Connection tracking is an essential security feature of Iptables. But, what is connection tracking?
It is the ability to maintain connection information in memory. This is new feature added in 2.4.xx Linux kernel. Eariler only commercial firewall has this feature but now it is part of Linux. It can remember connection states such as established & new connections along with protocol types, source and destination ip address. You can allow or deny access based upon state. Following are the states:
- NEW – A Client requesting new connection via firewall host
- ESTABLISHED – A connection that is part of already established connection
- RELATED - A connection that is requesting a new request but is part of an existing connection.
- INVALID - If none of the above three states can be referred or used then it is an INVAID state.
Let us try to understand four state with ftp example (our setup):
client FTP Server 126.96.36.199 188.8.131.52 client.me.com ftp.me.com
A) Connet to ftp server:
You have to use ftp command as follows:
$ ftp ftp.me.com
It opens NEW (STATE) connection at ftp server.
client NEW FTP Server 184.108.40.206 ---> 220.127.116.11 client.me.com ftp.me.com
B) Download files
> get bigfile.tar.gz
When client download files from ftp server we call it ESTABLISHED connection.
client ESTABLISHED FTP Server 18.104.22.168 22.214.171.124 client.me.com ftp.me.com
Please note that when you see username/password prompt your connection get established and access to ftp server is granted upon successful authentication.
C)Passive ftp connections
In A passive ftp connection, client connection port is 20, but the trasfer port can be any unused port 1024 or higher. To enable passive mode ftp client can send pass command:
Passive mode on.
You need to use RELATED state at firewall level if you wish to allow passive ftp access. Here is an example of SSH server, allow only new and established connection for SSH server IP 126.96.36.199.
iptables -A INPUT -p tcp -s 0/0 --sport 513:65535 -d 188.8.131.52 --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp -s 184.108.40.206 --sport 22 -d 0/0 --dport 513:65535 -m state --state ESTABLISHED -j ACCEPT
It also works with stateless protocol such as UDP. The following example allows connection tracking to forward only the packets that are associated with an established connection:
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ALLOW
In the past we have covered lots of examples related to iptables connection tracking.
Update You may need to put following two lines in your script to use connection tracking feature:
Please see complete example script here.