Beginning with V8.7,
sendmail began to support a flat text-file form
of database. The /etc/hosts file is an example
of such a flat file, in that it is organized in a line-by-line
When such files are read as databases (with the
text type, text), you need
to specify which column contains the key and which contains the
For nisplus, netinfo, and
ph database maps, the -k switch
specifies the name (text) of the desired column.
When the -k switch specifies which column contains
the key, its absence defaults to 0 for the text
type (which is indexed beginning with 0), and defaults to the name of
the first column for the nisplus type. See also
-v (-v) for the returned
value's column, and -z (-z) for the column delimiter.
Finally, note that for ldap database maps the
-k switch has a different meaning, one that is
particular to that type.