Chapter 11: Hosting Virtual Domain and Users

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Highlights

The term virtual appears in many areas of computer science and information technology. Generally, it refers to something that looks real, but isn't. In an operating system with virtual memory, for example, the available memory appears to be unconstrained by the amount of real, physical memory installed in the system. The operating system uses swap space—memory on larger, slower disk drives— to supplement the high-speed random-access memory (RAM). The mapping of virtual memory to RAM and swap space is transparent to jobs running under the operating system.

With e-mail systems, virtual domains and virtual users also appear to be something they aren't. A virtual domain looks exactly like a real domain with a dedicated mail server. Given an address such as [e-mail], you cannot tell whether it's real or virtual simply by examining the address. Example.com could be running its own dedicated mail server, or it could be a virtual domain hosted by a service provider.

So what's the big deal about virtual domains?

Without virtual domains, you have two choices: You can set up a dedicated server for each domain you host, or you can host the domains as aliases of the local domain. The first option is often prohibitively expensive because of the hardware and labor required. The second option is usually unacceptable because it requires sharing one namespace among all of the co-resident domains. For example, if example.com uses [e-mail], then no other domain hosted by the same system would be able to have an info address. Another problem is misdirected mail. Say example.com and fly.example.net are hosted on the same system. Further, one of the fly.example.net users is david, and one of the example.com users is dave. If somebody intending to send mail to [e-mail] misremembers his address as [e-mail], the message will be delivered to the wrong person instead of being returned as undeliverable.

With virtual domains, each co-resident domain can have its own, private namespace, without any danger of collision with another domain's addresses.

OK, so what's a virtual user? The term virtual user has two different usages, one specific to qmail and one applicable to e-mail systems in general. In qmail, a virtual user is simply a virtual domain with only one address. In general, a virtual user is a mail user without a real system account.

qmail includes native support for qmail-style virtual users and virtual domains through the virtualdomains control file (see Chapter 3, "Configuring qmail: The Basics"). Several add-on utilities provide enhanced support for virtual domains, allowing virtual domain managers to manage their domains via Web interfaces, and provide support for the more general style of virtual user, where all of a virtual domain's users can be hosted using a single Unix user ID (UID). The most popular and powerful of these add-ons are VMailMgr and Vpopmail.

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