passive mode ftp

passive mode ftp

[Eric Constantineau]

Hi !

I want to be able to use passive ftp under my firewall...

Right now passive mode does not function because I allow only packets
WITH port 21 as a destination OR source.  When in normal mode, packets
from the ftp server have 21 for the source port and the packets from the
client have 21 for the destination port so it works.  But, entering
passive mode, the destination and source ports are not 21, so my
firewall blocks.

Using a ftp client such as leechftp, cuteftp, wsftp is okay, but using
the browser (iexplore) does not because it use passive mode.

I don't want to open too much my ports, it would mean that someone could
use those ports to get access to inside networks...  But I want to be
able to use passive mode, some users do not want to use ftp client
software, or are not even able to install them...

Is there a specific range of ports that all passive ftp use ?

Thanks for helping me

Eric

Re: passive mode ftp

[Cedric Blancher]

Le ven 23/05/2003 à 10:56, Eric Constantineau a écrit :
> I want to be able to use passive ftp under my firewall...

Use Netfilter conntrack functionnalities :

# modprobe ip_conntrack_ftp

[...]
> Is there a specific range of ports that all passive ftp use ?

Using FTP conntrack helper, you can spot data connections with RELATED
state. A setup setup could be :

	iptables -A FORWARD -m state --state ESTABLISHED,RELATED \
		-j ACCEPT
	iptables -A FORWARD -m state --state NEW -p tcp -s $LAN \
		--dport 21 -o $EXTIF -j ACCEPT

Once a FTP connection is up, further packets are handled by ESTABLISHED
state rule, and connection data initiating is handled by RELATED state.

If you're using NAT, then you will need to modprobe ip_nat_ftp module.

-- 
Cédric Blancher  <blancher@cartel-securite.fr>
IT systems and networks security - Cartel Sécurité
Phone : +33 (0)1 44 06 97 87 - Fax: +33 (0)1 44 06 97 99
PGP KeyID:157E98EE  FingerPrint:FA62226DA9E72FA8AECAA240008B480E157E98EE

Re: passive mode ftp

[Ray Leach]

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On Fri, 2003-05-23 at 10:56, Eric Constantineau wrote:
> Hi !
> 
> I want to be able to use passive ftp under my firewall...
> 
> Right now passive mode does not function because I allow only packets
> WITH port 21 as a destination OR source.  When in normal mode, packets
> from the ftp server have 21 for the source port and the packets from the
> client have 21 for the destination port so it works.  But, entering
> passive mode, the destination and source ports are not 21, so my
> firewall blocks.
> 
> Using a ftp client such as leechftp, cuteftp, wsftp is okay, but using
> the browser (iexplore) does not because it use passive mode.
> 
> I don't want to open too much my ports, it would mean that someone could
> use those ports to get access to inside networks...  But I want to be
> able to use passive mode, some users do not want to use ftp client
> software, or are not even able to install them...
> 
> Is there a specific range of ports that all passive ftp use ?
No, passive mode uses random ports from 1024 upwards.

You could use a web proxy like squid. You can force squid to always use
active mode.

Alternatively, you could use the state inspection of iptables and the
ftp connection tracking module to configure passive ftp.

> 
> Thanks for helping me
> 
> Eric
> 
> 
> 
> 

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RE: passive mode ftp

[Patrick Ahler]

http://slacksite.com/other/ftp.html

Read the article above... about active vs. passive ftp.

Also, most ftp servers allow you to specify which ports to use for passive
ftp, that way you would only have to open those.

-Patrick

-----Original Message-----
From: netfilter-admin@lists.netfilter.org
[mailto:netfilter-admin@lists.netfilter.org]On Behalf Of Ray Leach
Sent: Friday, May 23, 2003 6:05 AM
To: Netfilter Mailing List
Subject: Re: passive mode ftp


On Fri, 2003-05-23 at 10:56, Eric Constantineau wrote:
> Hi !
>
> I want to be able to use passive ftp under my firewall...
>
> Right now passive mode does not function because I allow only packets
> WITH port 21 as a destination OR source.  When in normal mode, packets
> from the ftp server have 21 for the source port and the packets from the
> client have 21 for the destination port so it works.  But, entering
> passive mode, the destination and source ports are not 21, so my
> firewall blocks.
>
> Using a ftp client such as leechftp, cuteftp, wsftp is okay, but using
> the browser (iexplore) does not because it use passive mode.
>
> I don't want to open too much my ports, it would mean that someone could
> use those ports to get access to inside networks...  But I want to be
> able to use passive mode, some users do not want to use ftp client
> software, or are not even able to install them...
>
> Is there a specific range of ports that all passive ftp use ?
No, passive mode uses random ports from 1024 upwards.

You could use a web proxy like squid. You can force squid to always use
active mode.

Alternatively, you could use the state inspection of iptables and the
ftp connection tracking module to configure passive ftp.

>
> Thanks for helping me
>
> Eric
>
>
>
>

RE: passive mode ftp

[George Vieira]

You must load 
 
insmod ip_conntrack_ftp
 
otherwise it won't work without it.. you must also add a rule similar to this..
 
iptables -A INPUT -p tcp --m state RELATED -j ACCEPT
 
You must use RELATED as the connection is related to an existing ftp session on port 21. ip_conntrack_ftp knows the connection and port you negotiated with the ftp server and allows it..
 
hope that helps.. there's so many scripts out there which shows this info.

	-----Original Message----- 
	From: Eric Constantineau [mailto:mekanik@nerim.net] 
	Sent: Fri 23-May-03 6:56 PM 
	To: netfilter@lists.netfilter.org 
	Cc: 
	Subject: passive mode ftp