Документация ProFTPD: http://www.proftpd.org/docs/howto/TLS.html / http://www.proftpd.org/docs/contrib/mod_tls.html

Мой рабочий конфиг для CentOS 7, ProFTPD 1.3.5 и OpenSSL 1.0.2k

TLSEngine                   on
TLSRequired                 on
TLSProtocol                 TLSv1.2
TLSCipherSuite              ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384
TLSRenegotiate              none
TLSVerifyClient             off
TLSLog                      /var/log/proftpd/tls.log
TLSRSACertificateFile       /etc/letsencrypt/live/rtfm.wiki/cert.pem
TLSRSACertificateKeyFile    /etc/letsencrypt/live/rtfm.wiki/privkey.pem
TLSCertificateChainFile     /etc/letsencrypt/live/rtfm.wiki/chain.pem
TLSDHParamFile              /etc/ssl/certs/dhparam4096.pem

Cipher list взял с сайта Mozilla

Если нет файла DH

openssl dhparam -out /etc/ssl/certs/dhparam4096.pem 4096

Удобнее добавить эти настройки для TLS в отдельном файле через Include в /etc/proftpd.conf

Include /etc/proftpd_tls.conf

/var/log/proftpd/tls.log

2020-01-12 01:43:11,588 mod_tls/2.6[22465]: TLS/TLS-C negotiation failed on control channel
2020-01-12 02:25:10,230 mod_tls/2.6[24365]: TLS/TLS-C requested, starting TLS handshake
2020-01-12 02:25:10,557 mod_tls/2.6[24365]: client supports secure renegotiations
2020-01-12 02:25:10,558 mod_tls/2.6[24365]: TLSv1.2 connection accepted, using cipher ECDHE-RSA-AES256-GCM-SHA384 (256 bits)

Информация о подключении в WinSCP

Проблема в Filezilla

Что это значит (всё ок!)

• https://community.letsencrypt.org/t/installing-a-certificate-in-proftpd/49540/4
• https://community.letsencrypt.org/t/proftpd-over-tls-ssl/8184/4
• https://forum.filezilla-project.org/viewtopic.php?t=36799

<blockquote> Also, keep in mind that Filezilla doesn’t check the CAs that your OS trust so you need to manually trust it in the first connection. No matter whether it is a self-signed certificate, a Let’s Encrypt cert or a commercial one, in all of them, in the first connection, you need to trust the certificate. </blockquote>

<blockquote> The only thing you can do is trust the certificate. It is a Filezilla “feature” it doesn’t check the CAs that your OS trust so you need to manually trust it in the first connection. No matter whether it is a self-signed certificate, a let’s encrypt cert or a commercial one, in all of them, in the first connection, you need to trust it. </blockquote>

Самоподписанный сертификат. Описание ключей в документации openssl.

openssl req -x509 -nodes -sha256 -days 365 -newkey rsa:4096 -keyout /etc/ssl/certs/proftpd.key -out /etc/ssl/certs/proftpd.crt

На будущее осталось разобраться в опциях TLSCACertificateFile и TLSCertificateChainFile. Единственный топик в сети порождает еще больше вопросов.