapping hub. First it looks up firstname.lastname@example.org. When it finds a match, that address is changed to email@example.com. It then looks at firstname.lastname@example.org. There are three possible gateways for vikingfreight.com, gateway1, gateway2 and gateway3, corresponding to Tandem PS Mail, Microsoft Exchange and TAO/Emc2, respectively. The hub looks up the address and sees that Jane's mailbox (jdoe) is on PS Mail and is called vft1100. So mhub changes email@example.com to firstname.lastname@example.org. The message header now looks like this:
From: email@example.com To: firstname.lastname@example.org
The hub then hands the message off to gateway1.vikingfreight.com for delivery to Jane's mailbox. Now let's take a look a closer look at the hub.
The primary mail hub is a Compaq Proliant 1500 with a 133 MHz Pentium processor. The backup hub is a Dell OptiPlex with a 75 MHz Pentium. Both are running Linux Slackware version 3.0. The decision to run Linux was an easy one. I wanted a solid, dependable solution at the lowest possible cost—that meant Intel-based hardware and Linux. I've used Linux for various projects over the last three years and have never experienced a kernel crash. I can't say that for all the commercial operating systems I've used. The crucial piece of software for this project is sendmail, available for virtually all Unix variants. I knew if Linux didn't work out, I could swap in a Sun, IBM or HP workstation without having to make software changes. The risk of using Linux as the initial platform was very small. In the four months since the project went live, I'm happy to say that we've had zero problems with it. Now it's time to discuss the nitty-gritty details. Two primary pieces make the hub work: DNS and sendmail. I'll discuss each in turn.