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Reinstalled 19.03.2023

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!Info
{{{
proftpd -v
ps -ef|grep proftp
dpkg -l '*proftp*'
apt list --installed | less
    proftpd-basic/stable,now 1.3.7a+dfsg-12+deb11u2 all  [installiert]
    proftpd-core/stable,now 1.3.7a+dfsg-12+deb11u2 amd64  [Installiert,automatisch]
    proftpd-doc/stable,now 1.3.7a+dfsg-12+deb11u2 all  [Installiert,automatisch]
    proftpd-mod-crypto/stable,now 1.3.7a+dfsg-12+deb11u2 amd64  [Installiert,automatisch]
    proftpd-mod-wrap/stable,now 1.3.7a+dfsg-12+deb11u2 amd64  [Installiert,automatisch]    
}}}

! Analysis
{{{
$ service proftpd status      # ProFTPD Dienst läuft
$ netstat -tlp|grep proftp    # ProFTPD lauscht auf Port 21 
$ tail -20 /var/log/proftpd/proftpd.log    # Fehlermeldungen im ProFTPD Log
$ tail -20 /var/log/proftpd/tls.log    # Fehlermeldungen im ProFTPD TLS Log 
$ telnet 192.0.2.10 21    # Verbindungstest auf Port 21 mit telnet
$ openssl s_client -connect 192.0.2.10:21 -starttls ftp    # Verbindungstest auf Port 21 mit TLS
}}}

! Deinstall / Clean
{{{
/etc/init.d/proftpd stop
apt-get remove proftpd-basic  (removes all packages listed above)
apt install proftpd-basic
}}}

!Configuration
Main one:
{{{
/etc/proftpd/proftpd.conf
}}}
This has
{{{
Include /etc/proftpd/conf.d/
}}}
We create a custom one in that subfolder
{{{
vi /etc/proftpd/conf.d/custom.conf
# Ftp user doesn't need a valid shell
<Global>
    RequireValidShell off
</Global>
# If desired turn off IPv6
UseIPv6 off
# Default directory is ftpusers home
DefaultRoot ~ ftpuser
# Limit login to the ftpuser group
<Limit LOGIN>
    DenyGroup !ftpuser
</Limit>
}}}

!Restart
{{{
systemctl restart proftpd.service
}}}

! Activate TLS
{{{
$ vi /etc/proftpd/conf.d/tls.conf
<IfModule mod_tls.c>
TLSEngine                  on
TLSLog                     /var/log/proftpd/tls.log
TLSProtocol                TLSv1.2
#TLSOptions                 NoSessionReuseRequired NoCertRequest
#TLSRSACertificateFile      /etc/proftpd/ssl/proftpd.cert.pem
TLSRSACertificateFile      /etc/letsencrypt/live/www.inetone.de/cert.pem
#TLSRSACertificateKeyFile   /etc/proftpd/ssl/proftpd.key.pem
TLSRSACertificateKeyFile   /etc/letsencrypt/live/www.inetone.de/privkey.pem
TLSVerifyClient            off
TLSRequired                on
</IfModule>
}}}

Add a virtual user (in the specified path !)
{{{
> cd /etc/proftpd
> ftpasswd --passwd --name proftpd --uid 1001 --home /home/proftpd --shell /bin/false 
> ftpasswd --passwd --name ftp --uid 1001 --home /home/ftp --shell /bin/false 
}}}
The following output is not important:
{{{
ftpasswd: --passwd: missing --gid argument: default gid set to uid
ftpasswd: creating passwd entry for user proftpd

ftpasswd: /bin/false is not among the valid system shells.  Use of
ftpasswd: "RequireValidShell off" may be required, and the PAM
ftpasswd: module configuration may need to be adjusted.
}}}
This will create the file __/etc/proftpd/ftpd.passwd__ in the current path !

!Active/Passive
Generally FTP clients use passive connections. To enable passive connection you need to enable port 49152-65534 in the firewall (iptables) and in the conf. Alternatively use active connections.

! Logging
Add in proftpd.conf:
{{{
## Logging ##
LogFormat       default "%h %l %u %t \"%r\" %s %b"
LogFormat       auth "%v [%P] %h %t \"%r\" %s"
LogFormat       write "%h %l %u %t \"%r\" %s %b"

TransferLog     /var/log/proftpd/proftpd.xferlog
ExtendedLog     /var/log/proftpd/ftp_auth.log AUTH auth
ExtendedLog     /var/log/proftpd/ftp_access.log WRITE,READ write
# ExtendedLog   /var/log/proftpd/ftp_paranoid.log ALL default
}}}

Increase debug level by starting with -d:
{{{
proftpd -d 5
}}}
-d level\\
Set debugging level (0-10, 10 = most debugging)