Question

Passive mode on FTP-server behind IPTables with NAT

Asked by: yorthen

I have the following setup:
One computer running Slackware 9 with IPTables 1.2.7a protecting a small home network with 4 computers on it. One of these computers (192.168.10.2) is running Windows 2003 and an FTP-server (not IIS) hosting two "sites" one on port 1045 and the other on port 2069. But you can't connect to the FTP-server from the internet using passive mode.

I could open up some ports to allow passive FTP to work but I've heard/read somewhere that iptables could open ports for passive ftp on demand. I've tryed using the ip_conntrack_ftp and it gives me the abillity to use passive FTP from clients in the network to servers on the internet but I still can't use passive FTP from the internet to my server.

Is there anyplace where I can get some more information on using ip_conntrack_ftp and ip_nat_ftp?

My firewallscript:
eth0=internet
eth1=network

   # Load modules
   modprobe ip_conntrack_ftp ports=21,1045,2096
   modprobe ip_nat_ftp ports=21,1045,2069

   # Clear
   iptables -F
   iptables -t nat -F

   # NAT
   echo 1 > /proc/sys/net/ipv4/ip_forward
   iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE # Starta NAT
   iptables -A OUTPUT -o eth0 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT # Allow outgoing connections          <-- Are those lines correct?
   iptables -A INPUT -i eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT # Allow incomming traffic

   # FTP
   iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 1045 -j DNAT --to 192.168.10.2 # FTP 1
   iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 2069 -j DNAT --to 192.168.10.2 # FTP 2

There are some more but it's just forwarding some ports for a number of programs.

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Asked On
2003-06-18 at 04:32:27ID20652062
Tags

ftp

,

iptables

,

passive

,

mode

Topic

Linux Network Security

Participating Experts
1
Points
50
Comments
3

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Answers

 

by: mburdickPosted on 2003-06-18 at 09:30:12ID: 8750578

Couple of thigs here...

1) Is it imperative to support passive-mode clients? If so, why?

2) You mention that you are running a third-party FTP server. Have you verified that the server is compliant with the FTP RFC (RFC959)? Passive-mode clients are really a misnomer. The server is the only component of becoming "passive". If the server doesn't support it, it won't work.

3) The ability to use passive mode on the *inside* and connect to a server on the *outside* does not, and will not, imply that passive mode on an *outside* client can, should, or will work with a server on the *inside*.

 

by: yorthenPosted on 2003-06-18 at 11:37:21ID: 8751524

1) Most users who connect to the server are stupid and they uses windows FTP-clients which uses passive-mode as default and the users have no idea how to turn it of and I'm tired of explaining.

2) Yes, the server supports passive mode, as I said I could forward ports but rather not if it's not necessary.

3) I'm not trying to imply that either. What I was saying was that ip_conntrack_ftp (and possibly ip_nat_ftp, not sure if it's needed) makes passive connections to the outside possible without opening ports and I thought that it might work the other way around too.

I'll restate my questions:
1) Can ip_conntrack_ftp and/or ip_nat_ftp make a note when a server behind the firewall accepts a PASV command and responds with "227 Entering Passive Mode" and then forward and incomming connection to the specifyed port to the server?

2) Where can I find more information about ip_conntrack_ftp and ip_nat_ftp? I've searched but all I could come up with was the sourcecode, which I have neither the knowledge nor the time to analyze.

 

by: mburdickPosted on 2003-06-18 at 11:48:28ID: 8751640

Ok... Now that I'm armed with a deeper understanding of what you need to accomplish...

Have you looked at this information yet? http://www.sns.ias.edu/~jns/security/iptables/iptables_conntrack.html

I read through it [briefly], and believe that it may provide some assistance for you. There is some specific detail about using this with FTP at the very bottom.

20120131-EE-VQP-002

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