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FTP server says: "421 service not available, remote server has closed connection."

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This Error Reference is intended for: Any audience.

If an FTP server won't allow logins, it is probably configured correctly, but the problem is probably with authorizing users at login.

FTP servers in current distributions often authorize users with the Pluggable Authentication Modules library, in which case there should be an authorization file /etc/pam.d/ftp. A generic authorization file looks like this. (The line break on the first "auth" line is for readability. The entry is actually a single, long line).

 #%PAM-1.0
 auth                required /lib/security/pam_listfile.so item=user
                     sense=deny file=/etc/ftpusers onerr=succeed
 auth                required /lib/security/pam_pwdb.so shadow nullok
 auth                required /lib/security/pam_shells.so
 account             required /lib/security/pam_pwdb.so
 session             required /lib/security/pam_pwdb.so

Also, make sure the /etc/ftpusers file, or whatever users file is named in the first "auth" line, is configured correctly.

Btw, the sample ftp file above is actually the ftpd/ftp.pam.sample file from the ftpd-BSD-0.3.1.tar.gz package. Many thanks to David A. Madore for this much needed port.
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