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I’m experimenting with IPT's (iptables) in Xubuntu.

First experimentation wato allow all OUTPUT traffic and block all INPUT except already existing TCP connections can somebody verify if these are correct

enter image description here

To go a bit more advanced I'm trying to allow als TCP connections to active services on my workstation. My idea is to do a nmap scan and grep the listening/open ports but I'm probably over thinking it.

Finally I'm trying to allow FTP.
I used this additional rule to allow FTP but it seems I still get blocked

sudo iptables -A INPUT -p tcp --dport 21 -m state --state NEW,ESTABLISHED -j ACCEPT
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  • i do know that images can only be posted at 10 rep but i think it is more clear what i want achieved with a img Feb 26, 2013 at 18:34
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    It's better just copy and pste the text from terminal to your question. Feb 26, 2013 at 18:56
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    Can you try sudo iptables -A -p tcp --dport 21 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT. FTP opens another connection for data transfer. I think "RELATED" should handle that. Feb 26, 2013 at 18:58
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    Try to load the FTP connection tracking module sudo modprobe nf_conntrack_ftp. You can also try sudo iptables -A -p tcp --dport 21 -m conntrack --ctstate NEW,ESTABLISHED,RELATED. Feb 26, 2013 at 19:20
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    OK, looking at the screenshot I realise now that iptables is blocking nothing, the policy for all chains is ACCEPT, which means if no rule is matched the packet is accepted. The refused message usualy means the port is closed. I don't think you have an FTP server running (sudo netstat -tlnp to check, search for port 21). Feb 26, 2013 at 19:38

2 Answers 2

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FTP is a bit odd in that to allow inbound traffic on port 21 and outbound traffic on port 20 :

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

In addition ftp will use a random higher port. To allow this 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"

You will still need a way to save your iptables configuration and restore it when you boot. Ubuntu does not have a simple way of doing this. Basically you can either use /etc/rc.local or disable NetworkManager and use networking scripts.

First save your rules:

sudo iptables-save /etc/iptables.save

Method 1 : Edit /etc/rc.local and add the line

iptables-restore /etc/iptables.save

Method 2 : Edit /etc/network/interfaces and use "post-up" to bring our iptables rules up.

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp
post-up /sbin/iptables-restore /etc/iptables.save

Then reboot.

The preferred method is probably to use UFW

sudo ufw allow ftp

UFW is the fedault tool for Ubuntu, uses syntax very similar to iptables, and is enabled and restored on rebooting.

See:

https://serverfault.com/questions/38398/allowing-ftp-with-iptables

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

http://bodhizazen.com/Tutorials/iptables

https://help.ubuntu.com/community/UFW

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If you are new to iptables, you may want to use either gufw or ufw to set up rules initially. You can use rules as simple as "allow incoming ftp" instead of needing to understand all of the special flags to make it work. They will also allow you to set up advanced rules if you need to.

Both ufw and gufw create iptables rules behind the scenes.

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  • thnx for lettting my know those tool exists but i want to train my console kunfu Feb 27, 2013 at 12:41
  • you can still use them to set up a sample rule, then use console iptables to see what they spit out. Feb 28, 2013 at 18:59

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