Debian 2.6.8/bridge/iptables/passive ftp

[Arnd-Hendrik Mathias]

Hi Jo,
the first thing I am wondering about is that you open ports 1024:65535 
while I would expect the initial data connection at port 20.
Nevertheless, the main problem you are facing is that you try to 
conntrack FTP on your own. FTP is a little bit too complex for that so 
you'll get by with a little help from your friend: The helper module may 
be the solution for your problem.
I built my linux from scratch so I cannot tell you much about any 
distributions or util packages, but my PC serves as gateway for the both 
of my local home-networks to the internet and my ftp routing works well 
so I paste the corresponding section of my configuration in order to 
give an example. Since you don't seem to be masquerading you can omit 
the last rule and replace the IP-adresses and interface names. Note that 
these rules only accept outgoing FTP connections, so if you're driving a 
server you'll have to add NEW to the --ctstate of the second rule.



###########################
# forwarding tcp sessions to global net #
###########################
*filter
-A FORWARD -s 10.0.0.0/255.255.255.224 -d ! 10.0.0.0/255.255.255.224 -i 
! ppp0 -o ppp0 -p tcp --dport 21 -m conntrack --ctstate NEW,ESTABLISHED 
-j ACCEPT
-A FORWARD -d 10.0.0.0/255.255.255.224 -s ! 10.0.0.0/255.255.255.224 -i 
ppp0 -o ! ppp0 -p tcp --sport 21 -m conntrack --ctstate ESTABLISHED -j 
ACCEPT
-A FORWARD -d 10.0.0.0/255.255.255.224 -s ! 10.0.0.0/255.255.255.224 -i 
ppp0 -o ! ppp0 -p tcp -m helper --helper ftp-21 -m conntrack --ctstate 
RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -s 10.0.0.0/255.255.255.224 -d ! 10.0.0.0/255.255.255.224 -i 
! ppp0 -o ppp0 -p tcp -m helper --helper ftp-21 -m conntrack --ctstate 
RELATED,ESTABLISHED -j ACCEPT
COMMIT

*nat
-A POSTROUTING -s 10.0.0.0/255.255.255.224 -o ppp0 -p tcp --dport 21 -m 
conntrack --ctstate NEW -j MASQUERADE
COMMIT



Good luck

Arnd-Hendrik


spaminator at web.de wrote:

>Hi there,
>
>I'm experiencing a strange problem when trying to FTP through a firewalling bridge.
>
>My FTP client connects to the FTP server ok. But when the client switches to passive mode to get the directory's file list I get 
>
>stuck.
>
>The bridge is running on a Debian Sarge box with kernel 2.6.8-3, iptables 1.2.11-10 and bridge-utils 1.0.4-1. The bridge is built from the physical devices eth0 and eth1.
>
>The bridge is assigned an IP address too to be able to manage it remotely. Hence the INPUT and OUTPUT rules in my /etc/firewall.up.rules. As far as I understood, iptables only uses the FORWARD chain for the bridged packets.
>
>Here is my /etc/firewall.up.rules:
>#
># is invoked by /etc/network/interfaces as pre-up for br0
>#
>*filter
>#
>:INPUT DROP [0:0]
># some input rules
>#
>:FORWARD DROP [0:0]
>-A FORWARD -m state --state INVALID -j DROP
>-A FORWARD -p icmp -j ACCEPT
># client to server
>-A FORWARD -p tcp -s ! 217.17.69.18/255.255.255.224 --sport 1024:65535 \
>	-d 217.17.69.18/255.255.255.224 --dport 21 \
>	-m conntrack --ctstate NEW,RELATED,ESTABLISHED -j ACCEPT
>-A FORWARD -p tcp -s ! 217.17.69.18/255.255.255.224 --sport 1024:65535 \
>	-d 217.17.69.18/255.255.255.224 --dport 1024:65535 \
>	-m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
># server to client
>-A FORWARD -p tcp -s 217.17.69.18/255.255.255.224 --sport 21 \
>	-d ! 217.17.69.18/255.255.255.224 --dport 1024:65535 \
>	-m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
>-A FORWARD -p tcp -s 212.117.69.128/255.255.255.224 --sport 1024:65535 \
>	-d ! 217.17.69.18/255.255.255.224 --dport 1024:65535 \
>	-m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
># logging
>-A FORWARD -j ULOG --ulog-nlgroup 1
>#
>:OUTPUT DROP [0:0]
># some output rules
>#
>COMMIT
>#
>
>
>These are all rules in the FORWARD chain. Using "! --syn" or "-m state --state RELATED,ESTABLISHED" instead of "-m conntrack --ctstate RELATED,ESTABLISHED" leads to the same result:
>
>When I look into the logfile I find an entry where my client:somehighport tries to tcp the server:somehighport. To me it looks like the client seems to want to establish a data-connection and iptables does not recognize these packet as RELATED or ESTABLISHED.
>
>Just for the crack of it I temporarily added NEW to the second "client to server"-rule. With that it works fine, but leaves the boxes behind the bridge open for any attack on the high ports.
>
>http, https or anything else is working properly, if I implement them in the FORWARD chain.
>
>Any suggestions out there?
>
>bye and TIA
>Jo
>
>
>
>
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