Allowing FTP with IPTables

Here’s the document I refer people to so that they can following the FTP protocol:http://slacksite.com/other/ftp.html

  • To do active-mode FTP, you need to allow incoming connections to TCP port 21 and outgoing connections from port 20.
  • To do passive-mode FTP, you need to allow incoming connections to TCP port 21 and incoming connections to a randomly-generated port on the server computer (necessitating using a conntrack module in netfilter)

You don’t have anything re: your OUTPUT chain in your post, so I’ll include that here, too. If your OUTPUT chain is default-drop then this matters.

Add these rules to your iptables configuration:

iptables -A INPUT -p tcp --dport 21 -j ACCEPT
iptables -A OUTPUT -p tcp --sport 20 -j ACCEPT

To support passive mode FTP, then, you need to load the ip_conntrack_ftp module on boot. Uncomment and modify the IPTABLES_MODULES line in the /etc/sysconfig/iptables-config file to read:

IPTABLES_MODULES="ip_conntrack_ftp"

Save the iptables config and restart iptables.

service iptables save
service iptables restart

To completely rule out VSFTPD as being a problem, stop VSFTPD, verify that it’s not listening on port 21 with a “netstat -a” and then run a :

nc -l 21

This will start netcat listening on port 21 and will echo input to your shell. From another host, TELNET to port 21 of your server and verify that you get a TCP connection and that you see output in the shell when you type in the TELNET connection.

Finally, bring VSFTPD back up, verify that it is listening on port 21, and try to connect again. If the connection to netcat worked then your iptables rules are fine. If the connection to VSFTPD doesn’t work after netcat does then something is wrong w/ your VSFTPD configuration.

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