Instead of having each individual workstation in a network
handle its own mail, it can be advantageous to have a powerful
central machine that handles all mail.
Such a machine is called a
and is analogous to the way Federal Express originally handled package
In the old days, when you sent a package via Federal Express from, say,
San Francisco to Paris, or even from
San Francisco to Los Angeles,
that package was first sent to Memphis, Tennessee
The Memphis location was the Federal Express hub. All packages, no
matter what their origin or destination, were sent to the hub first
and then were transported outward from there.
The advantage to this approach is that the Memphis hub
was the only Federal Express office that needed the special knowledge
of how to deliver packages anywhere in the world.
Local offices needed to know only how to deliver to the hub.
In a similar way, your workstations can be thought of as
the outlying offices (client machines) and your central mail-handling machine
as the hub. For mail, the hub
performs the following functions for the entire site:
All incoming mail is sent to the hub, meaning that no mail is sent
directly to a client machine.
This hub approach has several advantages.
No client needs to run a
daemon to listen for mail. No client's name needs
to be known to the outside world, thus insulating client machines
for easier security.
Rather than having each client send its mail directly
to the outside world,
all outgoing mail from clients is sent to the
hub, and the hub then forwards that mail to its ultimate destination.
The advantages are
that clients do not need to be continually aware of changes in
the Internet; deferred mail queues on the hub, not on
the client machine, making management simpler;
and a single,
file may be shared by, or distributed
to, all the clients.
All outgoing mail is modified so
that it appears to have originated on the hub.
The alternative is to have
mail returned directly
to each client. The advantages of the hub approach are that
all mail appears to come from a single, giant machine (making
replying to mail easier).
All mail to local users is delivered to, and spooled on, the
hub. The alternative is to have one or more separate mail
spool directories on separate client machines. The advantages
of the hub approach are that all local delivery is handled
by one machine, that no client needs to accept mail for
local delivery, and that management is easier with a single
There are disadvantages to the hub approach when a
site is composed of differing machine architectures
and when machines are spread over many networks:
At sites where there is a huge amount of mail constantly flowing,
the load on the hub can become excessive. This can cause client
mail to be queued rather than sent, even when mail is destined for
For the hub to work, it needs to know the login names of all users on the system.
Either it must have a master
file that is
the union of all systems'
files, or it must use
NIS to access such a master file.
In the absence of universal
knowledge, it must have an
entry for every local user.
Because all mail passes through the hub, rather than being
sent to the recipient directly, there are unavoidable
delays. Those delays are negligible
at small sites (a couple of hundred or so users) but can become
significant at sites with a huge number of users.
If a client machine has direct UUCP connections or is connected to
multiple networks, a more complex configuration file is needed.
The purpose of this tutorial is to develop a small
file for the clients of a hub scheme.
As we did in the previous chapter,
we will call our file
We will be developing it in pieces
and examining those pieces individually in the chapters that follow.
In this chapter we will begin the process of developing a
file from scratch.
It will perform only two tasks:
It will instruct
to send all mail to another machine
that serves as a mail hub. (Recall that in the hub design, no workstation
receives mail, and all mail from clients is sent first to the hub
and from there to the outside world.)
It will make all outgoing mail appear as though it originally came from
that hub. (Thus, replies to mail will be delivered to the hub, not to the client.)
In this chapter we tackle the first task: how to get mail from the client machine to the hub.