Note: To begin it is necessary to run vsftpd (Very Secure File Transfer Protocol Daemon).
1. Generate CSR file using OpenSSL .
2. Select Other from menu, when prompted which type of server you are using.
3. Find a .zip file in the e-mail with your certificate. It contains Root certificate and Intermediate SSL. You have to concentrate it to one file (.pem). Use a command and stay with this order:
# cat your.server.crt UTNAddTrustExternalCA.crt AddTrustExternalCA.crt > your.server.pem
4. Move the file where you keep certificates (usually it is /usr/share/ssl/certs/ ).
5. Open vsftp.conf file (default location is /etc/vsftpd/vsftpd.conf) with text editor and add following line rsa_cert_file /path/to/your.server.pem
6. Restart vsftpd with
# /etc/init.d/vsftpd restart
# /sbin/service/vsftpd restart
# killall vsftpd
7. Your vsftp server is secure now.