sendmail, 4th Edition

Use Value As Is with $&

For situations in which a macro should not be recursively expanded when the configuration file is read, but rather should be used in rules as is, V8 sendmail offers the $& prefix. For example, consider the following RHS of a rule:

R...     $w.$&m

When sendmail encounters this RHS in the configuration file, it recursively expands $w into its final text value (where that text value is your hostname, such as lady). But because the m macro is prefixed with $&, it is not expanded until the rule is later evaluated at runtime.^[309]

To illustrate one application of $&, consider a client/hub setup. In such a setup, all mail sent from a client machine is forwarded to the hub for eventual delivery. If the client were to run a sendmail daemon to receive mail for local delivery, a mail loop could (in the absence of an MX record) develop where a message would bounce back and fourth between the client and the hub, eventually failing.

To break such a loop, a rule must be devised that recognizes that a received message is from the hub:

R $+              $: $&r @ $&s <$1>       Get protocol and host
R smtp @ $H <$+>  $#local $: $1           Local delivery breaks a loop
R $* <$+>         $#smtp  $@ $H $: $2     Punt to hub

These rules appear in the parse rule set 0. By the time they are reached, other rules have forwarded any nonlocal mail to the hub. What is left in the workspace is a lone username. The first rule in the preceding example matches the workspace and rewrites it to be the sending protocol ($&r; see $r on page 842), ...