Connecting to an FTP server can fail with the message "500 Illegal PORT command". It usually happens when the client is not using passive mode, or when the server is not set up to support it.
Key takeaways
- The client sends a PORT command to say where the server should connect for data.
- Servers or firewalls often reject this, so passive mode is needed.
- Configure the passive port range and server address in the FTP server settings.
- Open the same port range in your firewall and restart the FTP service.
The FTP connection issue
In active mode, the client uses the PORT command to tell the server which port to use for the data connection. The server may reject it if it expects a passive (PASV) connection, or if the port is blocked by a firewall or NAT.
The error log
COMMAND:> PORT 192,168,0,101,196,215
500 Illegal PORT command.
ERROR:> Syntax error: command unrecognized.
ERROR:> Failed to establish data socket.The solution: passive mode configuration
Configure the FTP server to handle passive mode properly. That means defining the ports used for passive connections and setting the server's public IP address. For vsftpd, edit vsftpd.conf:
pasv_enable=YES
pasv_min_port=[lowest port in your range]
pasv_max_port=[highest port in your range]
pasv_address=[public IP address of your server]- pasv_enable turns on passive mode.
- pasv_min_port and pasv_max_port set the range of ports for data connections. These ports must be open in the firewall.
- pasv_address is the server's public IP address.
Some guides also suggest pasv_promiscuous=YES. It relaxes a security check, so use it only if nothing else works and you understand the risk.
After the change
- Open the passive port range in the server firewall and any cloud security group.
- Restart the FTP service.
- Switch your FTP client to passive mode and test again.
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Frequently asked questions
What does 500 Illegal PORT command mean?
The server rejected the PORT command the client sent to set up the data connection. Using passive mode usually solves it.
Do I need to open ports in the firewall?
Yes. The passive port range you configure must be allowed in the server firewall and in any cloud security group.
Should I use SFTP instead?
SFTP is generally the safer choice because it encrypts the connection and uses a single port, which avoids passive-mode problems.
