sendmail, 4th Edition

$[ and $]: A Special Case

The special database-map type called host can be declared to modify name-server lookups with $[ and $]. The special symbolic name and type pair, host and host, is declared with the $( and $) operators like this:

Khost host -a.

The -a switch was discussed earlier in this chapter. Here, it is sufficient to note how it is used in resolving fully qualified domain names with the $[ and $] operators in the RHS of rules. Under V8 sendmail, $[ and $] are a special case of the following database lookup:

$(host lookuphost $)

A successful match will ordinarily append a dot to a successfully resolved hostname.

When a host type is declared with the K command, any suffix of the -a replaces the dot as the character or characters added.^[342] For example:

$[ lookuphost $]     ← found, so rewritten as lookuphost.domain.

Khost host -a
$[ lookuphost $]     ← found, so rewritten as lookuphost.domain

Khost host -a.yes
$[ lookuphost $]     ← found, so rewritten as lookuphost.domain.yes

The first line shows the default action of the $[ and $] operators in the RHS of the rules. If lookuphost can be fully qualified, its fully qualified name becomes the rewritten value of the RHS and has a dot appended. The next two lines show the -a with no suffix (note that with no suffix the -a is optional). In this configuration file, the fully qualified name has nothing (not even a dot) appended. The last two lines show a configuration file with a .yes as the suffix. This time, the fully qualified name has a ...