11.3.3 A Rule to Match Any Address

The special pattern “.” (a single period) will match any host/domain specification if no other rule matches and the host/domain specification cannot be found anywhere in the channel table. In other words, the “.” rule is used as a last resort when address rewriting would fail otherwise.

Regarding substitution sequences, when the match-all rule matches and its template is expanded, $H expands to the full host name and $D expands to a single dot “.”. Thus, $D is of limited use in a match-all rule template!