Thursday, 23 June 2016

Setting up IPTables from scratch on CentOS 7

This tutorial will provide you with a basic IPTables configuration to help you get up and running initially.

We will firstly start by flushing all of our IPTables rules.

** Warning: This could potentially lock you out of SSH if performed incorrectly - ensure you have console access to the server if something goes wrong! **

Ensure that a default-accept rule is in place on all of the default chains:

sudo iptables -P INPUT ACCEPT
sudo iptables -P FORWARD ACCEPT
sudo iptables -P OUTPUT ACCEPT

and then flush the chains as well as any non-standard chains:

sudo iptables -t nat -F
sudo iptables -t mangle -F
sudo iptables -F
sudo iptables -X

Now we will start by allowing traffic to freely flow out and in from our loopback interface:

sudo iptables -A INPUT -i lo -j ACCEPT
sudo iptables -A OUTPUT -o lo -j ACCEPT

We will also want to ensure that already established connections can get back to the server - i.e. allow stateful connections.

sudo iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

We will likely also want to allow all outbound traffic from connections that are currently established:

sudo iptables -A OUTPUT -m conntrack --ctstate ESTABLISHED -j ACCEPT

We will also likely wish to allow SSH access from a specific host network:

sudo iptables -A INPUT -p tcp -s 10.11.12.13/32 --dport 22 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT

and allow the incoming SSH connection outbound back to the SSH initiator:

sudo iptables -A OUTPUT -p tcp --sport 22 -m conntrack --ctstate ESTABLISHED -j ACCEPT

We might also wish to accept ICMP echo requests:

sudo iptables -A INPUT -p icmp -j ACCEPT

And also log and then drop any other packets:

sudo iptables -N LOGGING
sudo iptables -A INPUT -j LOGGING
sudo iptables -A FORWARD -j LOGGING
sudo iptables -A LOGGING -m limit --limit 2/min -j LOG --log-prefix "IPTables-Dropped: " --log-level 4
sudo iptables -A LOGGING -j DROP

By leaving the default chain policies as 'ACCEPT' we ensure that if someone accidentally flushes a chain they are not going to lock themselves out.

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