By default, any UNIX user will be allowed to log in remotely to the SSH server on your system, or use it to upload and download files. On a mail server system or one that hosts websites, however, this may not be appropriate. You might want to allow most users to only log in to your POP3, FTP, or Usermin servers instead. Although it is possible to achieve this by giving them a shell like /bin/false, this could cause other problems with user Cron or At jobs.
Fortunately, the SSH server can be configured to restrict who can log in. Just follow these steps:
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