The function of rule set 0 is to decide which mail delivery agent
will handle delivery for a particular recipient address.
Figure 9.1
shows how rule set 0 relates to the other rule sets.
Rule set 0 is called once for each recipient of a mail message.
Rule set 0 determines
a triple for each address. The triple is
composed of three parts: the symbolic name of the mail delivery agent,
the name of the
user to whom the mail is addressed, and the name of the
host to which the mail will be sent. We'll expand on these soon
and cover the other rule sets in later chapters.
Recall that one goal of the client.cf file is to cause sendmail
to forward all mail to a central hub machine for processing.
To lay the groundwork for this, we designed the hub delivery agent in
Chapter 6, The Mail Hub and Delivery Agents,
and declared it like this:
# Delivery agent definition to forward mail to hub
Mhub, P=[IPC], S=0, R=0, F=mDFMuXa, T=DNS/RFC822/SMTP, A=IPC $h
As this goal implies,
we don't want to design any fancy rules for rewriting
the recipient's address, we simply want to send it to the hub as is.
The rule set that we show below does just that. It may
look complicated, but recall that its one rule is basically just an if-then
statement:
S0
R$+ $#hub $@${REMOTE} $:$1 forward to hub
The first line (the S0) declares the start of rule set 0.
The R line in this example is the first of many possible rules that will form
rule set 0.
You saw the LHS of this rule in the preceding chapter. The $+ wildcard operator
is used to match one or more tokens in the workspace. Essentially,
this LHS will match anything in the workspace but will fail to match
an empty workspace.
