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valid users (S)
              This is a list of users that should be allowed to login to this service. Names starting with 
	      '@', '+' and '&' are interpreted using the  same  rules  as described  in the  invalid users 
	      parameter.

              If  this  is  empty  (the default) then any user can login.  If a username is in  both  this 
	      list and the invalid users list then access is denied for that user.

              The current servicename is substituted for %S . This is useful in the [homes] section.

              See also invalid users

              Default: No valid users list (anyone can login)

              Example: valid users = greg, @pcusers

IIRC there was a discussion recently on this list about the interaction
of this option with the "write list" and "read list" parameters, so you
should perhaps also play with this ones.

HTH,

Wolfi

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