FTP data transfer in active mode.
What is port 20 used for?
Data channel historically used by active FTP. The server establishes the data connection from source TCP port 20 to the client port negotiated on the control channel.
Transport protocol
TCP
Role and operation
FTP separates the control channel, usually TCP 21, from the data channel. In active mode, after the client sends a PORT/EPRT command, the server initiates the data connection from TCP source port 20 toward the negotiated client port. In passive mode, the server instead uses a negotiated data port and port 20 is generally not the data channel.
Security precautions
Should this port be opened in the firewall?
Do not open TCP 20 as a generic rule without confirming the FTP mode in use. In active mode, the firewall must allow the server-initiated data connection toward the client; an FTP helper/ALG or a tightly controlled rule set may be required depending on the design.
How to verify this port
Validate active FTP with a complete transfer in active mode and inspect the negotiated data channel in firewall logs or a packet capture. Test-NetConnection to port 20 only checks TCP connectivity to a listener and does not reproduce the actual active-FTP data-channel workflow.
Port/service mappings are common conventions. An application can be configured to use a different port.