Silicon Technix

There’s lots of advice on the net about how to setup a server with iptables to allow passive mode FTP. Below is the approach that we’ve found to be most effective.

Start by configuring your FTP daemon to use a fixed range of ports. We use 41361 to 65534 which is the IANA registered ephemeral port range. The exact config depends on what FTP software you’re using:

vsftpd

Edit /etc/vsftpd/vsftpd.conf and add the following lines:

pasv_min_port=49152
pasv_max_port=65534

proftpd

Edit /etc/proftpd.conf and add to the Global section:

 
...... 
PassivePorts 49152 65534
......

Now restart your FTP service so the changes take effect.

Next you’ll need to configure the ip_conntrack_ftp iptables module to load. On Redhat/CentOS just edit /etc/sysconfig/iptables-config and add “ip_conntrack_ftp” to the IPTABLES_MODULES like this:

IPTABLES_MODULES="ip_conntrack_ftp"

Next edit /etc/sysconfig/iptables and add a rule to allow TCP port 21.
The new line is marked in red:

*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 21 -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
COMMIT

Now restart the iptables service:

/sbin/service iptables restart

You can verify that the correct port range has been registered with lsmod like this:

lsmod | grep conntrack_ftp

and you’ll get something like this:

nf_conntrack_ftp       12913  0 
nf_conntrack           79645  4 nf_conntrack_ftp,nf_conntrack_ipv4,nf_conntrack_ipv6,xt_state

And that’s all it takes to get passive mode ftp working behind iptables.

P.S: If your server is behind a physical firewall and you are behind NAT, then you’ll probable need to load the “ip_nat_ftp” iptables module.