Port 20 (FTP data): Uses and Security

What uses port 20

Classic FTP uses two connections: commands go over port 21, and file contents and directory listings go over a separate data connection. In active mode the server opens that data connection from port 20 back to the client, which breaks behind NAT and most firewalls. Passive mode, where the client connects to a high port announced by the server, is what almost all clients use today.

Should port 20 be open to the internet?

Restrict access. Only open this port to known networks or users, for example your office IP, a VPN or behind authentication.

If listings hang after login, the data connection is being blocked. Switch the client to passive mode, and on the server open the passive port range (for vsftpd, pasv_min_port and pasv_max_port).

Check what is listening on port 20

Linux

sudo ss -ltnp 'sport = :20'

macOS

sudo lsof -nP -iTCP:20 -sTCP:LISTEN

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 20 -State Listen
netstat -ano | findstr :20

No output means nothing is listening. If a service should be, check that it is running and which address it binds to: 127.0.0.1 only accepts local connections, 0.0.0.0 or :: accepts them on every interface.

Fix “address already in use” on port 20

Another process already holds the port. Find it with the commands above, then stop that service properly, or end the process:

macOS / Linux

sudo kill $(sudo lsof -t -iTCP:20 -sTCP:LISTEN)

Windows (PowerShell)

Stop-Process -Id (Get-NetTCPConnection -LocalPort 20 -State Listen).OwningProcess

Allow port 20 from trusted networks only

The examples allow 20 only from 10.0.0.0/8; replace it with your own private network, office IP or VPN range.

Ubuntu / Debian (ufw)

sudo ufw allow from 10.0.0.0/8 to any port 20 proto tcp
# Close it again:
sudo ufw delete allow from 10.0.0.0/8 to any port 20 proto tcp

RHEL / Fedora / Rocky (firewalld)

sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="10.0.0.0/8" port port="20" protocol="tcp" accept'
sudo firewall-cmd --reload

Windows (PowerShell, as administrator)

New-NetFirewallRule -DisplayName "FTP data 20/TCP" -Direction Inbound -Protocol TCP -LocalPort 20 -Action Allow -RemoteAddress 10.0.0.0/8

On AWS, Google Cloud and Azure, also allow the port in the security group or network firewall rules; host firewalls and cloud firewalls are separate.

Test port 20 from another machine

macOS / Linux

nc -zv example.com 20

Windows (PowerShell)

Test-NetConnection example.com -Port 20

Related ports

Port 21 (FTP): the control port of FTP, the File Transfer Protocol.
Port 22 (SSH): used by SSH for secure remote logins, SFTP, SCP and Git over SSH.
Port 990 (FTPS): used by FTPS in implicit TLS mode.

More file sharing ports

69 (TFTP) , 139 (NetBIOS Session) , 445 (SMB) , 873 (rsync) , 2049 (NFS) . See all ports in the port lookup.

Sources

IANA port registry: 20

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Frequently asked questions

What is port 20 used for?

Port 20 is the data channel of FTP in active mode. Classic FTP uses two connections: commands go over port 21, and file contents and directory listings go over a separate data connection. In active mode the server opens that data connection from port 20 back to the client, which breaks behind NAT and most firewalls. Passive mode, where the client connects to a high port announced by the server, is what almost all clients use today.

Is it safe to open port 20?

Only open this port to known networks or users, for example your office IP, a VPN or behind authentication. Allow it only from the networks that need it, as in the firewall examples on this page.

How do I check if port 20 is open?

On the server, list listening sockets with "sudo ss -ltnp 'sport = :20'" on Linux or "sudo lsof -nP -i :20" on macOS. From another machine, run "nc -zv host 20" or, on Windows, "Test-NetConnection host -Port 20". Connection refused means nothing is listening; a timeout means a firewall is dropping the traffic.