Many administrators struggle with VSFTPD's implicit SSL configuration because the documentation isn't explicit about port handling. The key issue is that implicit SSL requires a separate listener on port 990, while explicit SSL (the default) uses port 21.

Here's a working configuration that enables both implicit and explicit SSL:

listen=YES
listen_port=21
implicit_ssl=YES
listen_port6=990
ssl_enable=YES
allow_anon_ssl=NO
force_local_data_ssl=YES
force_local_logins_ssl=YES
ssl_tlsv1=YES
ssl_sslv2=NO 
ssl_sslv3=NO
require_ssl_reuse=NO
rsa_cert_file=/etc/ssl/certs/vsftpd.pem
rsa_private_key_file=/etc/ssl/private/vsftpd.key

The most important configuration lines are:

implicit_ssl=YES      # Enables port 990 listener
listen_port6=990      # Explicitly sets IPv6 implicit SSL port
listen_port=21        # Explicit FTP port

Create your certificate with OpenSSL (Debian-specific paths):

openssl req -x509 -nodes -days 365 -newkey rsa:2048 \
-keyout /etc/ssl/private/vsftpd.key \
-out /etc/ssl/certs/vsftpd.pem

If the server fails to start:

1. Check /var/log/vsftpd.log for errors
2. Verify certificate permissions (chmod 600 for private key)
3. Test port availability: netstat -tulnp | grep 990
4. Ensure no other FTP server is running

For Debian's UFW firewall:

ufw allow 21/tcp
ufw allow 990/tcp
ufw allow 40000:50000/tcp  # Passive ports range

Many administrators struggle with enabling Implicit SSL in VSFTPD because its implementation differs significantly from Explicit SSL. While the documentation mentions the implicit_ssl parameter, simply setting it to "YES" often leads to service failures without clear error messages.

Before proceeding, ensure you have:

sudo apt update
sudo apt install vsftpd openssl

And a valid SSL certificate at /etc/ssl/certs/vsftpd.pem. If missing, create one:

sudo openssl req -x509 -nodes -days 365 -newkey rsa:2048 \
-keyout /etc/ssl/private/vsftpd.key \
-out /etc/ssl/certs/vsftpd.pem

Here's a tested configuration that enables both Explicit and Implicit SSL modes:

listen=YES
listen_port=21
listen_ipv6=NO
anonymous_enable=NO
local_enable=YES
write_enable=YES
dirmessage_enable=YES
use_localtime=YES
xferlog_enable=YES
connect_from_port_20=YES
seccomp_sandbox=NO

# SSL Configuration
ssl_enable=YES
implicit_ssl=YES
listen_port=990
allow_anon_ssl=NO
force_local_data_ssl=YES
force_local_logins_ssl=YES
ssl_tlsv1=YES
ssl_sslv2=NO
ssl_sslv3=NO
require_ssl_reuse=NO
ssl_ciphers=HIGH
rsa_cert_file=/etc/ssl/certs/vsftpd.pem
rsa_private_key_file=/etc/ssl/private/vsftpd.key

1. The listen_port directive becomes port 990 when implicit_ssl=YES is set
2. You must explicitly set ssl_enable=YES before implicit_ssl
3. IPv6 must be disabled (listen_ipv6=NO) for this configuration to work properly

After saving the configuration, restart VSFTPD:

sudo systemctl restart vsftpd

Verify both ports are listening:

sudo netstat -tulnp | grep vsftpd

Expected output should show both ports 21 and 990.

Problem: Service fails to start
Solution: Check journal logs with journalctl -xe and ensure:
- Certificate paths are correct
- No port conflicts exist
- SELinux/AppArmor isn't blocking access

Problem: Clients can't connect to port 990
Solution: Verify firewall rules:

sudo ufw allow 990/tcp
sudo ufw allow 21/tcp